1 This is doc/cpp.info, produced by makeinfo version 4.13 from 2 /Volumes/androidtc/androidtoolchain/./src/build/../gcc/gcc-4.6/gcc/doc/cpp.texi. 3 4 Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 5 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 6 2010, 2011 Free Software Foundation, Inc. 7 8 Permission is granted to copy, distribute and/or modify this document 9 under the terms of the GNU Free Documentation License, Version 1.3 or 10 any later version published by the Free Software Foundation. A copy of 11 the license is included in the section entitled "GNU Free Documentation 12 License". 13 14 This manual contains no Invariant Sections. The Front-Cover Texts 15 are (a) (see below), and the Back-Cover Texts are (b) (see below). 16 17 (a) The FSF's Front-Cover Text is: 18 19 A GNU Manual 20 21 (b) The FSF's Back-Cover Text is: 22 23 You have freedom to copy and modify this GNU Manual, like GNU 24 software. Copies published by the Free Software Foundation raise 25 funds for GNU development. 26 27 INFO-DIR-SECTION Software development 28 START-INFO-DIR-ENTRY 29 * Cpp: (cpp). The GNU C preprocessor. 30 END-INFO-DIR-ENTRY 31 32 33 File: cpp.info, Node: Top, Next: Overview, Up: (dir) 34 35 The C Preprocessor 36 ****************** 37 38 The C preprocessor implements the macro language used to transform C, 39 C++, and Objective-C programs before they are compiled. It can also be 40 useful on its own. 41 42 * Menu: 43 44 * Overview:: 45 * Header Files:: 46 * Macros:: 47 * Conditionals:: 48 * Diagnostics:: 49 * Line Control:: 50 * Pragmas:: 51 * Other Directives:: 52 * Preprocessor Output:: 53 * Traditional Mode:: 54 * Implementation Details:: 55 * Invocation:: 56 * Environment Variables:: 57 * GNU Free Documentation License:: 58 * Index of Directives:: 59 * Option Index:: 60 * Concept Index:: 61 62 --- The Detailed Node Listing --- 63 64 Overview 65 66 * Character sets:: 67 * Initial processing:: 68 * Tokenization:: 69 * The preprocessing language:: 70 71 Header Files 72 73 * Include Syntax:: 74 * Include Operation:: 75 * Search Path:: 76 * Once-Only Headers:: 77 * Alternatives to Wrapper #ifndef:: 78 * Computed Includes:: 79 * Wrapper Headers:: 80 * System Headers:: 81 82 Macros 83 84 * Object-like Macros:: 85 * Function-like Macros:: 86 * Macro Arguments:: 87 * Stringification:: 88 * Concatenation:: 89 * Variadic Macros:: 90 * Predefined Macros:: 91 * Undefining and Redefining Macros:: 92 * Directives Within Macro Arguments:: 93 * Macro Pitfalls:: 94 95 Predefined Macros 96 97 * Standard Predefined Macros:: 98 * Common Predefined Macros:: 99 * System-specific Predefined Macros:: 100 * C++ Named Operators:: 101 102 Macro Pitfalls 103 104 * Misnesting:: 105 * Operator Precedence Problems:: 106 * Swallowing the Semicolon:: 107 * Duplication of Side Effects:: 108 * Self-Referential Macros:: 109 * Argument Prescan:: 110 * Newlines in Arguments:: 111 112 Conditionals 113 114 * Conditional Uses:: 115 * Conditional Syntax:: 116 * Deleted Code:: 117 118 Conditional Syntax 119 120 * Ifdef:: 121 * If:: 122 * Defined:: 123 * Else:: 124 * Elif:: 125 126 Implementation Details 127 128 * Implementation-defined behavior:: 129 * Implementation limits:: 130 * Obsolete Features:: 131 * Differences from previous versions:: 132 133 Obsolete Features 134 135 * Obsolete Features:: 136 137 Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 138 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 139 2010, 2011 Free Software Foundation, Inc. 140 141 Permission is granted to copy, distribute and/or modify this document 142 under the terms of the GNU Free Documentation License, Version 1.3 or 143 any later version published by the Free Software Foundation. A copy of 144 the license is included in the section entitled "GNU Free Documentation 145 License". 146 147 This manual contains no Invariant Sections. The Front-Cover Texts 148 are (a) (see below), and the Back-Cover Texts are (b) (see below). 149 150 (a) The FSF's Front-Cover Text is: 151 152 A GNU Manual 153 154 (b) The FSF's Back-Cover Text is: 155 156 You have freedom to copy and modify this GNU Manual, like GNU 157 software. Copies published by the Free Software Foundation raise 158 funds for GNU development. 159 160 161 File: cpp.info, Node: Overview, Next: Header Files, Prev: Top, Up: Top 162 163 1 Overview 164 ********** 165 166 The C preprocessor, often known as "cpp", is a "macro processor" that 167 is used automatically by the C compiler to transform your program 168 before compilation. It is called a macro processor because it allows 169 you to define "macros", which are brief abbreviations for longer 170 constructs. 171 172 The C preprocessor is intended to be used only with C, C++, and 173 Objective-C source code. In the past, it has been abused as a general 174 text processor. It will choke on input which does not obey C's lexical 175 rules. For example, apostrophes will be interpreted as the beginning of 176 character constants, and cause errors. Also, you cannot rely on it 177 preserving characteristics of the input which are not significant to 178 C-family languages. If a Makefile is preprocessed, all the hard tabs 179 will be removed, and the Makefile will not work. 180 181 Having said that, you can often get away with using cpp on things 182 which are not C. Other Algol-ish programming languages are often safe 183 (Pascal, Ada, etc.) So is assembly, with caution. `-traditional-cpp' 184 mode preserves more white space, and is otherwise more permissive. Many 185 of the problems can be avoided by writing C or C++ style comments 186 instead of native language comments, and keeping macros simple. 187 188 Wherever possible, you should use a preprocessor geared to the 189 language you are writing in. Modern versions of the GNU assembler have 190 macro facilities. Most high level programming languages have their own 191 conditional compilation and inclusion mechanism. If all else fails, 192 try a true general text processor, such as GNU M4. 193 194 C preprocessors vary in some details. This manual discusses the GNU 195 C preprocessor, which provides a small superset of the features of ISO 196 Standard C. In its default mode, the GNU C preprocessor does not do a 197 few things required by the standard. These are features which are 198 rarely, if ever, used, and may cause surprising changes to the meaning 199 of a program which does not expect them. To get strict ISO Standard C, 200 you should use the `-std=c90', `-std=c99' or `-std=c1x' options, 201 depending on which version of the standard you want. To get all the 202 mandatory diagnostics, you must also use `-pedantic'. *Note 203 Invocation::. 204 205 This manual describes the behavior of the ISO preprocessor. To 206 minimize gratuitous differences, where the ISO preprocessor's behavior 207 does not conflict with traditional semantics, the traditional 208 preprocessor should behave the same way. The various differences that 209 do exist are detailed in the section *note Traditional Mode::. 210 211 For clarity, unless noted otherwise, references to `CPP' in this 212 manual refer to GNU CPP. 213 214 * Menu: 215 216 * Character sets:: 217 * Initial processing:: 218 * Tokenization:: 219 * The preprocessing language:: 220 221 222 File: cpp.info, Node: Character sets, Next: Initial processing, Up: Overview 223 224 1.1 Character sets 225 ================== 226 227 Source code character set processing in C and related languages is 228 rather complicated. The C standard discusses two character sets, but 229 there are really at least four. 230 231 The files input to CPP might be in any character set at all. CPP's 232 very first action, before it even looks for line boundaries, is to 233 convert the file into the character set it uses for internal 234 processing. That set is what the C standard calls the "source" 235 character set. It must be isomorphic with ISO 10646, also known as 236 Unicode. CPP uses the UTF-8 encoding of Unicode. 237 238 The character sets of the input files are specified using the 239 `-finput-charset=' option. 240 241 All preprocessing work (the subject of the rest of this manual) is 242 carried out in the source character set. If you request textual output 243 from the preprocessor with the `-E' option, it will be in UTF-8. 244 245 After preprocessing is complete, string and character constants are 246 converted again, into the "execution" character set. This character 247 set is under control of the user; the default is UTF-8, matching the 248 source character set. Wide string and character constants have their 249 own character set, which is not called out specifically in the 250 standard. Again, it is under control of the user. The default is 251 UTF-16 or UTF-32, whichever fits in the target's `wchar_t' type, in the 252 target machine's byte order.(1) Octal and hexadecimal escape sequences 253 do not undergo conversion; '\x12' has the value 0x12 regardless of the 254 currently selected execution character set. All other escapes are 255 replaced by the character in the source character set that they 256 represent, then converted to the execution character set, just like 257 unescaped characters. 258 259 Unless the experimental `-fextended-identifiers' option is used, GCC 260 does not permit the use of characters outside the ASCII range, nor `\u' 261 and `\U' escapes, in identifiers. Even with that option, characters 262 outside the ASCII range can only be specified with the `\u' and `\U' 263 escapes, not used directly in identifiers. 264 265 ---------- Footnotes ---------- 266 267 (1) UTF-16 does not meet the requirements of the C standard for a 268 wide character set, but the choice of 16-bit `wchar_t' is enshrined in 269 some system ABIs so we cannot fix this. 270 271 272 File: cpp.info, Node: Initial processing, Next: Tokenization, Prev: Character sets, Up: Overview 273 274 1.2 Initial processing 275 ====================== 276 277 The preprocessor performs a series of textual transformations on its 278 input. These happen before all other processing. Conceptually, they 279 happen in a rigid order, and the entire file is run through each 280 transformation before the next one begins. CPP actually does them all 281 at once, for performance reasons. These transformations correspond 282 roughly to the first three "phases of translation" described in the C 283 standard. 284 285 1. The input file is read into memory and broken into lines. 286 287 Different systems use different conventions to indicate the end of 288 a line. GCC accepts the ASCII control sequences `LF', `CR LF' and 289 `CR' as end-of-line markers. These are the canonical sequences 290 used by Unix, DOS and VMS, and the classic Mac OS (before OSX) 291 respectively. You may therefore safely copy source code written 292 on any of those systems to a different one and use it without 293 conversion. (GCC may lose track of the current line number if a 294 file doesn't consistently use one convention, as sometimes happens 295 when it is edited on computers with different conventions that 296 share a network file system.) 297 298 If the last line of any input file lacks an end-of-line marker, 299 the end of the file is considered to implicitly supply one. The C 300 standard says that this condition provokes undefined behavior, so 301 GCC will emit a warning message. 302 303 2. If trigraphs are enabled, they are replaced by their corresponding 304 single characters. By default GCC ignores trigraphs, but if you 305 request a strictly conforming mode with the `-std' option, or you 306 specify the `-trigraphs' option, then it converts them. 307 308 These are nine three-character sequences, all starting with `??', 309 that are defined by ISO C to stand for single characters. They 310 permit obsolete systems that lack some of C's punctuation to use 311 C. For example, `??/' stands for `\', so '??/n' is a character 312 constant for a newline. 313 314 Trigraphs are not popular and many compilers implement them 315 incorrectly. Portable code should not rely on trigraphs being 316 either converted or ignored. With `-Wtrigraphs' GCC will warn you 317 when a trigraph may change the meaning of your program if it were 318 converted. *Note Wtrigraphs::. 319 320 In a string constant, you can prevent a sequence of question marks 321 from being confused with a trigraph by inserting a backslash 322 between the question marks, or by separating the string literal at 323 the trigraph and making use of string literal concatenation. 324 "(??\?)" is the string `(???)', not `(?]'. Traditional C 325 compilers do not recognize these idioms. 326 327 The nine trigraphs and their replacements are 328 329 Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- 330 Replacement: [ ] { } # \ ^ | ~ 331 332 3. Continued lines are merged into one long line. 333 334 A continued line is a line which ends with a backslash, `\'. The 335 backslash is removed and the following line is joined with the 336 current one. No space is inserted, so you may split a line 337 anywhere, even in the middle of a word. (It is generally more 338 readable to split lines only at white space.) 339 340 The trailing backslash on a continued line is commonly referred to 341 as a "backslash-newline". 342 343 If there is white space between a backslash and the end of a line, 344 that is still a continued line. However, as this is usually the 345 result of an editing mistake, and many compilers will not accept 346 it as a continued line, GCC will warn you about it. 347 348 4. All comments are replaced with single spaces. 349 350 There are two kinds of comments. "Block comments" begin with `/*' 351 and continue until the next `*/'. Block comments do not nest: 352 353 /* this is /* one comment */ text outside comment 354 355 "Line comments" begin with `//' and continue to the end of the 356 current line. Line comments do not nest either, but it does not 357 matter, because they would end in the same place anyway. 358 359 // this is // one comment 360 text outside comment 361 362 It is safe to put line comments inside block comments, or vice versa. 363 364 /* block comment 365 // contains line comment 366 yet more comment 367 */ outside comment 368 369 // line comment /* contains block comment */ 370 371 But beware of commenting out one end of a block comment with a line 372 comment. 373 374 // l.c. /* block comment begins 375 oops! this isn't a comment anymore */ 376 377 Comments are not recognized within string literals. "/* blah */" is 378 the string constant `/* blah */', not an empty string. 379 380 Line comments are not in the 1989 edition of the C standard, but they 381 are recognized by GCC as an extension. In C++ and in the 1999 edition 382 of the C standard, they are an official part of the language. 383 384 Since these transformations happen before all other processing, you 385 can split a line mechanically with backslash-newline anywhere. You can 386 comment out the end of a line. You can continue a line comment onto the 387 next line with backslash-newline. You can even split `/*', `*/', and 388 `//' onto multiple lines with backslash-newline. For example: 389 390 /\ 391 * 392 */ # /* 393 */ defi\ 394 ne FO\ 395 O 10\ 396 20 397 398 is equivalent to `#define FOO 1020'. All these tricks are extremely 399 confusing and should not be used in code intended to be readable. 400 401 There is no way to prevent a backslash at the end of a line from 402 being interpreted as a backslash-newline. This cannot affect any 403 correct program, however. 404 405 406 File: cpp.info, Node: Tokenization, Next: The preprocessing language, Prev: Initial processing, Up: Overview 407 408 1.3 Tokenization 409 ================ 410 411 After the textual transformations are finished, the input file is 412 converted into a sequence of "preprocessing tokens". These mostly 413 correspond to the syntactic tokens used by the C compiler, but there are 414 a few differences. White space separates tokens; it is not itself a 415 token of any kind. Tokens do not have to be separated by white space, 416 but it is often necessary to avoid ambiguities. 417 418 When faced with a sequence of characters that has more than one 419 possible tokenization, the preprocessor is greedy. It always makes 420 each token, starting from the left, as big as possible before moving on 421 to the next token. For instance, `a+++++b' is interpreted as 422 `a ++ ++ + b', not as `a ++ + ++ b', even though the latter 423 tokenization could be part of a valid C program and the former could 424 not. 425 426 Once the input file is broken into tokens, the token boundaries never 427 change, except when the `##' preprocessing operator is used to paste 428 tokens together. *Note Concatenation::. For example, 429 430 #define foo() bar 431 foo()baz 432 ==> bar baz 433 _not_ 434 ==> barbaz 435 436 The compiler does not re-tokenize the preprocessor's output. Each 437 preprocessing token becomes one compiler token. 438 439 Preprocessing tokens fall into five broad classes: identifiers, 440 preprocessing numbers, string literals, punctuators, and other. An 441 "identifier" is the same as an identifier in C: any sequence of 442 letters, digits, or underscores, which begins with a letter or 443 underscore. Keywords of C have no significance to the preprocessor; 444 they are ordinary identifiers. You can define a macro whose name is a 445 keyword, for instance. The only identifier which can be considered a 446 preprocessing keyword is `defined'. *Note Defined::. 447 448 This is mostly true of other languages which use the C preprocessor. 449 However, a few of the keywords of C++ are significant even in the 450 preprocessor. *Note C++ Named Operators::. 451 452 In the 1999 C standard, identifiers may contain letters which are not 453 part of the "basic source character set", at the implementation's 454 discretion (such as accented Latin letters, Greek letters, or Chinese 455 ideograms). This may be done with an extended character set, or the 456 `\u' and `\U' escape sequences. The implementation of this feature in 457 GCC is experimental; such characters are only accepted in the `\u' and 458 `\U' forms and only if `-fextended-identifiers' is used. 459 460 As an extension, GCC treats `$' as a letter. This is for 461 compatibility with some systems, such as VMS, where `$' is commonly 462 used in system-defined function and object names. `$' is not a letter 463 in strictly conforming mode, or if you specify the `-$' option. *Note 464 Invocation::. 465 466 A "preprocessing number" has a rather bizarre definition. The 467 category includes all the normal integer and floating point constants 468 one expects of C, but also a number of other things one might not 469 initially recognize as a number. Formally, preprocessing numbers begin 470 with an optional period, a required decimal digit, and then continue 471 with any sequence of letters, digits, underscores, periods, and 472 exponents. Exponents are the two-character sequences `e+', `e-', `E+', 473 `E-', `p+', `p-', `P+', and `P-'. (The exponents that begin with `p' 474 or `P' are new to C99. They are used for hexadecimal floating-point 475 constants.) 476 477 The purpose of this unusual definition is to isolate the preprocessor 478 from the full complexity of numeric constants. It does not have to 479 distinguish between lexically valid and invalid floating-point numbers, 480 which is complicated. The definition also permits you to split an 481 identifier at any position and get exactly two tokens, which can then be 482 pasted back together with the `##' operator. 483 484 It's possible for preprocessing numbers to cause programs to be 485 misinterpreted. For example, `0xE+12' is a preprocessing number which 486 does not translate to any valid numeric constant, therefore a syntax 487 error. It does not mean `0xE + 12', which is what you might have 488 intended. 489 490 "String literals" are string constants, character constants, and 491 header file names (the argument of `#include').(1) String constants 492 and character constants are straightforward: "..." or '...'. In either 493 case embedded quotes should be escaped with a backslash: '\'' is the 494 character constant for `''. There is no limit on the length of a 495 character constant, but the value of a character constant that contains 496 more than one character is implementation-defined. *Note 497 Implementation Details::. 498 499 Header file names either look like string constants, "...", or are 500 written with angle brackets instead, <...>. In either case, backslash 501 is an ordinary character. There is no way to escape the closing quote 502 or angle bracket. The preprocessor looks for the header file in 503 different places depending on which form you use. *Note Include 504 Operation::. 505 506 No string literal may extend past the end of a line. Older versions 507 of GCC accepted multi-line string constants. You may use continued 508 lines instead, or string constant concatenation. *Note Differences 509 from previous versions::. 510 511 "Punctuators" are all the usual bits of punctuation which are 512 meaningful to C and C++. All but three of the punctuation characters in 513 ASCII are C punctuators. The exceptions are `@', `$', and ``'. In 514 addition, all the two- and three-character operators are punctuators. 515 There are also six "digraphs", which the C++ standard calls 516 "alternative tokens", which are merely alternate ways to spell other 517 punctuators. This is a second attempt to work around missing 518 punctuation in obsolete systems. It has no negative side effects, 519 unlike trigraphs, but does not cover as much ground. The digraphs and 520 their corresponding normal punctuators are: 521 522 Digraph: <% %> <: :> %: %:%: 523 Punctuator: { } [ ] # ## 524 525 Any other single character is considered "other". It is passed on to 526 the preprocessor's output unmolested. The C compiler will almost 527 certainly reject source code containing "other" tokens. In ASCII, the 528 only other characters are `@', `$', ``', and control characters other 529 than NUL (all bits zero). (Note that `$' is normally considered a 530 letter.) All characters with the high bit set (numeric range 531 0x7F-0xFF) are also "other" in the present implementation. This will 532 change when proper support for international character sets is added to 533 GCC. 534 535 NUL is a special case because of the high probability that its 536 appearance is accidental, and because it may be invisible to the user 537 (many terminals do not display NUL at all). Within comments, NULs are 538 silently ignored, just as any other character would be. In running 539 text, NUL is considered white space. For example, these two directives 540 have the same meaning. 541 542 #define X^@1 543 #define X 1 544 545 (where `^@' is ASCII NUL). Within string or character constants, NULs 546 are preserved. In the latter two cases the preprocessor emits a 547 warning message. 548 549 ---------- Footnotes ---------- 550 551 (1) The C standard uses the term "string literal" to refer only to 552 what we are calling "string constants". 553 554 555 File: cpp.info, Node: The preprocessing language, Prev: Tokenization, Up: Overview 556 557 1.4 The preprocessing language 558 ============================== 559 560 After tokenization, the stream of tokens may simply be passed straight 561 to the compiler's parser. However, if it contains any operations in the 562 "preprocessing language", it will be transformed first. This stage 563 corresponds roughly to the standard's "translation phase 4" and is what 564 most people think of as the preprocessor's job. 565 566 The preprocessing language consists of "directives" to be executed 567 and "macros" to be expanded. Its primary capabilities are: 568 569 * Inclusion of header files. These are files of declarations that 570 can be substituted into your program. 571 572 * Macro expansion. You can define "macros", which are abbreviations 573 for arbitrary fragments of C code. The preprocessor will replace 574 the macros with their definitions throughout the program. Some 575 macros are automatically defined for you. 576 577 * Conditional compilation. You can include or exclude parts of the 578 program according to various conditions. 579 580 * Line control. If you use a program to combine or rearrange source 581 files into an intermediate file which is then compiled, you can 582 use line control to inform the compiler where each source line 583 originally came from. 584 585 * Diagnostics. You can detect problems at compile time and issue 586 errors or warnings. 587 588 There are a few more, less useful, features. 589 590 Except for expansion of predefined macros, all these operations are 591 triggered with "preprocessing directives". Preprocessing directives 592 are lines in your program that start with `#'. Whitespace is allowed 593 before and after the `#'. The `#' is followed by an identifier, the 594 "directive name". It specifies the operation to perform. Directives 595 are commonly referred to as `#NAME' where NAME is the directive name. 596 For example, `#define' is the directive that defines a macro. 597 598 The `#' which begins a directive cannot come from a macro expansion. 599 Also, the directive name is not macro expanded. Thus, if `foo' is 600 defined as a macro expanding to `define', that does not make `#foo' a 601 valid preprocessing directive. 602 603 The set of valid directive names is fixed. Programs cannot define 604 new preprocessing directives. 605 606 Some directives require arguments; these make up the rest of the 607 directive line and must be separated from the directive name by 608 whitespace. For example, `#define' must be followed by a macro name 609 and the intended expansion of the macro. 610 611 A preprocessing directive cannot cover more than one line. The line 612 may, however, be continued with backslash-newline, or by a block comment 613 which extends past the end of the line. In either case, when the 614 directive is processed, the continuations have already been merged with 615 the first line to make one long line. 616 617 618 File: cpp.info, Node: Header Files, Next: Macros, Prev: Overview, Up: Top 619 620 2 Header Files 621 ************** 622 623 A header file is a file containing C declarations and macro definitions 624 (*note Macros::) to be shared between several source files. You request 625 the use of a header file in your program by "including" it, with the C 626 preprocessing directive `#include'. 627 628 Header files serve two purposes. 629 630 * System header files declare the interfaces to parts of the 631 operating system. You include them in your program to supply the 632 definitions and declarations you need to invoke system calls and 633 libraries. 634 635 * Your own header files contain declarations for interfaces between 636 the source files of your program. Each time you have a group of 637 related declarations and macro definitions all or most of which 638 are needed in several different source files, it is a good idea to 639 create a header file for them. 640 641 Including a header file produces the same results as copying the 642 header file into each source file that needs it. Such copying would be 643 time-consuming and error-prone. With a header file, the related 644 declarations appear in only one place. If they need to be changed, they 645 can be changed in one place, and programs that include the header file 646 will automatically use the new version when next recompiled. The header 647 file eliminates the labor of finding and changing all the copies as well 648 as the risk that a failure to find one copy will result in 649 inconsistencies within a program. 650 651 In C, the usual convention is to give header files names that end 652 with `.h'. It is most portable to use only letters, digits, dashes, and 653 underscores in header file names, and at most one dot. 654 655 * Menu: 656 657 * Include Syntax:: 658 * Include Operation:: 659 * Search Path:: 660 * Once-Only Headers:: 661 * Alternatives to Wrapper #ifndef:: 662 * Computed Includes:: 663 * Wrapper Headers:: 664 * System Headers:: 665 666 667 File: cpp.info, Node: Include Syntax, Next: Include Operation, Up: Header Files 668 669 2.1 Include Syntax 670 ================== 671 672 Both user and system header files are included using the preprocessing 673 directive `#include'. It has two variants: 674 675 `#include <FILE>' 676 This variant is used for system header files. It searches for a 677 file named FILE in a standard list of system directories. You can 678 prepend directories to this list with the `-I' option (*note 679 Invocation::). 680 681 `#include "FILE"' 682 This variant is used for header files of your own program. It 683 searches for a file named FILE first in the directory containing 684 the current file, then in the quote directories and then the same 685 directories used for `<FILE>'. You can prepend directories to the 686 list of quote directories with the `-iquote' option. 687 688 The argument of `#include', whether delimited with quote marks or 689 angle brackets, behaves like a string constant in that comments are not 690 recognized, and macro names are not expanded. Thus, `#include <x/*y>' 691 specifies inclusion of a system header file named `x/*y'. 692 693 However, if backslashes occur within FILE, they are considered 694 ordinary text characters, not escape characters. None of the character 695 escape sequences appropriate to string constants in C are processed. 696 Thus, `#include "x\n\\y"' specifies a filename containing three 697 backslashes. (Some systems interpret `\' as a pathname separator. All 698 of these also interpret `/' the same way. It is most portable to use 699 only `/'.) 700 701 It is an error if there is anything (other than comments) on the line 702 after the file name. 703 704 705 File: cpp.info, Node: Include Operation, Next: Search Path, Prev: Include Syntax, Up: Header Files 706 707 2.2 Include Operation 708 ===================== 709 710 The `#include' directive works by directing the C preprocessor to scan 711 the specified file as input before continuing with the rest of the 712 current file. The output from the preprocessor contains the output 713 already generated, followed by the output resulting from the included 714 file, followed by the output that comes from the text after the 715 `#include' directive. For example, if you have a header file 716 `header.h' as follows, 717 718 char *test (void); 719 720 and a main program called `program.c' that uses the header file, like 721 this, 722 723 int x; 724 #include "header.h" 725 726 int 727 main (void) 728 { 729 puts (test ()); 730 } 731 732 the compiler will see the same token stream as it would if `program.c' 733 read 734 735 int x; 736 char *test (void); 737 738 int 739 main (void) 740 { 741 puts (test ()); 742 } 743 744 Included files are not limited to declarations and macro definitions; 745 those are merely the typical uses. Any fragment of a C program can be 746 included from another file. The include file could even contain the 747 beginning of a statement that is concluded in the containing file, or 748 the end of a statement that was started in the including file. However, 749 an included file must consist of complete tokens. Comments and string 750 literals which have not been closed by the end of an included file are 751 invalid. For error recovery, they are considered to end at the end of 752 the file. 753 754 To avoid confusion, it is best if header files contain only complete 755 syntactic units--function declarations or definitions, type 756 declarations, etc. 757 758 The line following the `#include' directive is always treated as a 759 separate line by the C preprocessor, even if the included file lacks a 760 final newline. 761 762 763 File: cpp.info, Node: Search Path, Next: Once-Only Headers, Prev: Include Operation, Up: Header Files 764 765 2.3 Search Path 766 =============== 767 768 GCC looks in several different places for headers. On a normal Unix 769 system, if you do not instruct it otherwise, it will look for headers 770 requested with `#include <FILE>' in: 771 772 /usr/local/include 773 LIBDIR/gcc/TARGET/VERSION/include 774 /usr/TARGET/include 775 /usr/include 776 777 For C++ programs, it will also look in `/usr/include/g++-v3', first. 778 In the above, TARGET is the canonical name of the system GCC was 779 configured to compile code for; often but not always the same as the 780 canonical name of the system it runs on. VERSION is the version of GCC 781 in use. 782 783 You can add to this list with the `-IDIR' command line option. All 784 the directories named by `-I' are searched, in left-to-right order, 785 _before_ the default directories. The only exception is when `dir' is 786 already searched by default. In this case, the option is ignored and 787 the search order for system directories remains unchanged. 788 789 Duplicate directories are removed from the quote and bracket search 790 chains before the two chains are merged to make the final search chain. 791 Thus, it is possible for a directory to occur twice in the final search 792 chain if it was specified in both the quote and bracket chains. 793 794 You can prevent GCC from searching any of the default directories 795 with the `-nostdinc' option. This is useful when you are compiling an 796 operating system kernel or some other program that does not use the 797 standard C library facilities, or the standard C library itself. `-I' 798 options are not ignored as described above when `-nostdinc' is in 799 effect. 800 801 GCC looks for headers requested with `#include "FILE"' first in the 802 directory containing the current file, then in the directories as 803 specified by `-iquote' options, then in the same places it would have 804 looked for a header requested with angle brackets. For example, if 805 `/usr/include/sys/stat.h' contains `#include "types.h"', GCC looks for 806 `types.h' first in `/usr/include/sys', then in its usual search path. 807 808 `#line' (*note Line Control::) does not change GCC's idea of the 809 directory containing the current file. 810 811 You may put `-I-' at any point in your list of `-I' options. This 812 has two effects. First, directories appearing before the `-I-' in the 813 list are searched only for headers requested with quote marks. 814 Directories after `-I-' are searched for all headers. Second, the 815 directory containing the current file is not searched for anything, 816 unless it happens to be one of the directories named by an `-I' switch. 817 `-I-' is deprecated, `-iquote' should be used instead. 818 819 `-I. -I-' is not the same as no `-I' options at all, and does not 820 cause the same behavior for `<>' includes that `""' includes get with 821 no special options. `-I.' searches the compiler's current working 822 directory for header files. That may or may not be the same as the 823 directory containing the current file. 824 825 If you need to look for headers in a directory named `-', write 826 `-I./-'. 827 828 There are several more ways to adjust the header search path. They 829 are generally less useful. *Note Invocation::. 830 831 832 File: cpp.info, Node: Once-Only Headers, Next: Alternatives to Wrapper #ifndef, Prev: Search Path, Up: Header Files 833 834 2.4 Once-Only Headers 835 ===================== 836 837 If a header file happens to be included twice, the compiler will process 838 its contents twice. This is very likely to cause an error, e.g. when 839 the compiler sees the same structure definition twice. Even if it does 840 not, it will certainly waste time. 841 842 The standard way to prevent this is to enclose the entire real 843 contents of the file in a conditional, like this: 844 845 /* File foo. */ 846 #ifndef FILE_FOO_SEEN 847 #define FILE_FOO_SEEN 848 849 THE ENTIRE FILE 850 851 #endif /* !FILE_FOO_SEEN */ 852 853 This construct is commonly known as a "wrapper #ifndef". When the 854 header is included again, the conditional will be false, because 855 `FILE_FOO_SEEN' is defined. The preprocessor will skip over the entire 856 contents of the file, and the compiler will not see it twice. 857 858 CPP optimizes even further. It remembers when a header file has a 859 wrapper `#ifndef'. If a subsequent `#include' specifies that header, 860 and the macro in the `#ifndef' is still defined, it does not bother to 861 rescan the file at all. 862 863 You can put comments outside the wrapper. They will not interfere 864 with this optimization. 865 866 The macro `FILE_FOO_SEEN' is called the "controlling macro" or 867 "guard macro". In a user header file, the macro name should not begin 868 with `_'. In a system header file, it should begin with `__' to avoid 869 conflicts with user programs. In any kind of header file, the macro 870 name should contain the name of the file and some additional text, to 871 avoid conflicts with other header files. 872 873 874 File: cpp.info, Node: Alternatives to Wrapper #ifndef, Next: Computed Includes, Prev: Once-Only Headers, Up: Header Files 875 876 2.5 Alternatives to Wrapper #ifndef 877 =================================== 878 879 CPP supports two more ways of indicating that a header file should be 880 read only once. Neither one is as portable as a wrapper `#ifndef' and 881 we recommend you do not use them in new programs, with the caveat that 882 `#import' is standard practice in Objective-C. 883 884 CPP supports a variant of `#include' called `#import' which includes 885 a file, but does so at most once. If you use `#import' instead of 886 `#include', then you don't need the conditionals inside the header file 887 to prevent multiple inclusion of the contents. `#import' is standard 888 in Objective-C, but is considered a deprecated extension in C and C++. 889 890 `#import' is not a well designed feature. It requires the users of 891 a header file to know that it should only be included once. It is much 892 better for the header file's implementor to write the file so that users 893 don't need to know this. Using a wrapper `#ifndef' accomplishes this 894 goal. 895 896 In the present implementation, a single use of `#import' will 897 prevent the file from ever being read again, by either `#import' or 898 `#include'. You should not rely on this; do not use both `#import' and 899 `#include' to refer to the same header file. 900 901 Another way to prevent a header file from being included more than 902 once is with the `#pragma once' directive. If `#pragma once' is seen 903 when scanning a header file, that file will never be read again, no 904 matter what. 905 906 `#pragma once' does not have the problems that `#import' does, but 907 it is not recognized by all preprocessors, so you cannot rely on it in 908 a portable program. 909 910 911 File: cpp.info, Node: Computed Includes, Next: Wrapper Headers, Prev: Alternatives to Wrapper #ifndef, Up: Header Files 912 913 2.6 Computed Includes 914 ===================== 915 916 Sometimes it is necessary to select one of several different header 917 files to be included into your program. They might specify 918 configuration parameters to be used on different sorts of operating 919 systems, for instance. You could do this with a series of conditionals, 920 921 #if SYSTEM_1 922 # include "system_1.h" 923 #elif SYSTEM_2 924 # include "system_2.h" 925 #elif SYSTEM_3 926 ... 927 #endif 928 929 That rapidly becomes tedious. Instead, the preprocessor offers the 930 ability to use a macro for the header name. This is called a "computed 931 include". Instead of writing a header name as the direct argument of 932 `#include', you simply put a macro name there instead: 933 934 #define SYSTEM_H "system_1.h" 935 ... 936 #include SYSTEM_H 937 938 `SYSTEM_H' will be expanded, and the preprocessor will look for 939 `system_1.h' as if the `#include' had been written that way originally. 940 `SYSTEM_H' could be defined by your Makefile with a `-D' option. 941 942 You must be careful when you define the macro. `#define' saves 943 tokens, not text. The preprocessor has no way of knowing that the macro 944 will be used as the argument of `#include', so it generates ordinary 945 tokens, not a header name. This is unlikely to cause problems if you 946 use double-quote includes, which are close enough to string constants. 947 If you use angle brackets, however, you may have trouble. 948 949 The syntax of a computed include is actually a bit more general than 950 the above. If the first non-whitespace character after `#include' is 951 not `"' or `<', then the entire line is macro-expanded like running 952 text would be. 953 954 If the line expands to a single string constant, the contents of that 955 string constant are the file to be included. CPP does not re-examine 956 the string for embedded quotes, but neither does it process backslash 957 escapes in the string. Therefore 958 959 #define HEADER "a\"b" 960 #include HEADER 961 962 looks for a file named `a\"b'. CPP searches for the file according to 963 the rules for double-quoted includes. 964 965 If the line expands to a token stream beginning with a `<' token and 966 including a `>' token, then the tokens between the `<' and the first 967 `>' are combined to form the filename to be included. Any whitespace 968 between tokens is reduced to a single space; then any space after the 969 initial `<' is retained, but a trailing space before the closing `>' is 970 ignored. CPP searches for the file according to the rules for 971 angle-bracket includes. 972 973 In either case, if there are any tokens on the line after the file 974 name, an error occurs and the directive is not processed. It is also 975 an error if the result of expansion does not match either of the two 976 expected forms. 977 978 These rules are implementation-defined behavior according to the C 979 standard. To minimize the risk of different compilers interpreting your 980 computed includes differently, we recommend you use only a single 981 object-like macro which expands to a string constant. This will also 982 minimize confusion for people reading your program. 983 984 985 File: cpp.info, Node: Wrapper Headers, Next: System Headers, Prev: Computed Includes, Up: Header Files 986 987 2.7 Wrapper Headers 988 =================== 989 990 Sometimes it is necessary to adjust the contents of a system-provided 991 header file without editing it directly. GCC's `fixincludes' operation 992 does this, for example. One way to do that would be to create a new 993 header file with the same name and insert it in the search path before 994 the original header. That works fine as long as you're willing to 995 replace the old header entirely. But what if you want to refer to the 996 old header from the new one? 997 998 You cannot simply include the old header with `#include'. That will 999 start from the beginning, and find your new header again. If your 1000 header is not protected from multiple inclusion (*note Once-Only 1001 Headers::), it will recurse infinitely and cause a fatal error. 1002 1003 You could include the old header with an absolute pathname: 1004 #include "/usr/include/old-header.h" 1005 This works, but is not clean; should the system headers ever move, 1006 you would have to edit the new headers to match. 1007 1008 There is no way to solve this problem within the C standard, but you 1009 can use the GNU extension `#include_next'. It means, "Include the 1010 _next_ file with this name". This directive works like `#include' 1011 except in searching for the specified file: it starts searching the 1012 list of header file directories _after_ the directory in which the 1013 current file was found. 1014 1015 Suppose you specify `-I /usr/local/include', and the list of 1016 directories to search also includes `/usr/include'; and suppose both 1017 directories contain `signal.h'. Ordinary `#include <signal.h>' finds 1018 the file under `/usr/local/include'. If that file contains 1019 `#include_next <signal.h>', it starts searching after that directory, 1020 and finds the file in `/usr/include'. 1021 1022 `#include_next' does not distinguish between `<FILE>' and `"FILE"' 1023 inclusion, nor does it check that the file you specify has the same 1024 name as the current file. It simply looks for the file named, starting 1025 with the directory in the search path after the one where the current 1026 file was found. 1027 1028 The use of `#include_next' can lead to great confusion. We 1029 recommend it be used only when there is no other alternative. In 1030 particular, it should not be used in the headers belonging to a specific 1031 program; it should be used only to make global corrections along the 1032 lines of `fixincludes'. 1033 1034 1035 File: cpp.info, Node: System Headers, Prev: Wrapper Headers, Up: Header Files 1036 1037 2.8 System Headers 1038 ================== 1039 1040 The header files declaring interfaces to the operating system and 1041 runtime libraries often cannot be written in strictly conforming C. 1042 Therefore, GCC gives code found in "system headers" special treatment. 1043 All warnings, other than those generated by `#warning' (*note 1044 Diagnostics::), are suppressed while GCC is processing a system header. 1045 Macros defined in a system header are immune to a few warnings wherever 1046 they are expanded. This immunity is granted on an ad-hoc basis, when 1047 we find that a warning generates lots of false positives because of 1048 code in macros defined in system headers. 1049 1050 Normally, only the headers found in specific directories are 1051 considered system headers. These directories are determined when GCC 1052 is compiled. There are, however, two ways to make normal headers into 1053 system headers. 1054 1055 The `-isystem' command line option adds its argument to the list of 1056 directories to search for headers, just like `-I'. Any headers found 1057 in that directory will be considered system headers. 1058 1059 All directories named by `-isystem' are searched _after_ all 1060 directories named by `-I', no matter what their order was on the 1061 command line. If the same directory is named by both `-I' and 1062 `-isystem', the `-I' option is ignored. GCC provides an informative 1063 message when this occurs if `-v' is used. 1064 1065 There is also a directive, `#pragma GCC system_header', which tells 1066 GCC to consider the rest of the current include file a system header, 1067 no matter where it was found. Code that comes before the `#pragma' in 1068 the file will not be affected. `#pragma GCC system_header' has no 1069 effect in the primary source file. 1070 1071 On very old systems, some of the pre-defined system header 1072 directories get even more special treatment. GNU C++ considers code in 1073 headers found in those directories to be surrounded by an `extern "C"' 1074 block. There is no way to request this behavior with a `#pragma', or 1075 from the command line. 1076 1077 1078 File: cpp.info, Node: Macros, Next: Conditionals, Prev: Header Files, Up: Top 1079 1080 3 Macros 1081 ******** 1082 1083 A "macro" is a fragment of code which has been given a name. Whenever 1084 the name is used, it is replaced by the contents of the macro. There 1085 are two kinds of macros. They differ mostly in what they look like 1086 when they are used. "Object-like" macros resemble data objects when 1087 used, "function-like" macros resemble function calls. 1088 1089 You may define any valid identifier as a macro, even if it is a C 1090 keyword. The preprocessor does not know anything about keywords. This 1091 can be useful if you wish to hide a keyword such as `const' from an 1092 older compiler that does not understand it. However, the preprocessor 1093 operator `defined' (*note Defined::) can never be defined as a macro, 1094 and C++'s named operators (*note C++ Named Operators::) cannot be 1095 macros when you are compiling C++. 1096 1097 * Menu: 1098 1099 * Object-like Macros:: 1100 * Function-like Macros:: 1101 * Macro Arguments:: 1102 * Stringification:: 1103 * Concatenation:: 1104 * Variadic Macros:: 1105 * Predefined Macros:: 1106 * Undefining and Redefining Macros:: 1107 * Directives Within Macro Arguments:: 1108 * Macro Pitfalls:: 1109 1110 1111 File: cpp.info, Node: Object-like Macros, Next: Function-like Macros, Up: Macros 1112 1113 3.1 Object-like Macros 1114 ====================== 1115 1116 An "object-like macro" is a simple identifier which will be replaced by 1117 a code fragment. It is called object-like because it looks like a data 1118 object in code that uses it. They are most commonly used to give 1119 symbolic names to numeric constants. 1120 1121 You create macros with the `#define' directive. `#define' is 1122 followed by the name of the macro and then the token sequence it should 1123 be an abbreviation for, which is variously referred to as the macro's 1124 "body", "expansion" or "replacement list". For example, 1125 1126 #define BUFFER_SIZE 1024 1127 1128 defines a macro named `BUFFER_SIZE' as an abbreviation for the token 1129 `1024'. If somewhere after this `#define' directive there comes a C 1130 statement of the form 1131 1132 foo = (char *) malloc (BUFFER_SIZE); 1133 1134 then the C preprocessor will recognize and "expand" the macro 1135 `BUFFER_SIZE'. The C compiler will see the same tokens as it would if 1136 you had written 1137 1138 foo = (char *) malloc (1024); 1139 1140 By convention, macro names are written in uppercase. Programs are 1141 easier to read when it is possible to tell at a glance which names are 1142 macros. 1143 1144 The macro's body ends at the end of the `#define' line. You may 1145 continue the definition onto multiple lines, if necessary, using 1146 backslash-newline. When the macro is expanded, however, it will all 1147 come out on one line. For example, 1148 1149 #define NUMBERS 1, \ 1150 2, \ 1151 3 1152 int x[] = { NUMBERS }; 1153 ==> int x[] = { 1, 2, 3 }; 1154 1155 The most common visible consequence of this is surprising line numbers 1156 in error messages. 1157 1158 There is no restriction on what can go in a macro body provided it 1159 decomposes into valid preprocessing tokens. Parentheses need not 1160 balance, and the body need not resemble valid C code. (If it does not, 1161 you may get error messages from the C compiler when you use the macro.) 1162 1163 The C preprocessor scans your program sequentially. Macro 1164 definitions take effect at the place you write them. Therefore, the 1165 following input to the C preprocessor 1166 1167 foo = X; 1168 #define X 4 1169 bar = X; 1170 1171 produces 1172 1173 foo = X; 1174 bar = 4; 1175 1176 When the preprocessor expands a macro name, the macro's expansion 1177 replaces the macro invocation, then the expansion is examined for more 1178 macros to expand. For example, 1179 1180 #define TABLESIZE BUFSIZE 1181 #define BUFSIZE 1024 1182 TABLESIZE 1183 ==> BUFSIZE 1184 ==> 1024 1185 1186 `TABLESIZE' is expanded first to produce `BUFSIZE', then that macro is 1187 expanded to produce the final result, `1024'. 1188 1189 Notice that `BUFSIZE' was not defined when `TABLESIZE' was defined. 1190 The `#define' for `TABLESIZE' uses exactly the expansion you 1191 specify--in this case, `BUFSIZE'--and does not check to see whether it 1192 too contains macro names. Only when you _use_ `TABLESIZE' is the 1193 result of its expansion scanned for more macro names. 1194 1195 This makes a difference if you change the definition of `BUFSIZE' at 1196 some point in the source file. `TABLESIZE', defined as shown, will 1197 always expand using the definition of `BUFSIZE' that is currently in 1198 effect: 1199 1200 #define BUFSIZE 1020 1201 #define TABLESIZE BUFSIZE 1202 #undef BUFSIZE 1203 #define BUFSIZE 37 1204 1205 Now `TABLESIZE' expands (in two stages) to `37'. 1206 1207 If the expansion of a macro contains its own name, either directly or 1208 via intermediate macros, it is not expanded again when the expansion is 1209 examined for more macros. This prevents infinite recursion. *Note 1210 Self-Referential Macros::, for the precise details. 1211 1212 1213 File: cpp.info, Node: Function-like Macros, Next: Macro Arguments, Prev: Object-like Macros, Up: Macros 1214 1215 3.2 Function-like Macros 1216 ======================== 1217 1218 You can also define macros whose use looks like a function call. These 1219 are called "function-like macros". To define a function-like macro, 1220 you use the same `#define' directive, but you put a pair of parentheses 1221 immediately after the macro name. For example, 1222 1223 #define lang_init() c_init() 1224 lang_init() 1225 ==> c_init() 1226 1227 A function-like macro is only expanded if its name appears with a 1228 pair of parentheses after it. If you write just the name, it is left 1229 alone. This can be useful when you have a function and a macro of the 1230 same name, and you wish to use the function sometimes. 1231 1232 extern void foo(void); 1233 #define foo() /* optimized inline version */ 1234 ... 1235 foo(); 1236 funcptr = foo; 1237 1238 Here the call to `foo()' will use the macro, but the function 1239 pointer will get the address of the real function. If the macro were to 1240 be expanded, it would cause a syntax error. 1241 1242 If you put spaces between the macro name and the parentheses in the 1243 macro definition, that does not define a function-like macro, it defines 1244 an object-like macro whose expansion happens to begin with a pair of 1245 parentheses. 1246 1247 #define lang_init () c_init() 1248 lang_init() 1249 ==> () c_init()() 1250 1251 The first two pairs of parentheses in this expansion come from the 1252 macro. The third is the pair that was originally after the macro 1253 invocation. Since `lang_init' is an object-like macro, it does not 1254 consume those parentheses. 1255 1256 1257 File: cpp.info, Node: Macro Arguments, Next: Stringification, Prev: Function-like Macros, Up: Macros 1258 1259 3.3 Macro Arguments 1260 =================== 1261 1262 Function-like macros can take "arguments", just like true functions. 1263 To define a macro that uses arguments, you insert "parameters" between 1264 the pair of parentheses in the macro definition that make the macro 1265 function-like. The parameters must be valid C identifiers, separated 1266 by commas and optionally whitespace. 1267 1268 To invoke a macro that takes arguments, you write the name of the 1269 macro followed by a list of "actual arguments" in parentheses, separated 1270 by commas. The invocation of the macro need not be restricted to a 1271 single logical line--it can cross as many lines in the source file as 1272 you wish. The number of arguments you give must match the number of 1273 parameters in the macro definition. When the macro is expanded, each 1274 use of a parameter in its body is replaced by the tokens of the 1275 corresponding argument. (You need not use all of the parameters in the 1276 macro body.) 1277 1278 As an example, here is a macro that computes the minimum of two 1279 numeric values, as it is defined in many C programs, and some uses. 1280 1281 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1282 x = min(a, b); ==> x = ((a) < (b) ? (a) : (b)); 1283 y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2)); 1284 z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1285 1286 (In this small example you can already see several of the dangers of 1287 macro arguments. *Note Macro Pitfalls::, for detailed explanations.) 1288 1289 Leading and trailing whitespace in each argument is dropped, and all 1290 whitespace between the tokens of an argument is reduced to a single 1291 space. Parentheses within each argument must balance; a comma within 1292 such parentheses does not end the argument. However, there is no 1293 requirement for square brackets or braces to balance, and they do not 1294 prevent a comma from separating arguments. Thus, 1295 1296 macro (array[x = y, x + 1]) 1297 1298 passes two arguments to `macro': `array[x = y' and `x + 1]'. If you 1299 want to supply `array[x = y, x + 1]' as an argument, you can write it 1300 as `array[(x = y, x + 1)]', which is equivalent C code. 1301 1302 All arguments to a macro are completely macro-expanded before they 1303 are substituted into the macro body. After substitution, the complete 1304 text is scanned again for macros to expand, including the arguments. 1305 This rule may seem strange, but it is carefully designed so you need 1306 not worry about whether any function call is actually a macro 1307 invocation. You can run into trouble if you try to be too clever, 1308 though. *Note Argument Prescan::, for detailed discussion. 1309 1310 For example, `min (min (a, b), c)' is first expanded to 1311 1312 min (((a) < (b) ? (a) : (b)), (c)) 1313 1314 and then to 1315 1316 ((((a) < (b) ? (a) : (b))) < (c) 1317 ? (((a) < (b) ? (a) : (b))) 1318 : (c)) 1319 1320 (Line breaks shown here for clarity would not actually be generated.) 1321 1322 You can leave macro arguments empty; this is not an error to the 1323 preprocessor (but many macros will then expand to invalid code). You 1324 cannot leave out arguments entirely; if a macro takes two arguments, 1325 there must be exactly one comma at the top level of its argument list. 1326 Here are some silly examples using `min': 1327 1328 min(, b) ==> (( ) < (b) ? ( ) : (b)) 1329 min(a, ) ==> ((a ) < ( ) ? (a ) : ( )) 1330 min(,) ==> (( ) < ( ) ? ( ) : ( )) 1331 min((,),) ==> (((,)) < ( ) ? ((,)) : ( )) 1332 1333 min() error--> macro "min" requires 2 arguments, but only 1 given 1334 min(,,) error--> macro "min" passed 3 arguments, but takes just 2 1335 1336 Whitespace is not a preprocessing token, so if a macro `foo' takes 1337 one argument, `foo ()' and `foo ( )' both supply it an empty argument. 1338 Previous GNU preprocessor implementations and documentation were 1339 incorrect on this point, insisting that a function-like macro that 1340 takes a single argument be passed a space if an empty argument was 1341 required. 1342 1343 Macro parameters appearing inside string literals are not replaced by 1344 their corresponding actual arguments. 1345 1346 #define foo(x) x, "x" 1347 foo(bar) ==> bar, "x" 1348 1349 1350 File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Macro Arguments, Up: Macros 1351 1352 3.4 Stringification 1353 =================== 1354 1355 Sometimes you may want to convert a macro argument into a string 1356 constant. Parameters are not replaced inside string constants, but you 1357 can use the `#' preprocessing operator instead. When a macro parameter 1358 is used with a leading `#', the preprocessor replaces it with the 1359 literal text of the actual argument, converted to a string constant. 1360 Unlike normal parameter replacement, the argument is not macro-expanded 1361 first. This is called "stringification". 1362 1363 There is no way to combine an argument with surrounding text and 1364 stringify it all together. Instead, you can write a series of adjacent 1365 string constants and stringified arguments. The preprocessor will 1366 replace the stringified arguments with string constants. The C 1367 compiler will then combine all the adjacent string constants into one 1368 long string. 1369 1370 Here is an example of a macro definition that uses stringification: 1371 1372 #define WARN_IF(EXP) \ 1373 do { if (EXP) \ 1374 fprintf (stderr, "Warning: " #EXP "\n"); } \ 1375 while (0) 1376 WARN_IF (x == 0); 1377 ==> do { if (x == 0) 1378 fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0); 1379 1380 The argument for `EXP' is substituted once, as-is, into the `if' 1381 statement, and once, stringified, into the argument to `fprintf'. If 1382 `x' were a macro, it would be expanded in the `if' statement, but not 1383 in the string. 1384 1385 The `do' and `while (0)' are a kludge to make it possible to write 1386 `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function 1387 would make C programmers want to do; see *note Swallowing the 1388 Semicolon::. 1389 1390 Stringification in C involves more than putting double-quote 1391 characters around the fragment. The preprocessor backslash-escapes the 1392 quotes surrounding embedded string constants, and all backslashes 1393 within string and character constants, in order to get a valid C string 1394 constant with the proper contents. Thus, stringifying `p = "foo\n";' 1395 results in "p = \"foo\\n\";". However, backslashes that are not inside 1396 string or character constants are not duplicated: `\n' by itself 1397 stringifies to "\n". 1398 1399 All leading and trailing whitespace in text being stringified is 1400 ignored. Any sequence of whitespace in the middle of the text is 1401 converted to a single space in the stringified result. Comments are 1402 replaced by whitespace long before stringification happens, so they 1403 never appear in stringified text. 1404 1405 There is no way to convert a macro argument into a character 1406 constant. 1407 1408 If you want to stringify the result of expansion of a macro argument, 1409 you have to use two levels of macros. 1410 1411 #define xstr(s) str(s) 1412 #define str(s) #s 1413 #define foo 4 1414 str (foo) 1415 ==> "foo" 1416 xstr (foo) 1417 ==> xstr (4) 1418 ==> str (4) 1419 ==> "4" 1420 1421 `s' is stringified when it is used in `str', so it is not 1422 macro-expanded first. But `s' is an ordinary argument to `xstr', so it 1423 is completely macro-expanded before `xstr' itself is expanded (*note 1424 Argument Prescan::). Therefore, by the time `str' gets to its 1425 argument, it has already been macro-expanded. 1426 1427 1428 File: cpp.info, Node: Concatenation, Next: Variadic Macros, Prev: Stringification, Up: Macros 1429 1430 3.5 Concatenation 1431 ================= 1432 1433 It is often useful to merge two tokens into one while expanding macros. 1434 This is called "token pasting" or "token concatenation". The `##' 1435 preprocessing operator performs token pasting. When a macro is 1436 expanded, the two tokens on either side of each `##' operator are 1437 combined into a single token, which then replaces the `##' and the two 1438 original tokens in the macro expansion. Usually both will be 1439 identifiers, or one will be an identifier and the other a preprocessing 1440 number. When pasted, they make a longer identifier. This isn't the 1441 only valid case. It is also possible to concatenate two numbers (or a 1442 number and a name, such as `1.5' and `e3') into a number. Also, 1443 multi-character operators such as `+=' can be formed by token pasting. 1444 1445 However, two tokens that don't together form a valid token cannot be 1446 pasted together. For example, you cannot concatenate `x' with `+' in 1447 either order. If you try, the preprocessor issues a warning and emits 1448 the two tokens. Whether it puts white space between the tokens is 1449 undefined. It is common to find unnecessary uses of `##' in complex 1450 macros. If you get this warning, it is likely that you can simply 1451 remove the `##'. 1452 1453 Both the tokens combined by `##' could come from the macro body, but 1454 you could just as well write them as one token in the first place. 1455 Token pasting is most useful when one or both of the tokens comes from a 1456 macro argument. If either of the tokens next to an `##' is a parameter 1457 name, it is replaced by its actual argument before `##' executes. As 1458 with stringification, the actual argument is not macro-expanded first. 1459 If the argument is empty, that `##' has no effect. 1460 1461 Keep in mind that the C preprocessor converts comments to whitespace 1462 before macros are even considered. Therefore, you cannot create a 1463 comment by concatenating `/' and `*'. You can put as much whitespace 1464 between `##' and its operands as you like, including comments, and you 1465 can put comments in arguments that will be concatenated. However, it 1466 is an error if `##' appears at either end of a macro body. 1467 1468 Consider a C program that interprets named commands. There probably 1469 needs to be a table of commands, perhaps an array of structures declared 1470 as follows: 1471 1472 struct command 1473 { 1474 char *name; 1475 void (*function) (void); 1476 }; 1477 1478 struct command commands[] = 1479 { 1480 { "quit", quit_command }, 1481 { "help", help_command }, 1482 ... 1483 }; 1484 1485 It would be cleaner not to have to give each command name twice, 1486 once in the string constant and once in the function name. A macro 1487 which takes the name of a command as an argument can make this 1488 unnecessary. The string constant can be created with stringification, 1489 and the function name by concatenating the argument with `_command'. 1490 Here is how it is done: 1491 1492 #define COMMAND(NAME) { #NAME, NAME ## _command } 1493 1494 struct command commands[] = 1495 { 1496 COMMAND (quit), 1497 COMMAND (help), 1498 ... 1499 }; 1500 1501 1502 File: cpp.info, Node: Variadic Macros, Next: Predefined Macros, Prev: Concatenation, Up: Macros 1503 1504 3.6 Variadic Macros 1505 =================== 1506 1507 A macro can be declared to accept a variable number of arguments much as 1508 a function can. The syntax for defining the macro is similar to that of 1509 a function. Here is an example: 1510 1511 #define eprintf(...) fprintf (stderr, __VA_ARGS__) 1512 1513 This kind of macro is called "variadic". When the macro is invoked, 1514 all the tokens in its argument list after the last named argument (this 1515 macro has none), including any commas, become the "variable argument". 1516 This sequence of tokens replaces the identifier `__VA_ARGS__' in the 1517 macro body wherever it appears. Thus, we have this expansion: 1518 1519 eprintf ("%s:%d: ", input_file, lineno) 1520 ==> fprintf (stderr, "%s:%d: ", input_file, lineno) 1521 1522 The variable argument is completely macro-expanded before it is 1523 inserted into the macro expansion, just like an ordinary argument. You 1524 may use the `#' and `##' operators to stringify the variable argument 1525 or to paste its leading or trailing token with another token. (But see 1526 below for an important special case for `##'.) 1527 1528 If your macro is complicated, you may want a more descriptive name 1529 for the variable argument than `__VA_ARGS__'. CPP permits this, as an 1530 extension. You may write an argument name immediately before the 1531 `...'; that name is used for the variable argument. The `eprintf' 1532 macro above could be written 1533 1534 #define eprintf(args...) fprintf (stderr, args) 1535 1536 using this extension. You cannot use `__VA_ARGS__' and this extension 1537 in the same macro. 1538 1539 You can have named arguments as well as variable arguments in a 1540 variadic macro. We could define `eprintf' like this, instead: 1541 1542 #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__) 1543 1544 This formulation looks more descriptive, but unfortunately it is less 1545 flexible: you must now supply at least one argument after the format 1546 string. In standard C, you cannot omit the comma separating the named 1547 argument from the variable arguments. Furthermore, if you leave the 1548 variable argument empty, you will get a syntax error, because there 1549 will be an extra comma after the format string. 1550 1551 eprintf("success!\n", ); 1552 ==> fprintf(stderr, "success!\n", ); 1553 1554 GNU CPP has a pair of extensions which deal with this problem. 1555 First, you are allowed to leave the variable argument out entirely: 1556 1557 eprintf ("success!\n") 1558 ==> fprintf(stderr, "success!\n", ); 1559 1560 Second, the `##' token paste operator has a special meaning when placed 1561 between a comma and a variable argument. If you write 1562 1563 #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__) 1564 1565 and the variable argument is left out when the `eprintf' macro is used, 1566 then the comma before the `##' will be deleted. This does _not_ happen 1567 if you pass an empty argument, nor does it happen if the token 1568 preceding `##' is anything other than a comma. 1569 1570 eprintf ("success!\n") 1571 ==> fprintf(stderr, "success!\n"); 1572 1573 The above explanation is ambiguous about the case where the only macro 1574 parameter is a variable arguments parameter, as it is meaningless to 1575 try to distinguish whether no argument at all is an empty argument or a 1576 missing argument. In this case the C99 standard is clear that the 1577 comma must remain, however the existing GCC extension used to swallow 1578 the comma. So CPP retains the comma when conforming to a specific C 1579 standard, and drops it otherwise. 1580 1581 C99 mandates that the only place the identifier `__VA_ARGS__' can 1582 appear is in the replacement list of a variadic macro. It may not be 1583 used as a macro name, macro argument name, or within a different type 1584 of macro. It may also be forbidden in open text; the standard is 1585 ambiguous. We recommend you avoid using it except for its defined 1586 purpose. 1587 1588 Variadic macros are a new feature in C99. GNU CPP has supported them 1589 for a long time, but only with a named variable argument (`args...', 1590 not `...' and `__VA_ARGS__'). If you are concerned with portability to 1591 previous versions of GCC, you should use only named variable arguments. 1592 On the other hand, if you are concerned with portability to other 1593 conforming implementations of C99, you should use only `__VA_ARGS__'. 1594 1595 Previous versions of CPP implemented the comma-deletion extension 1596 much more generally. We have restricted it in this release to minimize 1597 the differences from C99. To get the same effect with both this and 1598 previous versions of GCC, the token preceding the special `##' must be 1599 a comma, and there must be white space between that comma and whatever 1600 comes immediately before it: 1601 1602 #define eprintf(format, args...) fprintf (stderr, format , ##args) 1603 1604 *Note Differences from previous versions::, for the gory details. 1605 1606 1607 File: cpp.info, Node: Predefined Macros, Next: Undefining and Redefining Macros, Prev: Variadic Macros, Up: Macros 1608 1609 3.7 Predefined Macros 1610 ===================== 1611 1612 Several object-like macros are predefined; you use them without 1613 supplying their definitions. They fall into three classes: standard, 1614 common, and system-specific. 1615 1616 In C++, there is a fourth category, the named operators. They act 1617 like predefined macros, but you cannot undefine them. 1618 1619 * Menu: 1620 1621 * Standard Predefined Macros:: 1622 * Common Predefined Macros:: 1623 * System-specific Predefined Macros:: 1624 * C++ Named Operators:: 1625 1626 1627 File: cpp.info, Node: Standard Predefined Macros, Next: Common Predefined Macros, Up: Predefined Macros 1628 1629 3.7.1 Standard Predefined Macros 1630 -------------------------------- 1631 1632 The standard predefined macros are specified by the relevant language 1633 standards, so they are available with all compilers that implement 1634 those standards. Older compilers may not provide all of them. Their 1635 names all start with double underscores. 1636 1637 `__FILE__' 1638 This macro expands to the name of the current input file, in the 1639 form of a C string constant. This is the path by which the 1640 preprocessor opened the file, not the short name specified in 1641 `#include' or as the input file name argument. For example, 1642 `"/usr/local/include/myheader.h"' is a possible expansion of this 1643 macro. 1644 1645 `__LINE__' 1646 This macro expands to the current input line number, in the form 1647 of a decimal integer constant. While we call it a predefined 1648 macro, it's a pretty strange macro, since its "definition" changes 1649 with each new line of source code. 1650 1651 `__FILE__' and `__LINE__' are useful in generating an error message 1652 to report an inconsistency detected by the program; the message can 1653 state the source line at which the inconsistency was detected. For 1654 example, 1655 1656 fprintf (stderr, "Internal error: " 1657 "negative string length " 1658 "%d at %s, line %d.", 1659 length, __FILE__, __LINE__); 1660 1661 An `#include' directive changes the expansions of `__FILE__' and 1662 `__LINE__' to correspond to the included file. At the end of that 1663 file, when processing resumes on the input file that contained the 1664 `#include' directive, the expansions of `__FILE__' and `__LINE__' 1665 revert to the values they had before the `#include' (but `__LINE__' is 1666 then incremented by one as processing moves to the line after the 1667 `#include'). 1668 1669 A `#line' directive changes `__LINE__', and may change `__FILE__' as 1670 well. *Note Line Control::. 1671 1672 C99 introduces `__func__', and GCC has provided `__FUNCTION__' for a 1673 long time. Both of these are strings containing the name of the 1674 current function (there are slight semantic differences; see the GCC 1675 manual). Neither of them is a macro; the preprocessor does not know the 1676 name of the current function. They tend to be useful in conjunction 1677 with `__FILE__' and `__LINE__', though. 1678 1679 `__DATE__' 1680 This macro expands to a string constant that describes the date on 1681 which the preprocessor is being run. The string constant contains 1682 eleven characters and looks like `"Feb 12 1996"'. If the day of 1683 the month is less than 10, it is padded with a space on the left. 1684 1685 If GCC cannot determine the current date, it will emit a warning 1686 message (once per compilation) and `__DATE__' will expand to 1687 `"??? ?? ????"'. 1688 1689 `__TIME__' 1690 This macro expands to a string constant that describes the time at 1691 which the preprocessor is being run. The string constant contains 1692 eight characters and looks like `"23:59:01"'. 1693 1694 If GCC cannot determine the current time, it will emit a warning 1695 message (once per compilation) and `__TIME__' will expand to 1696 `"??:??:??"'. 1697 1698 `__STDC__' 1699 In normal operation, this macro expands to the constant 1, to 1700 signify that this compiler conforms to ISO Standard C. If GNU CPP 1701 is used with a compiler other than GCC, this is not necessarily 1702 true; however, the preprocessor always conforms to the standard 1703 unless the `-traditional-cpp' option is used. 1704 1705 This macro is not defined if the `-traditional-cpp' option is used. 1706 1707 On some hosts, the system compiler uses a different convention, 1708 where `__STDC__' is normally 0, but is 1 if the user specifies 1709 strict conformance to the C Standard. CPP follows the host 1710 convention when processing system header files, but when 1711 processing user files `__STDC__' is always 1. This has been 1712 reported to cause problems; for instance, some versions of Solaris 1713 provide X Windows headers that expect `__STDC__' to be either 1714 undefined or 1. *Note Invocation::. 1715 1716 `__STDC_VERSION__' 1717 This macro expands to the C Standard's version number, a long 1718 integer constant of the form `YYYYMML' where YYYY and MM are the 1719 year and month of the Standard version. This signifies which 1720 version of the C Standard the compiler conforms to. Like 1721 `__STDC__', this is not necessarily accurate for the entire 1722 implementation, unless GNU CPP is being used with GCC. 1723 1724 The value `199409L' signifies the 1989 C standard as amended in 1725 1994, which is the current default; the value `199901L' signifies 1726 the 1999 revision of the C standard. Support for the 1999 1727 revision is not yet complete. 1728 1729 This macro is not defined if the `-traditional-cpp' option is 1730 used, nor when compiling C++ or Objective-C. 1731 1732 `__STDC_HOSTED__' 1733 This macro is defined, with value 1, if the compiler's target is a 1734 "hosted environment". A hosted environment has the complete 1735 facilities of the standard C library available. 1736 1737 `__cplusplus' 1738 This macro is defined when the C++ compiler is in use. You can use 1739 `__cplusplus' to test whether a header is compiled by a C compiler 1740 or a C++ compiler. This macro is similar to `__STDC_VERSION__', in 1741 that it expands to a version number. A fully conforming 1742 implementation of the 1998 C++ standard will define this macro to 1743 `199711L'. The GNU C++ compiler is not yet fully conforming, so 1744 it uses `1' instead. It is hoped to complete the implementation 1745 of standard C++ in the near future. 1746 1747 `__OBJC__' 1748 This macro is defined, with value 1, when the Objective-C compiler 1749 is in use. You can use `__OBJC__' to test whether a header is 1750 compiled by a C compiler or an Objective-C compiler. 1751 1752 `__ASSEMBLER__' 1753 This macro is defined with value 1 when preprocessing assembly 1754 language. 1755 1756 1757 1758 File: cpp.info, Node: Common Predefined Macros, Next: System-specific Predefined Macros, Prev: Standard Predefined Macros, Up: Predefined Macros 1759 1760 3.7.2 Common Predefined Macros 1761 ------------------------------ 1762 1763 The common predefined macros are GNU C extensions. They are available 1764 with the same meanings regardless of the machine or operating system on 1765 which you are using GNU C or GNU Fortran. Their names all start with 1766 double underscores. 1767 1768 `__COUNTER__' 1769 This macro expands to sequential integral values starting from 0. 1770 In conjunction with the `##' operator, this provides a convenient 1771 means to generate unique identifiers. Care must be taken to 1772 ensure that `__COUNTER__' is not expanded prior to inclusion of 1773 precompiled headers which use it. Otherwise, the precompiled 1774 headers will not be used. 1775 1776 `__GFORTRAN__' 1777 The GNU Fortran compiler defines this. 1778 1779 `__GNUC__' 1780 `__GNUC_MINOR__' 1781 `__GNUC_PATCHLEVEL__' 1782 These macros are defined by all GNU compilers that use the C 1783 preprocessor: C, C++, Objective-C and Fortran. Their values are 1784 the major version, minor version, and patch level of the compiler, 1785 as integer constants. For example, GCC 3.2.1 will define 1786 `__GNUC__' to 3, `__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__' 1787 to 1. These macros are also defined if you invoke the 1788 preprocessor directly. 1789 1790 `__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the 1791 widely-used development snapshots leading up to 3.0 (which identify 1792 themselves as GCC 2.96 or 2.97, depending on which snapshot you 1793 have). 1794 1795 If all you need to know is whether or not your program is being 1796 compiled by GCC, or a non-GCC compiler that claims to accept the 1797 GNU C dialects, you can simply test `__GNUC__'. If you need to 1798 write code which depends on a specific version, you must be more 1799 careful. Each time the minor version is increased, the patch 1800 level is reset to zero; each time the major version is increased 1801 (which happens rarely), the minor version and patch level are 1802 reset. If you wish to use the predefined macros directly in the 1803 conditional, you will need to write it like this: 1804 1805 /* Test for GCC > 3.2.0 */ 1806 #if __GNUC__ > 3 || \ 1807 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1808 (__GNUC_MINOR__ == 2 && \ 1809 __GNUC_PATCHLEVEL__ > 0)) 1810 1811 Another approach is to use the predefined macros to calculate a 1812 single number, then compare that against a threshold: 1813 1814 #define GCC_VERSION (__GNUC__ * 10000 \ 1815 + __GNUC_MINOR__ * 100 \ 1816 + __GNUC_PATCHLEVEL__) 1817 ... 1818 /* Test for GCC > 3.2.0 */ 1819 #if GCC_VERSION > 30200 1820 1821 Many people find this form easier to understand. 1822 1823 `__GNUG__' 1824 The GNU C++ compiler defines this. Testing it is equivalent to 1825 testing `(__GNUC__ && __cplusplus)'. 1826 1827 `__STRICT_ANSI__' 1828 GCC defines this macro if and only if the `-ansi' switch, or a 1829 `-std' switch specifying strict conformance to some version of ISO 1830 C, was specified when GCC was invoked. It is defined to `1'. 1831 This macro exists primarily to direct GNU libc's header files to 1832 restrict their definitions to the minimal set found in the 1989 C 1833 standard. 1834 1835 `__BASE_FILE__' 1836 This macro expands to the name of the main input file, in the form 1837 of a C string constant. This is the source file that was specified 1838 on the command line of the preprocessor or C compiler. 1839 1840 `__INCLUDE_LEVEL__' 1841 This macro expands to a decimal integer constant that represents 1842 the depth of nesting in include files. The value of this macro is 1843 incremented on every `#include' directive and decremented at the 1844 end of every included file. It starts out at 0, its value within 1845 the base file specified on the command line. 1846 1847 `__ELF__' 1848 This macro is defined if the target uses the ELF object format. 1849 1850 `__VERSION__' 1851 This macro expands to a string constant which describes the 1852 version of the compiler in use. You should not rely on its 1853 contents having any particular form, but it can be counted on to 1854 contain at least the release number. 1855 1856 `__OPTIMIZE__' 1857 `__OPTIMIZE_SIZE__' 1858 `__NO_INLINE__' 1859 These macros describe the compilation mode. `__OPTIMIZE__' is 1860 defined in all optimizing compilations. `__OPTIMIZE_SIZE__' is 1861 defined if the compiler is optimizing for size, not speed. 1862 `__NO_INLINE__' is defined if no functions will be inlined into 1863 their callers (when not optimizing, or when inlining has been 1864 specifically disabled by `-fno-inline'). 1865 1866 These macros cause certain GNU header files to provide optimized 1867 definitions, using macros or inline functions, of system library 1868 functions. You should not use these macros in any way unless you 1869 make sure that programs will execute with the same effect whether 1870 or not they are defined. If they are defined, their value is 1. 1871 1872 `__GNUC_GNU_INLINE__' 1873 GCC defines this macro if functions declared `inline' will be 1874 handled in GCC's traditional gnu90 mode. Object files will contain 1875 externally visible definitions of all functions declared `inline' 1876 without `extern' or `static'. They will not contain any 1877 definitions of any functions declared `extern inline'. 1878 1879 `__GNUC_STDC_INLINE__' 1880 GCC defines this macro if functions declared `inline' will be 1881 handled according to the ISO C99 standard. Object files will 1882 contain externally visible definitions of all functions declared 1883 `extern inline'. They will not contain definitions of any 1884 functions declared `inline' without `extern'. 1885 1886 If this macro is defined, GCC supports the `gnu_inline' function 1887 attribute as a way to always get the gnu90 behavior. Support for 1888 this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3. If neither 1889 macro is defined, an older version of GCC is being used: `inline' 1890 functions will be compiled in gnu90 mode, and the `gnu_inline' 1891 function attribute will not be recognized. 1892 1893 `__CHAR_UNSIGNED__' 1894 GCC defines this macro if and only if the data type `char' is 1895 unsigned on the target machine. It exists to cause the standard 1896 header file `limits.h' to work correctly. You should not use this 1897 macro yourself; instead, refer to the standard macros defined in 1898 `limits.h'. 1899 1900 `__WCHAR_UNSIGNED__' 1901 Like `__CHAR_UNSIGNED__', this macro is defined if and only if the 1902 data type `wchar_t' is unsigned and the front-end is in C++ mode. 1903 1904 `__REGISTER_PREFIX__' 1905 This macro expands to a single token (not a string constant) which 1906 is the prefix applied to CPU register names in assembly language 1907 for this target. You can use it to write assembly that is usable 1908 in multiple environments. For example, in the `m68k-aout' 1909 environment it expands to nothing, but in the `m68k-coff' 1910 environment it expands to a single `%'. 1911 1912 `__USER_LABEL_PREFIX__' 1913 This macro expands to a single token which is the prefix applied to 1914 user labels (symbols visible to C code) in assembly. For example, 1915 in the `m68k-aout' environment it expands to an `_', but in the 1916 `m68k-coff' environment it expands to nothing. 1917 1918 This macro will have the correct definition even if 1919 `-f(no-)underscores' is in use, but it will not be correct if 1920 target-specific options that adjust this prefix are used (e.g. the 1921 OSF/rose `-mno-underscores' option). 1922 1923 `__SIZE_TYPE__' 1924 `__PTRDIFF_TYPE__' 1925 `__WCHAR_TYPE__' 1926 `__WINT_TYPE__' 1927 `__INTMAX_TYPE__' 1928 `__UINTMAX_TYPE__' 1929 `__SIG_ATOMIC_TYPE__' 1930 `__INT8_TYPE__' 1931 `__INT16_TYPE__' 1932 `__INT32_TYPE__' 1933 `__INT64_TYPE__' 1934 `__UINT8_TYPE__' 1935 `__UINT16_TYPE__' 1936 `__UINT32_TYPE__' 1937 `__UINT64_TYPE__' 1938 `__INT_LEAST8_TYPE__' 1939 `__INT_LEAST16_TYPE__' 1940 `__INT_LEAST32_TYPE__' 1941 `__INT_LEAST64_TYPE__' 1942 `__UINT_LEAST8_TYPE__' 1943 `__UINT_LEAST16_TYPE__' 1944 `__UINT_LEAST32_TYPE__' 1945 `__UINT_LEAST64_TYPE__' 1946 `__INT_FAST8_TYPE__' 1947 `__INT_FAST16_TYPE__' 1948 `__INT_FAST32_TYPE__' 1949 `__INT_FAST64_TYPE__' 1950 `__UINT_FAST8_TYPE__' 1951 `__UINT_FAST16_TYPE__' 1952 `__UINT_FAST32_TYPE__' 1953 `__UINT_FAST64_TYPE__' 1954 `__INTPTR_TYPE__' 1955 `__UINTPTR_TYPE__' 1956 These macros are defined to the correct underlying types for the 1957 `size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', 1958 `uintmax_t', `sig_atomic_t', `int8_t', `int16_t', `int32_t', 1959 `int64_t', `uint8_t', `uint16_t', `uint32_t', `uint64_t', 1960 `int_least8_t', `int_least16_t', `int_least32_t', `int_least64_t', 1961 `uint_least8_t', `uint_least16_t', `uint_least32_t', 1962 `uint_least64_t', `int_fast8_t', `int_fast16_t', `int_fast32_t', 1963 `int_fast64_t', `uint_fast8_t', `uint_fast16_t', `uint_fast32_t', 1964 `uint_fast64_t', `intptr_t', and `uintptr_t' typedefs, 1965 respectively. They exist to make the standard header files 1966 `stddef.h', `stdint.h', and `wchar.h' work correctly. You should 1967 not use these macros directly; instead, include the appropriate 1968 headers and use the typedefs. Some of these macros may not be 1969 defined on particular systems if GCC does not provide a `stdint.h' 1970 header on those systems. 1971 1972 `__CHAR_BIT__' 1973 Defined to the number of bits used in the representation of the 1974 `char' data type. It exists to make the standard header given 1975 numerical limits work correctly. You should not use this macro 1976 directly; instead, include the appropriate headers. 1977 1978 `__SCHAR_MAX__' 1979 `__WCHAR_MAX__' 1980 `__SHRT_MAX__' 1981 `__INT_MAX__' 1982 `__LONG_MAX__' 1983 `__LONG_LONG_MAX__' 1984 `__WINT_MAX__' 1985 `__SIZE_MAX__' 1986 `__PTRDIFF_MAX__' 1987 `__INTMAX_MAX__' 1988 `__UINTMAX_MAX__' 1989 `__SIG_ATOMIC_MAX__' 1990 `__INT8_MAX__' 1991 `__INT16_MAX__' 1992 `__INT32_MAX__' 1993 `__INT64_MAX__' 1994 `__UINT8_MAX__' 1995 `__UINT16_MAX__' 1996 `__UINT32_MAX__' 1997 `__UINT64_MAX__' 1998 `__INT_LEAST8_MAX__' 1999 `__INT_LEAST16_MAX__' 2000 `__INT_LEAST32_MAX__' 2001 `__INT_LEAST64_MAX__' 2002 `__UINT_LEAST8_MAX__' 2003 `__UINT_LEAST16_MAX__' 2004 `__UINT_LEAST32_MAX__' 2005 `__UINT_LEAST64_MAX__' 2006 `__INT_FAST8_MAX__' 2007 `__INT_FAST16_MAX__' 2008 `__INT_FAST32_MAX__' 2009 `__INT_FAST64_MAX__' 2010 `__UINT_FAST8_MAX__' 2011 `__UINT_FAST16_MAX__' 2012 `__UINT_FAST32_MAX__' 2013 `__UINT_FAST64_MAX__' 2014 `__INTPTR_MAX__' 2015 `__UINTPTR_MAX__' 2016 `__WCHAR_MIN__' 2017 `__WINT_MIN__' 2018 `__SIG_ATOMIC_MIN__' 2019 Defined to the maximum value of the `signed char', `wchar_t', 2020 `signed short', `signed int', `signed long', `signed long long', 2021 `wint_t', `size_t', `ptrdiff_t', `intmax_t', `uintmax_t', 2022 `sig_atomic_t', `int8_t', `int16_t', `int32_t', `int64_t', 2023 `uint8_t', `uint16_t', `uint32_t', `uint64_t', `int_least8_t', 2024 `int_least16_t', `int_least32_t', `int_least64_t', 2025 `uint_least8_t', `uint_least16_t', `uint_least32_t', 2026 `uint_least64_t', `int_fast8_t', `int_fast16_t', `int_fast32_t', 2027 `int_fast64_t', `uint_fast8_t', `uint_fast16_t', `uint_fast32_t', 2028 `uint_fast64_t', `intptr_t', and `uintptr_t' types and to the 2029 minimum value of the `wchar_t', `wint_t', and `sig_atomic_t' types 2030 respectively. They exist to make the standard header given 2031 numerical limits work correctly. You should not use these macros 2032 directly; instead, include the appropriate headers. Some of these 2033 macros may not be defined on particular systems if GCC does not 2034 provide a `stdint.h' header on those systems. 2035 2036 `__INT8_C' 2037 `__INT16_C' 2038 `__INT32_C' 2039 `__INT64_C' 2040 `__UINT8_C' 2041 `__UINT16_C' 2042 `__UINT32_C' 2043 `__UINT64_C' 2044 `__INTMAX_C' 2045 `__UINTMAX_C' 2046 Defined to implementations of the standard `stdint.h' macros with 2047 the same names without the leading `__'. They exist the make the 2048 implementation of that header work correctly. You should not use 2049 these macros directly; instead, include the appropriate headers. 2050 Some of these macros may not be defined on particular systems if 2051 GCC does not provide a `stdint.h' header on those systems. 2052 2053 `__SIZEOF_INT__' 2054 `__SIZEOF_LONG__' 2055 `__SIZEOF_LONG_LONG__' 2056 `__SIZEOF_SHORT__' 2057 `__SIZEOF_POINTER__' 2058 `__SIZEOF_FLOAT__' 2059 `__SIZEOF_DOUBLE__' 2060 `__SIZEOF_LONG_DOUBLE__' 2061 `__SIZEOF_SIZE_T__' 2062 `__SIZEOF_WCHAR_T__' 2063 `__SIZEOF_WINT_T__' 2064 `__SIZEOF_PTRDIFF_T__' 2065 Defined to the number of bytes of the C standard data types: `int', 2066 `long', `long long', `short', `void *', `float', `double', `long 2067 double', `size_t', `wchar_t', `wint_t' and `ptrdiff_t'. 2068 2069 `__BYTE_ORDER__' 2070 `__ORDER_LITTLE_ENDIAN__' 2071 `__ORDER_BIG_ENDIAN__' 2072 `__ORDER_PDP_ENDIAN__' 2073 `__BYTE_ORDER__' is defined to one of the values 2074 `__ORDER_LITTLE_ENDIAN__', `__ORDER_BIG_ENDIAN__', or 2075 `__ORDER_PDP_ENDIAN__' to reflect the layout of multi-byte and 2076 multi-word quantities in memory. If `__BYTE_ORDER__' is equal to 2077 `__ORDER_LITTLE_ENDIAN__' or `__ORDER_BIG_ENDIAN__', then 2078 multi-byte and multi-word quantities are laid out identically: the 2079 byte (word) at the lowest address is the least significant or most 2080 significant byte (word) of the quantity, respectively. If 2081 `__BYTE_ORDER__' is equal to `__ORDER_PDP_ENDIAN__', then bytes in 2082 16-bit words are laid out in a little-endian fashion, whereas the 2083 16-bit subwords of a 32-bit quantity are laid out in big-endian 2084 fashion. 2085 2086 You should use these macros for testing like this: 2087 2088 /* Test for a little-endian machine */ 2089 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ 2090 2091 `__FLOAT_WORD_ORDER__' 2092 `__FLOAT_WORD_ORDER__' is defined to one of the values 2093 `__ORDER_LITTLE_ENDIAN__' or `__ORDER_BIG_ENDIAN__' to reflect the 2094 layout of the words of multi-word floating-point quantities. 2095 2096 `__DEPRECATED' 2097 This macro is defined, with value 1, when compiling a C++ source 2098 file with warnings about deprecated constructs enabled. These 2099 warnings are enabled by default, but can be disabled with 2100 `-Wno-deprecated'. 2101 2102 `__EXCEPTIONS' 2103 This macro is defined, with value 1, when compiling a C++ source 2104 file with exceptions enabled. If `-fno-exceptions' is used when 2105 compiling the file, then this macro is not defined. 2106 2107 `__GXX_RTTI' 2108 This macro is defined, with value 1, when compiling a C++ source 2109 file with runtime type identification enabled. If `-fno-rtti' is 2110 used when compiling the file, then this macro is not defined. 2111 2112 `__USING_SJLJ_EXCEPTIONS__' 2113 This macro is defined, with value 1, if the compiler uses the old 2114 mechanism based on `setjmp' and `longjmp' for exception handling. 2115 2116 `__GXX_EXPERIMENTAL_CXX0X__' 2117 This macro is defined when compiling a C++ source file with the 2118 option `-std=c++0x' or `-std=gnu++0x'. It indicates that some 2119 features likely to be included in C++0x are available. Note that 2120 these features are experimental, and may change or be removed in 2121 future versions of GCC. 2122 2123 `__GXX_WEAK__' 2124 This macro is defined when compiling a C++ source file. It has the 2125 value 1 if the compiler will use weak symbols, COMDAT sections, or 2126 other similar techniques to collapse symbols with "vague linkage" 2127 that are defined in multiple translation units. If the compiler 2128 will not collapse such symbols, this macro is defined with value 2129 0. In general, user code should not need to make use of this 2130 macro; the purpose of this macro is to ease implementation of the 2131 C++ runtime library provided with G++. 2132 2133 `__NEXT_RUNTIME__' 2134 This macro is defined, with value 1, if (and only if) the NeXT 2135 runtime (as in `-fnext-runtime') is in use for Objective-C. If 2136 the GNU runtime is used, this macro is not defined, so that you 2137 can use this macro to determine which runtime (NeXT or GNU) is 2138 being used. 2139 2140 `__LP64__' 2141 `_LP64' 2142 These macros are defined, with value 1, if (and only if) the 2143 compilation is for a target where `long int' and pointer both use 2144 64-bits and `int' uses 32-bit. 2145 2146 `__SSP__' 2147 This macro is defined, with value 1, when `-fstack-protector' is in 2148 use. 2149 2150 `__SSP_ALL__' 2151 This macro is defined, with value 2, when `-fstack-protector-all' 2152 is in use. 2153 2154 `__TIMESTAMP__' 2155 This macro expands to a string constant that describes the date 2156 and time of the last modification of the current source file. The 2157 string constant contains abbreviated day of the week, month, day 2158 of the month, time in hh:mm:ss form, year and looks like 2159 `"Sun Sep 16 01:03:52 1973"'. If the day of the month is less 2160 than 10, it is padded with a space on the left. 2161 2162 If GCC cannot determine the current date, it will emit a warning 2163 message (once per compilation) and `__TIMESTAMP__' will expand to 2164 `"??? ??? ?? ??:??:?? ????"'. 2165 2166 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1' 2167 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2' 2168 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4' 2169 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8' 2170 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16' 2171 These macros are defined when the target processor supports atomic 2172 compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in 2173 length, respectively. 2174 2175 `__GCC_HAVE_DWARF2_CFI_ASM' 2176 This macro is defined when the compiler is emitting Dwarf2 CFI 2177 directives to the assembler. When this is defined, it is possible 2178 to emit those same directives in inline assembly. 2179 2180 `__FP_FAST_FMA' 2181 `__FP_FAST_FMAF' 2182 `__FP_FAST_FMAL' 2183 These macros are defined with value 1 if the backend supports the 2184 `fma', `fmaf', and `fmal' builtin functions, so that the include 2185 file `math.h' can define the macros `FP_FAST_FMA', `FP_FAST_FMAF', 2186 and `FP_FAST_FMAL' for compatibility with the 1999 C standard. 2187 2188 2189 File: cpp.info, Node: System-specific Predefined Macros, Next: C++ Named Operators, Prev: Common Predefined Macros, Up: Predefined Macros 2190 2191 3.7.3 System-specific Predefined Macros 2192 --------------------------------------- 2193 2194 The C preprocessor normally predefines several macros that indicate what 2195 type of system and machine is in use. They are obviously different on 2196 each target supported by GCC. This manual, being for all systems and 2197 machines, cannot tell you what their names are, but you can use `cpp 2198 -dM' to see them all. *Note Invocation::. All system-specific 2199 predefined macros expand to the constant 1, so you can test them with 2200 either `#ifdef' or `#if'. 2201 2202 The C standard requires that all system-specific macros be part of 2203 the "reserved namespace". All names which begin with two underscores, 2204 or an underscore and a capital letter, are reserved for the compiler and 2205 library to use as they wish. However, historically system-specific 2206 macros have had names with no special prefix; for instance, it is common 2207 to find `unix' defined on Unix systems. For all such macros, GCC 2208 provides a parallel macro with two underscores added at the beginning 2209 and the end. If `unix' is defined, `__unix__' will be defined too. 2210 There will never be more than two underscores; the parallel of `_mips' 2211 is `__mips__'. 2212 2213 When the `-ansi' option, or any `-std' option that requests strict 2214 conformance, is given to the compiler, all the system-specific 2215 predefined macros outside the reserved namespace are suppressed. The 2216 parallel macros, inside the reserved namespace, remain defined. 2217 2218 We are slowly phasing out all predefined macros which are outside the 2219 reserved namespace. You should never use them in new programs, and we 2220 encourage you to correct older code to use the parallel macros whenever 2221 you find it. We don't recommend you use the system-specific macros that 2222 are in the reserved namespace, either. It is better in the long run to 2223 check specifically for features you need, using a tool such as 2224 `autoconf'. 2225 2226 2227 File: cpp.info, Node: C++ Named Operators, Prev: System-specific Predefined Macros, Up: Predefined Macros 2228 2229 3.7.4 C++ Named Operators 2230 ------------------------- 2231 2232 In C++, there are eleven keywords which are simply alternate spellings 2233 of operators normally written with punctuation. These keywords are 2234 treated as such even in the preprocessor. They function as operators in 2235 `#if', and they cannot be defined as macros or poisoned. In C, you can 2236 request that those keywords take their C++ meaning by including 2237 `iso646.h'. That header defines each one as a normal object-like macro 2238 expanding to the appropriate punctuator. 2239 2240 These are the named operators and their corresponding punctuators: 2241 2242 Named Operator Punctuator 2243 `and' `&&' 2244 `and_eq' `&=' 2245 `bitand' `&' 2246 `bitor' `|' 2247 `compl' `~' 2248 `not' `!' 2249 `not_eq' `!=' 2250 `or' `||' 2251 `or_eq' `|=' 2252 `xor' `^' 2253 `xor_eq' `^=' 2254 2255 2256 File: cpp.info, Node: Undefining and Redefining Macros, Next: Directives Within Macro Arguments, Prev: Predefined Macros, Up: Macros 2257 2258 3.8 Undefining and Redefining Macros 2259 ==================================== 2260 2261 If a macro ceases to be useful, it may be "undefined" with the `#undef' 2262 directive. `#undef' takes a single argument, the name of the macro to 2263 undefine. You use the bare macro name, even if the macro is 2264 function-like. It is an error if anything appears on the line after 2265 the macro name. `#undef' has no effect if the name is not a macro. 2266 2267 #define FOO 4 2268 x = FOO; ==> x = 4; 2269 #undef FOO 2270 x = FOO; ==> x = FOO; 2271 2272 Once a macro has been undefined, that identifier may be "redefined" 2273 as a macro by a subsequent `#define' directive. The new definition 2274 need not have any resemblance to the old definition. 2275 2276 However, if an identifier which is currently a macro is redefined, 2277 then the new definition must be "effectively the same" as the old one. 2278 Two macro definitions are effectively the same if: 2279 * Both are the same type of macro (object- or function-like). 2280 2281 * All the tokens of the replacement list are the same. 2282 2283 * If there are any parameters, they are the same. 2284 2285 * Whitespace appears in the same places in both. It need not be 2286 exactly the same amount of whitespace, though. Remember that 2287 comments count as whitespace. 2288 2289 These definitions are effectively the same: 2290 #define FOUR (2 + 2) 2291 #define FOUR (2 + 2) 2292 #define FOUR (2 /* two */ + 2) 2293 but these are not: 2294 #define FOUR (2 + 2) 2295 #define FOUR ( 2+2 ) 2296 #define FOUR (2 * 2) 2297 #define FOUR(score,and,seven,years,ago) (2 + 2) 2298 2299 If a macro is redefined with a definition that is not effectively the 2300 same as the old one, the preprocessor issues a warning and changes the 2301 macro to use the new definition. If the new definition is effectively 2302 the same, the redefinition is silently ignored. This allows, for 2303 instance, two different headers to define a common macro. The 2304 preprocessor will only complain if the definitions do not match. 2305 2306 2307 File: cpp.info, Node: Directives Within Macro Arguments, Next: Macro Pitfalls, Prev: Undefining and Redefining Macros, Up: Macros 2308 2309 3.9 Directives Within Macro Arguments 2310 ===================================== 2311 2312 Occasionally it is convenient to use preprocessor directives within the 2313 arguments of a macro. The C and C++ standards declare that behavior in 2314 these cases is undefined. 2315 2316 Versions of CPP prior to 3.2 would reject such constructs with an 2317 error message. This was the only syntactic difference between normal 2318 functions and function-like macros, so it seemed attractive to remove 2319 this limitation, and people would often be surprised that they could 2320 not use macros in this way. Moreover, sometimes people would use 2321 conditional compilation in the argument list to a normal library 2322 function like `printf', only to find that after a library upgrade 2323 `printf' had changed to be a function-like macro, and their code would 2324 no longer compile. So from version 3.2 we changed CPP to successfully 2325 process arbitrary directives within macro arguments in exactly the same 2326 way as it would have processed the directive were the function-like 2327 macro invocation not present. 2328 2329 If, within a macro invocation, that macro is redefined, then the new 2330 definition takes effect in time for argument pre-expansion, but the 2331 original definition is still used for argument replacement. Here is a 2332 pathological example: 2333 2334 #define f(x) x x 2335 f (1 2336 #undef f 2337 #define f 2 2338 f) 2339 2340 which expands to 2341 2342 1 2 1 2 2343 2344 with the semantics described above. 2345 2346 2347 File: cpp.info, Node: Macro Pitfalls, Prev: Directives Within Macro Arguments, Up: Macros 2348 2349 3.10 Macro Pitfalls 2350 =================== 2351 2352 In this section we describe some special rules that apply to macros and 2353 macro expansion, and point out certain cases in which the rules have 2354 counter-intuitive consequences that you must watch out for. 2355 2356 * Menu: 2357 2358 * Misnesting:: 2359 * Operator Precedence Problems:: 2360 * Swallowing the Semicolon:: 2361 * Duplication of Side Effects:: 2362 * Self-Referential Macros:: 2363 * Argument Prescan:: 2364 * Newlines in Arguments:: 2365 2366 2367 File: cpp.info, Node: Misnesting, Next: Operator Precedence Problems, Up: Macro Pitfalls 2368 2369 3.10.1 Misnesting 2370 ----------------- 2371 2372 When a macro is called with arguments, the arguments are substituted 2373 into the macro body and the result is checked, together with the rest of 2374 the input file, for more macro calls. It is possible to piece together 2375 a macro call coming partially from the macro body and partially from the 2376 arguments. For example, 2377 2378 #define twice(x) (2*(x)) 2379 #define call_with_1(x) x(1) 2380 call_with_1 (twice) 2381 ==> twice(1) 2382 ==> (2*(1)) 2383 2384 Macro definitions do not have to have balanced parentheses. By 2385 writing an unbalanced open parenthesis in a macro body, it is possible 2386 to create a macro call that begins inside the macro body but ends 2387 outside of it. For example, 2388 2389 #define strange(file) fprintf (file, "%s %d", 2390 ... 2391 strange(stderr) p, 35) 2392 ==> fprintf (stderr, "%s %d", p, 35) 2393 2394 The ability to piece together a macro call can be useful, but the 2395 use of unbalanced open parentheses in a macro body is just confusing, 2396 and should be avoided. 2397 2398 2399 File: cpp.info, Node: Operator Precedence Problems, Next: Swallowing the Semicolon, Prev: Misnesting, Up: Macro Pitfalls 2400 2401 3.10.2 Operator Precedence Problems 2402 ----------------------------------- 2403 2404 You may have noticed that in most of the macro definition examples shown 2405 above, each occurrence of a macro argument name had parentheses around 2406 it. In addition, another pair of parentheses usually surround the 2407 entire macro definition. Here is why it is best to write macros that 2408 way. 2409 2410 Suppose you define a macro as follows, 2411 2412 #define ceil_div(x, y) (x + y - 1) / y 2413 2414 whose purpose is to divide, rounding up. (One use for this operation is 2415 to compute how many `int' objects are needed to hold a certain number 2416 of `char' objects.) Then suppose it is used as follows: 2417 2418 a = ceil_div (b & c, sizeof (int)); 2419 ==> a = (b & c + sizeof (int) - 1) / sizeof (int); 2420 2421 This does not do what is intended. The operator-precedence rules of C 2422 make it equivalent to this: 2423 2424 a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2425 2426 What we want is this: 2427 2428 a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2429 2430 Defining the macro as 2431 2432 #define ceil_div(x, y) ((x) + (y) - 1) / (y) 2433 2434 provides the desired result. 2435 2436 Unintended grouping can result in another way. Consider `sizeof 2437 ceil_div(1, 2)'. That has the appearance of a C expression that would 2438 compute the size of the type of `ceil_div (1, 2)', but in fact it means 2439 something very different. Here is what it expands to: 2440 2441 sizeof ((1) + (2) - 1) / (2) 2442 2443 This would take the size of an integer and divide it by two. The 2444 precedence rules have put the division outside the `sizeof' when it was 2445 intended to be inside. 2446 2447 Parentheses around the entire macro definition prevent such problems. 2448 Here, then, is the recommended way to define `ceil_div': 2449 2450 #define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2451 2452 2453 File: cpp.info, Node: Swallowing the Semicolon, Next: Duplication of Side Effects, Prev: Operator Precedence Problems, Up: Macro Pitfalls 2454 2455 3.10.3 Swallowing the Semicolon 2456 ------------------------------- 2457 2458 Often it is desirable to define a macro that expands into a compound 2459 statement. Consider, for example, the following macro, that advances a 2460 pointer (the argument `p' says where to find it) across whitespace 2461 characters: 2462 2463 #define SKIP_SPACES(p, limit) \ 2464 { char *lim = (limit); \ 2465 while (p < lim) { \ 2466 if (*p++ != ' ') { \ 2467 p--; break; }}} 2468 2469 Here backslash-newline is used to split the macro definition, which must 2470 be a single logical line, so that it resembles the way such code would 2471 be laid out if not part of a macro definition. 2472 2473 A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly 2474 speaking, the call expands to a compound statement, which is a complete 2475 statement with no need for a semicolon to end it. However, since it 2476 looks like a function call, it minimizes confusion if you can use it 2477 like a function call, writing a semicolon afterward, as in `SKIP_SPACES 2478 (p, lim);' 2479 2480 This can cause trouble before `else' statements, because the 2481 semicolon is actually a null statement. Suppose you write 2482 2483 if (*p != 0) 2484 SKIP_SPACES (p, lim); 2485 else ... 2486 2487 The presence of two statements--the compound statement and a null 2488 statement--in between the `if' condition and the `else' makes invalid C 2489 code. 2490 2491 The definition of the macro `SKIP_SPACES' can be altered to solve 2492 this problem, using a `do ... while' statement. Here is how: 2493 2494 #define SKIP_SPACES(p, limit) \ 2495 do { char *lim = (limit); \ 2496 while (p < lim) { \ 2497 if (*p++ != ' ') { \ 2498 p--; break; }}} \ 2499 while (0) 2500 2501 Now `SKIP_SPACES (p, lim);' expands into 2502 2503 do {...} while (0); 2504 2505 which is one statement. The loop executes exactly once; most compilers 2506 generate no extra code for it. 2507 2508 2509 File: cpp.info, Node: Duplication of Side Effects, Next: Self-Referential Macros, Prev: Swallowing the Semicolon, Up: Macro Pitfalls 2510 2511 3.10.4 Duplication of Side Effects 2512 ---------------------------------- 2513 2514 Many C programs define a macro `min', for "minimum", like this: 2515 2516 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2517 2518 When you use this macro with an argument containing a side effect, 2519 as shown here, 2520 2521 next = min (x + y, foo (z)); 2522 2523 it expands as follows: 2524 2525 next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2526 2527 where `x + y' has been substituted for `X' and `foo (z)' for `Y'. 2528 2529 The function `foo' is used only once in the statement as it appears 2530 in the program, but the expression `foo (z)' has been substituted twice 2531 into the macro expansion. As a result, `foo' might be called two times 2532 when the statement is executed. If it has side effects or if it takes 2533 a long time to compute, the results might not be what you intended. We 2534 say that `min' is an "unsafe" macro. 2535 2536 The best solution to this problem is to define `min' in a way that 2537 computes the value of `foo (z)' only once. The C language offers no 2538 standard way to do this, but it can be done with GNU extensions as 2539 follows: 2540 2541 #define min(X, Y) \ 2542 ({ typeof (X) x_ = (X); \ 2543 typeof (Y) y_ = (Y); \ 2544 (x_ < y_) ? x_ : y_; }) 2545 2546 The `({ ... })' notation produces a compound statement that acts as 2547 an expression. Its value is the value of its last statement. This 2548 permits us to define local variables and assign each argument to one. 2549 The local variables have underscores after their names to reduce the 2550 risk of conflict with an identifier of wider scope (it is impossible to 2551 avoid this entirely). Now each argument is evaluated exactly once. 2552 2553 If you do not wish to use GNU C extensions, the only solution is to 2554 be careful when _using_ the macro `min'. For example, you can 2555 calculate the value of `foo (z)', save it in a variable, and use that 2556 variable in `min': 2557 2558 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2559 ... 2560 { 2561 int tem = foo (z); 2562 next = min (x + y, tem); 2563 } 2564 2565 (where we assume that `foo' returns type `int'). 2566 2567 2568 File: cpp.info, Node: Self-Referential Macros, Next: Argument Prescan, Prev: Duplication of Side Effects, Up: Macro Pitfalls 2569 2570 3.10.5 Self-Referential Macros 2571 ------------------------------ 2572 2573 A "self-referential" macro is one whose name appears in its definition. 2574 Recall that all macro definitions are rescanned for more macros to 2575 replace. If the self-reference were considered a use of the macro, it 2576 would produce an infinitely large expansion. To prevent this, the 2577 self-reference is not considered a macro call. It is passed into the 2578 preprocessor output unchanged. Consider an example: 2579 2580 #define foo (4 + foo) 2581 2582 where `foo' is also a variable in your program. 2583 2584 Following the ordinary rules, each reference to `foo' will expand 2585 into `(4 + foo)'; then this will be rescanned and will expand into `(4 2586 + (4 + foo))'; and so on until the computer runs out of memory. 2587 2588 The self-reference rule cuts this process short after one step, at 2589 `(4 + foo)'. Therefore, this macro definition has the possibly useful 2590 effect of causing the program to add 4 to the value of `foo' wherever 2591 `foo' is referred to. 2592 2593 In most cases, it is a bad idea to take advantage of this feature. A 2594 person reading the program who sees that `foo' is a variable will not 2595 expect that it is a macro as well. The reader will come across the 2596 identifier `foo' in the program and think its value should be that of 2597 the variable `foo', whereas in fact the value is four greater. 2598 2599 One common, useful use of self-reference is to create a macro which 2600 expands to itself. If you write 2601 2602 #define EPERM EPERM 2603 2604 then the macro `EPERM' expands to `EPERM'. Effectively, it is left 2605 alone by the preprocessor whenever it's used in running text. You can 2606 tell that it's a macro with `#ifdef'. You might do this if you want to 2607 define numeric constants with an `enum', but have `#ifdef' be true for 2608 each constant. 2609 2610 If a macro `x' expands to use a macro `y', and the expansion of `y' 2611 refers to the macro `x', that is an "indirect self-reference" of `x'. 2612 `x' is not expanded in this case either. Thus, if we have 2613 2614 #define x (4 + y) 2615 #define y (2 * x) 2616 2617 then `x' and `y' expand as follows: 2618 2619 x ==> (4 + y) 2620 ==> (4 + (2 * x)) 2621 2622 y ==> (2 * x) 2623 ==> (2 * (4 + y)) 2624 2625 Each macro is expanded when it appears in the definition of the other 2626 macro, but not when it indirectly appears in its own definition. 2627 2628 2629 File: cpp.info, Node: Argument Prescan, Next: Newlines in Arguments, Prev: Self-Referential Macros, Up: Macro Pitfalls 2630 2631 3.10.6 Argument Prescan 2632 ----------------------- 2633 2634 Macro arguments are completely macro-expanded before they are 2635 substituted into a macro body, unless they are stringified or pasted 2636 with other tokens. After substitution, the entire macro body, including 2637 the substituted arguments, is scanned again for macros to be expanded. 2638 The result is that the arguments are scanned _twice_ to expand macro 2639 calls in them. 2640 2641 Most of the time, this has no effect. If the argument contained any 2642 macro calls, they are expanded during the first scan. The result 2643 therefore contains no macro calls, so the second scan does not change 2644 it. If the argument were substituted as given, with no prescan, the 2645 single remaining scan would find the same macro calls and produce the 2646 same results. 2647 2648 You might expect the double scan to change the results when a 2649 self-referential macro is used in an argument of another macro (*note 2650 Self-Referential Macros::): the self-referential macro would be 2651 expanded once in the first scan, and a second time in the second scan. 2652 However, this is not what happens. The self-references that do not 2653 expand in the first scan are marked so that they will not expand in the 2654 second scan either. 2655 2656 You might wonder, "Why mention the prescan, if it makes no 2657 difference? And why not skip it and make the preprocessor faster?" 2658 The answer is that the prescan does make a difference in three special 2659 cases: 2660 2661 * Nested calls to a macro. 2662 2663 We say that "nested" calls to a macro occur when a macro's argument 2664 contains a call to that very macro. For example, if `f' is a macro 2665 that expects one argument, `f (f (1))' is a nested pair of calls to 2666 `f'. The desired expansion is made by expanding `f (1)' and 2667 substituting that into the definition of `f'. The prescan causes 2668 the expected result to happen. Without the prescan, `f (1)' itself 2669 would be substituted as an argument, and the inner use of `f' would 2670 appear during the main scan as an indirect self-reference and 2671 would not be expanded. 2672 2673 * Macros that call other macros that stringify or concatenate. 2674 2675 If an argument is stringified or concatenated, the prescan does not 2676 occur. If you _want_ to expand a macro, then stringify or 2677 concatenate its expansion, you can do that by causing one macro to 2678 call another macro that does the stringification or concatenation. 2679 For instance, if you have 2680 2681 #define AFTERX(x) X_ ## x 2682 #define XAFTERX(x) AFTERX(x) 2683 #define TABLESIZE 1024 2684 #define BUFSIZE TABLESIZE 2685 2686 then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and 2687 `XAFTERX(BUFSIZE)' expands to `X_1024'. (Not to `X_TABLESIZE'. 2688 Prescan always does a complete expansion.) 2689 2690 * Macros used in arguments, whose expansions contain unshielded 2691 commas. 2692 2693 This can cause a macro expanded on the second scan to be called 2694 with the wrong number of arguments. Here is an example: 2695 2696 #define foo a,b 2697 #define bar(x) lose(x) 2698 #define lose(x) (1 + (x)) 2699 2700 We would like `bar(foo)' to turn into `(1 + (foo))', which would 2701 then turn into `(1 + (a,b))'. Instead, `bar(foo)' expands into 2702 `lose(a,b)', and you get an error because `lose' requires a single 2703 argument. In this case, the problem is easily solved by the same 2704 parentheses that ought to be used to prevent misnesting of 2705 arithmetic operations: 2706 2707 #define foo (a,b) 2708 or 2709 #define bar(x) lose((x)) 2710 2711 The extra pair of parentheses prevents the comma in `foo''s 2712 definition from being interpreted as an argument separator. 2713 2714 2715 2716 File: cpp.info, Node: Newlines in Arguments, Prev: Argument Prescan, Up: Macro Pitfalls 2717 2718 3.10.7 Newlines in Arguments 2719 ---------------------------- 2720 2721 The invocation of a function-like macro can extend over many logical 2722 lines. However, in the present implementation, the entire expansion 2723 comes out on one line. Thus line numbers emitted by the compiler or 2724 debugger refer to the line the invocation started on, which might be 2725 different to the line containing the argument causing the problem. 2726 2727 Here is an example illustrating this: 2728 2729 #define ignore_second_arg(a,b,c) a; c 2730 2731 ignore_second_arg (foo (), 2732 ignored (), 2733 syntax error); 2734 2735 The syntax error triggered by the tokens `syntax error' results in an 2736 error message citing line three--the line of ignore_second_arg-- even 2737 though the problematic code comes from line five. 2738 2739 We consider this a bug, and intend to fix it in the near future. 2740 2741 2742 File: cpp.info, Node: Conditionals, Next: Diagnostics, Prev: Macros, Up: Top 2743 2744 4 Conditionals 2745 ************** 2746 2747 A "conditional" is a directive that instructs the preprocessor to 2748 select whether or not to include a chunk of code in the final token 2749 stream passed to the compiler. Preprocessor conditionals can test 2750 arithmetic expressions, or whether a name is defined as a macro, or both 2751 simultaneously using the special `defined' operator. 2752 2753 A conditional in the C preprocessor resembles in some ways an `if' 2754 statement in C, but it is important to understand the difference between 2755 them. The condition in an `if' statement is tested during the 2756 execution of your program. Its purpose is to allow your program to 2757 behave differently from run to run, depending on the data it is 2758 operating on. The condition in a preprocessing conditional directive is 2759 tested when your program is compiled. Its purpose is to allow different 2760 code to be included in the program depending on the situation at the 2761 time of compilation. 2762 2763 However, the distinction is becoming less clear. Modern compilers 2764 often do test `if' statements when a program is compiled, if their 2765 conditions are known not to vary at run time, and eliminate code which 2766 can never be executed. If you can count on your compiler to do this, 2767 you may find that your program is more readable if you use `if' 2768 statements with constant conditions (perhaps determined by macros). Of 2769 course, you can only use this to exclude code, not type definitions or 2770 other preprocessing directives, and you can only do it if the code 2771 remains syntactically valid when it is not to be used. 2772 2773 GCC version 3 eliminates this kind of never-executed code even when 2774 not optimizing. Older versions did it only when optimizing. 2775 2776 * Menu: 2777 2778 * Conditional Uses:: 2779 * Conditional Syntax:: 2780 * Deleted Code:: 2781 2782 2783 File: cpp.info, Node: Conditional Uses, Next: Conditional Syntax, Up: Conditionals 2784 2785 4.1 Conditional Uses 2786 ==================== 2787 2788 There are three general reasons to use a conditional. 2789 2790 * A program may need to use different code depending on the machine 2791 or operating system it is to run on. In some cases the code for 2792 one operating system may be erroneous on another operating system; 2793 for example, it might refer to data types or constants that do not 2794 exist on the other system. When this happens, it is not enough to 2795 avoid executing the invalid code. Its mere presence will cause 2796 the compiler to reject the program. With a preprocessing 2797 conditional, the offending code can be effectively excised from 2798 the program when it is not valid. 2799 2800 * You may want to be able to compile the same source file into two 2801 different programs. One version might make frequent time-consuming 2802 consistency checks on its intermediate data, or print the values of 2803 those data for debugging, and the other not. 2804 2805 * A conditional whose condition is always false is one way to 2806 exclude code from the program but keep it as a sort of comment for 2807 future reference. 2808 2809 Simple programs that do not need system-specific logic or complex 2810 debugging hooks generally will not need to use preprocessing 2811 conditionals. 2812 2813 2814 File: cpp.info, Node: Conditional Syntax, Next: Deleted Code, Prev: Conditional Uses, Up: Conditionals 2815 2816 4.2 Conditional Syntax 2817 ====================== 2818 2819 A conditional in the C preprocessor begins with a "conditional 2820 directive": `#if', `#ifdef' or `#ifndef'. 2821 2822 * Menu: 2823 2824 * Ifdef:: 2825 * If:: 2826 * Defined:: 2827 * Else:: 2828 * Elif:: 2829 2830 2831 File: cpp.info, Node: Ifdef, Next: If, Up: Conditional Syntax 2832 2833 4.2.1 Ifdef 2834 ----------- 2835 2836 The simplest sort of conditional is 2837 2838 #ifdef MACRO 2839 2840 CONTROLLED TEXT 2841 2842 #endif /* MACRO */ 2843 2844 This block is called a "conditional group". CONTROLLED TEXT will be 2845 included in the output of the preprocessor if and only if MACRO is 2846 defined. We say that the conditional "succeeds" if MACRO is defined, 2847 "fails" if it is not. 2848 2849 The CONTROLLED TEXT inside of a conditional can include 2850 preprocessing directives. They are executed only if the conditional 2851 succeeds. You can nest conditional groups inside other conditional 2852 groups, but they must be completely nested. In other words, `#endif' 2853 always matches the nearest `#ifdef' (or `#ifndef', or `#if'). Also, 2854 you cannot start a conditional group in one file and end it in another. 2855 2856 Even if a conditional fails, the CONTROLLED TEXT inside it is still 2857 run through initial transformations and tokenization. Therefore, it 2858 must all be lexically valid C. Normally the only way this matters is 2859 that all comments and string literals inside a failing conditional group 2860 must still be properly ended. 2861 2862 The comment following the `#endif' is not required, but it is a good 2863 practice if there is a lot of CONTROLLED TEXT, because it helps people 2864 match the `#endif' to the corresponding `#ifdef'. Older programs 2865 sometimes put MACRO directly after the `#endif' without enclosing it in 2866 a comment. This is invalid code according to the C standard. CPP 2867 accepts it with a warning. It never affects which `#ifndef' the 2868 `#endif' matches. 2869 2870 Sometimes you wish to use some code if a macro is _not_ defined. 2871 You can do this by writing `#ifndef' instead of `#ifdef'. One common 2872 use of `#ifndef' is to include code only the first time a header file 2873 is included. *Note Once-Only Headers::. 2874 2875 Macro definitions can vary between compilations for several reasons. 2876 Here are some samples. 2877 2878 * Some macros are predefined on each kind of machine (*note 2879 System-specific Predefined Macros::). This allows you to provide 2880 code specially tuned for a particular machine. 2881 2882 * System header files define more macros, associated with the 2883 features they implement. You can test these macros with 2884 conditionals to avoid using a system feature on a machine where it 2885 is not implemented. 2886 2887 * Macros can be defined or undefined with the `-D' and `-U' command 2888 line options when you compile the program. You can arrange to 2889 compile the same source file into two different programs by 2890 choosing a macro name to specify which program you want, writing 2891 conditionals to test whether or how this macro is defined, and 2892 then controlling the state of the macro with command line options, 2893 perhaps set in the Makefile. *Note Invocation::. 2894 2895 * Your program might have a special header file (often called 2896 `config.h') that is adjusted when the program is compiled. It can 2897 define or not define macros depending on the features of the 2898 system and the desired capabilities of the program. The 2899 adjustment can be automated by a tool such as `autoconf', or done 2900 by hand. 2901 2902 2903 File: cpp.info, Node: If, Next: Defined, Prev: Ifdef, Up: Conditional Syntax 2904 2905 4.2.2 If 2906 -------- 2907 2908 The `#if' directive allows you to test the value of an arithmetic 2909 expression, rather than the mere existence of one macro. Its syntax is 2910 2911 #if EXPRESSION 2912 2913 CONTROLLED TEXT 2914 2915 #endif /* EXPRESSION */ 2916 2917 EXPRESSION is a C expression of integer type, subject to stringent 2918 restrictions. It may contain 2919 2920 * Integer constants. 2921 2922 * Character constants, which are interpreted as they would be in 2923 normal code. 2924 2925 * Arithmetic operators for addition, subtraction, multiplication, 2926 division, bitwise operations, shifts, comparisons, and logical 2927 operations (`&&' and `||'). The latter two obey the usual 2928 short-circuiting rules of standard C. 2929 2930 * Macros. All macros in the expression are expanded before actual 2931 computation of the expression's value begins. 2932 2933 * Uses of the `defined' operator, which lets you check whether macros 2934 are defined in the middle of an `#if'. 2935 2936 * Identifiers that are not macros, which are all considered to be the 2937 number zero. This allows you to write `#if MACRO' instead of 2938 `#ifdef MACRO', if you know that MACRO, when defined, will always 2939 have a nonzero value. Function-like macros used without their 2940 function call parentheses are also treated as zero. 2941 2942 In some contexts this shortcut is undesirable. The `-Wundef' 2943 option causes GCC to warn whenever it encounters an identifier 2944 which is not a macro in an `#if'. 2945 2946 The preprocessor does not know anything about types in the language. 2947 Therefore, `sizeof' operators are not recognized in `#if', and neither 2948 are `enum' constants. They will be taken as identifiers which are not 2949 macros, and replaced by zero. In the case of `sizeof', this is likely 2950 to cause the expression to be invalid. 2951 2952 The preprocessor calculates the value of EXPRESSION. It carries out 2953 all calculations in the widest integer type known to the compiler; on 2954 most machines supported by GCC this is 64 bits. This is not the same 2955 rule as the compiler uses to calculate the value of a constant 2956 expression, and may give different results in some cases. If the value 2957 comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is 2958 included; otherwise it is skipped. 2959 2960 2961 File: cpp.info, Node: Defined, Next: Else, Prev: If, Up: Conditional Syntax 2962 2963 4.2.3 Defined 2964 ------------- 2965 2966 The special operator `defined' is used in `#if' and `#elif' expressions 2967 to test whether a certain name is defined as a macro. `defined NAME' 2968 and `defined (NAME)' are both expressions whose value is 1 if NAME is 2969 defined as a macro at the current point in the program, and 0 2970 otherwise. Thus, `#if defined MACRO' is precisely equivalent to 2971 `#ifdef MACRO'. 2972 2973 `defined' is useful when you wish to test more than one macro for 2974 existence at once. For example, 2975 2976 #if defined (__vax__) || defined (__ns16000__) 2977 2978 would succeed if either of the names `__vax__' or `__ns16000__' is 2979 defined as a macro. 2980 2981 Conditionals written like this: 2982 2983 #if defined BUFSIZE && BUFSIZE >= 1024 2984 2985 can generally be simplified to just `#if BUFSIZE >= 1024', since if 2986 `BUFSIZE' is not defined, it will be interpreted as having the value 2987 zero. 2988 2989 If the `defined' operator appears as a result of a macro expansion, 2990 the C standard says the behavior is undefined. GNU cpp treats it as a 2991 genuine `defined' operator and evaluates it normally. It will warn 2992 wherever your code uses this feature if you use the command-line option 2993 `-pedantic', since other compilers may handle it differently. 2994 2995 2996 File: cpp.info, Node: Else, Next: Elif, Prev: Defined, Up: Conditional Syntax 2997 2998 4.2.4 Else 2999 ---------- 3000 3001 The `#else' directive can be added to a conditional to provide 3002 alternative text to be used if the condition fails. This is what it 3003 looks like: 3004 3005 #if EXPRESSION 3006 TEXT-IF-TRUE 3007 #else /* Not EXPRESSION */ 3008 TEXT-IF-FALSE 3009 #endif /* Not EXPRESSION */ 3010 3011 If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the 3012 TEXT-IF-FALSE is skipped. If EXPRESSION is zero, the opposite happens. 3013 3014 You can use `#else' with `#ifdef' and `#ifndef', too. 3015 3016 3017 File: cpp.info, Node: Elif, Prev: Else, Up: Conditional Syntax 3018 3019 4.2.5 Elif 3020 ---------- 3021 3022 One common case of nested conditionals is used to check for more than 3023 two possible alternatives. For example, you might have 3024 3025 #if X == 1 3026 ... 3027 #else /* X != 1 */ 3028 #if X == 2 3029 ... 3030 #else /* X != 2 */ 3031 ... 3032 #endif /* X != 2 */ 3033 #endif /* X != 1 */ 3034 3035 Another conditional directive, `#elif', allows this to be 3036 abbreviated as follows: 3037 3038 #if X == 1 3039 ... 3040 #elif X == 2 3041 ... 3042 #else /* X != 2 and X != 1*/ 3043 ... 3044 #endif /* X != 2 and X != 1*/ 3045 3046 `#elif' stands for "else if". Like `#else', it goes in the middle 3047 of a conditional group and subdivides it; it does not require a 3048 matching `#endif' of its own. Like `#if', the `#elif' directive 3049 includes an expression to be tested. The text following the `#elif' is 3050 processed only if the original `#if'-condition failed and the `#elif' 3051 condition succeeds. 3052 3053 More than one `#elif' can go in the same conditional group. Then 3054 the text after each `#elif' is processed only if the `#elif' condition 3055 succeeds after the original `#if' and all previous `#elif' directives 3056 within it have failed. 3057 3058 `#else' is allowed after any number of `#elif' directives, but 3059 `#elif' may not follow `#else'. 3060 3061 3062 File: cpp.info, Node: Deleted Code, Prev: Conditional Syntax, Up: Conditionals 3063 3064 4.3 Deleted Code 3065 ================ 3066 3067 If you replace or delete a part of the program but want to keep the old 3068 code around for future reference, you often cannot simply comment it 3069 out. Block comments do not nest, so the first comment inside the old 3070 code will end the commenting-out. The probable result is a flood of 3071 syntax errors. 3072 3073 One way to avoid this problem is to use an always-false conditional 3074 instead. For instance, put `#if 0' before the deleted code and 3075 `#endif' after it. This works even if the code being turned off 3076 contains conditionals, but they must be entire conditionals (balanced 3077 `#if' and `#endif'). 3078 3079 Some people use `#ifdef notdef' instead. This is risky, because 3080 `notdef' might be accidentally defined as a macro, and then the 3081 conditional would succeed. `#if 0' can be counted on to fail. 3082 3083 Do not use `#if 0' for comments which are not C code. Use a real 3084 comment, instead. The interior of `#if 0' must consist of complete 3085 tokens; in particular, single-quote characters must balance. Comments 3086 often contain unbalanced single-quote characters (known in English as 3087 apostrophes). These confuse `#if 0'. They don't confuse `/*'. 3088 3089 3090 File: cpp.info, Node: Diagnostics, Next: Line Control, Prev: Conditionals, Up: Top 3091 3092 5 Diagnostics 3093 ************* 3094 3095 The directive `#error' causes the preprocessor to report a fatal error. 3096 The tokens forming the rest of the line following `#error' are used as 3097 the error message. 3098 3099 You would use `#error' inside of a conditional that detects a 3100 combination of parameters which you know the program does not properly 3101 support. For example, if you know that the program will not run 3102 properly on a VAX, you might write 3103 3104 #ifdef __vax__ 3105 #error "Won't work on VAXen. See comments at get_last_object." 3106 #endif 3107 3108 If you have several configuration parameters that must be set up by 3109 the installation in a consistent way, you can use conditionals to detect 3110 an inconsistency and report it with `#error'. For example, 3111 3112 #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO) 3113 #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP." 3114 #endif 3115 3116 The directive `#warning' is like `#error', but causes the 3117 preprocessor to issue a warning and continue preprocessing. The tokens 3118 following `#warning' are used as the warning message. 3119 3120 You might use `#warning' in obsolete header files, with a message 3121 directing the user to the header file which should be used instead. 3122 3123 Neither `#error' nor `#warning' macro-expands its argument. 3124 Internal whitespace sequences are each replaced with a single space. 3125 The line must consist of complete tokens. It is wisest to make the 3126 argument of these directives be a single string constant; this avoids 3127 problems with apostrophes and the like. 3128 3129 3130 File: cpp.info, Node: Line Control, Next: Pragmas, Prev: Diagnostics, Up: Top 3131 3132 6 Line Control 3133 ************** 3134 3135 The C preprocessor informs the C compiler of the location in your source 3136 code where each token came from. Presently, this is just the file name 3137 and line number. All the tokens resulting from macro expansion are 3138 reported as having appeared on the line of the source file where the 3139 outermost macro was used. We intend to be more accurate in the future. 3140 3141 If you write a program which generates source code, such as the 3142 `bison' parser generator, you may want to adjust the preprocessor's 3143 notion of the current file name and line number by hand. Parts of the 3144 output from `bison' are generated from scratch, other parts come from a 3145 standard parser file. The rest are copied verbatim from `bison''s 3146 input. You would like compiler error messages and symbolic debuggers 3147 to be able to refer to `bison''s input file. 3148 3149 `bison' or any such program can arrange this by writing `#line' 3150 directives into the output file. `#line' is a directive that specifies 3151 the original line number and source file name for subsequent input in 3152 the current preprocessor input file. `#line' has three variants: 3153 3154 `#line LINENUM' 3155 LINENUM is a non-negative decimal integer constant. It specifies 3156 the line number which should be reported for the following line of 3157 input. Subsequent lines are counted from LINENUM. 3158 3159 `#line LINENUM FILENAME' 3160 LINENUM is the same as for the first form, and has the same 3161 effect. In addition, FILENAME is a string constant. The 3162 following line and all subsequent lines are reported to come from 3163 the file it specifies, until something else happens to change that. 3164 FILENAME is interpreted according to the normal rules for a string 3165 constant: backslash escapes are interpreted. This is different 3166 from `#include'. 3167 3168 Previous versions of CPP did not interpret escapes in `#line'; we 3169 have changed it because the standard requires they be interpreted, 3170 and most other compilers do. 3171 3172 `#line ANYTHING ELSE' 3173 ANYTHING ELSE is checked for macro calls, which are expanded. The 3174 result should match one of the above two forms. 3175 3176 `#line' directives alter the results of the `__FILE__' and 3177 `__LINE__' predefined macros from that point on. *Note Standard 3178 Predefined Macros::. They do not have any effect on `#include''s idea 3179 of the directory containing the current file. This is a change from 3180 GCC 2.95. Previously, a file reading 3181 3182 #line 1 "../src/gram.y" 3183 #include "gram.h" 3184 3185 would search for `gram.h' in `../src', then the `-I' chain; the 3186 directory containing the physical source file would not be searched. 3187 In GCC 3.0 and later, the `#include' is not affected by the presence of 3188 a `#line' referring to a different directory. 3189 3190 We made this change because the old behavior caused problems when 3191 generated source files were transported between machines. For instance, 3192 it is common practice to ship generated parsers with a source release, 3193 so that people building the distribution do not need to have yacc or 3194 Bison installed. These files frequently have `#line' directives 3195 referring to the directory tree of the system where the distribution was 3196 created. If GCC tries to search for headers in those directories, the 3197 build is likely to fail. 3198 3199 The new behavior can cause failures too, if the generated file is not 3200 in the same directory as its source and it attempts to include a header 3201 which would be visible searching from the directory containing the 3202 source file. However, this problem is easily solved with an additional 3203 `-I' switch on the command line. The failures caused by the old 3204 semantics could sometimes be corrected only by editing the generated 3205 files, which is difficult and error-prone. 3206 3207 3208 File: cpp.info, Node: Pragmas, Next: Other Directives, Prev: Line Control, Up: Top 3209 3210 7 Pragmas 3211 ********* 3212 3213 The `#pragma' directive is the method specified by the C standard for 3214 providing additional information to the compiler, beyond what is 3215 conveyed in the language itself. Three forms of this directive 3216 (commonly known as "pragmas") are specified by the 1999 C standard. A 3217 C compiler is free to attach any meaning it likes to other pragmas. 3218 3219 GCC has historically preferred to use extensions to the syntax of the 3220 language, such as `__attribute__', for this purpose. However, GCC does 3221 define a few pragmas of its own. These mostly have effects on the 3222 entire translation unit or source file. 3223 3224 In GCC version 3, all GNU-defined, supported pragmas have been given 3225 a `GCC' prefix. This is in line with the `STDC' prefix on all pragmas 3226 defined by C99. For backward compatibility, pragmas which were 3227 recognized by previous versions are still recognized without the `GCC' 3228 prefix, but that usage is deprecated. Some older pragmas are 3229 deprecated in their entirety. They are not recognized with the `GCC' 3230 prefix. *Note Obsolete Features::. 3231 3232 C99 introduces the `_Pragma' operator. This feature addresses a 3233 major problem with `#pragma': being a directive, it cannot be produced 3234 as the result of macro expansion. `_Pragma' is an operator, much like 3235 `sizeof' or `defined', and can be embedded in a macro. 3236 3237 Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can 3238 be either a normal or wide-character string literal. It is 3239 destringized, by replacing all `\\' with a single `\' and all `\"' with 3240 a `"'. The result is then processed as if it had appeared as the right 3241 hand side of a `#pragma' directive. For example, 3242 3243 _Pragma ("GCC dependency \"parse.y\"") 3244 3245 has the same effect as `#pragma GCC dependency "parse.y"'. The same 3246 effect could be achieved using macros, for example 3247 3248 #define DO_PRAGMA(x) _Pragma (#x) 3249 DO_PRAGMA (GCC dependency "parse.y") 3250 3251 The standard is unclear on where a `_Pragma' operator can appear. 3252 The preprocessor does not accept it within a preprocessing conditional 3253 directive like `#if'. To be safe, you are probably best keeping it out 3254 of directives other than `#define', and putting it on a line of its own. 3255 3256 This manual documents the pragmas which are meaningful to the 3257 preprocessor itself. Other pragmas are meaningful to the C or C++ 3258 compilers. They are documented in the GCC manual. 3259 3260 GCC plugins may provide their own pragmas. 3261 3262 `#pragma GCC dependency' 3263 `#pragma GCC dependency' allows you to check the relative dates of 3264 the current file and another file. If the other file is more 3265 recent than the current file, a warning is issued. This is useful 3266 if the current file is derived from the other file, and should be 3267 regenerated. The other file is searched for using the normal 3268 include search path. Optional trailing text can be used to give 3269 more information in the warning message. 3270 3271 #pragma GCC dependency "parse.y" 3272 #pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3273 3274 `#pragma GCC poison' 3275 Sometimes, there is an identifier that you want to remove 3276 completely from your program, and make sure that it never creeps 3277 back in. To enforce this, you can "poison" the identifier with 3278 this pragma. `#pragma GCC poison' is followed by a list of 3279 identifiers to poison. If any of those identifiers appears 3280 anywhere in the source after the directive, it is a hard error. 3281 For example, 3282 3283 #pragma GCC poison printf sprintf fprintf 3284 sprintf(some_string, "hello"); 3285 3286 will produce an error. 3287 3288 If a poisoned identifier appears as part of the expansion of a 3289 macro which was defined before the identifier was poisoned, it 3290 will _not_ cause an error. This lets you poison an identifier 3291 without worrying about system headers defining macros that use it. 3292 3293 For example, 3294 3295 #define strrchr rindex 3296 #pragma GCC poison rindex 3297 strrchr(some_string, 'h'); 3298 3299 will not produce an error. 3300 3301 `#pragma GCC system_header' 3302 This pragma takes no arguments. It causes the rest of the code in 3303 the current file to be treated as if it came from a system header. 3304 *Note System Headers::. 3305 3306 3307 3308 File: cpp.info, Node: Other Directives, Next: Preprocessor Output, Prev: Pragmas, Up: Top 3309 3310 8 Other Directives 3311 ****************** 3312 3313 The `#ident' directive takes one argument, a string constant. On some 3314 systems, that string constant is copied into a special segment of the 3315 object file. On other systems, the directive is ignored. The `#sccs' 3316 directive is a synonym for `#ident'. 3317 3318 These directives are not part of the C standard, but they are not 3319 official GNU extensions either. What historical information we have 3320 been able to find, suggests they originated with System V. 3321 3322 The "null directive" consists of a `#' followed by a newline, with 3323 only whitespace (including comments) in between. A null directive is 3324 understood as a preprocessing directive but has no effect on the 3325 preprocessor output. The primary significance of the existence of the 3326 null directive is that an input line consisting of just a `#' will 3327 produce no output, rather than a line of output containing just a `#'. 3328 Supposedly some old C programs contain such lines. 3329 3330 3331 File: cpp.info, Node: Preprocessor Output, Next: Traditional Mode, Prev: Other Directives, Up: Top 3332 3333 9 Preprocessor Output 3334 ********************* 3335 3336 When the C preprocessor is used with the C, C++, or Objective-C 3337 compilers, it is integrated into the compiler and communicates a stream 3338 of binary tokens directly to the compiler's parser. However, it can 3339 also be used in the more conventional standalone mode, where it produces 3340 textual output. 3341 3342 The output from the C preprocessor looks much like the input, except 3343 that all preprocessing directive lines have been replaced with blank 3344 lines and all comments with spaces. Long runs of blank lines are 3345 discarded. 3346 3347 The ISO standard specifies that it is implementation defined whether 3348 a preprocessor preserves whitespace between tokens, or replaces it with 3349 e.g. a single space. In GNU CPP, whitespace between tokens is collapsed 3350 to become a single space, with the exception that the first token on a 3351 non-directive line is preceded with sufficient spaces that it appears in 3352 the same column in the preprocessed output that it appeared in the 3353 original source file. This is so the output is easy to read. *Note 3354 Differences from previous versions::. CPP does not insert any 3355 whitespace where there was none in the original source, except where 3356 necessary to prevent an accidental token paste. 3357 3358 Source file name and line number information is conveyed by lines of 3359 the form 3360 3361 # LINENUM FILENAME FLAGS 3362 3363 These are called "linemarkers". They are inserted as needed into the 3364 output (but never within a string or character constant). They mean 3365 that the following line originated in file FILENAME at line LINENUM. 3366 FILENAME will never contain any non-printing characters; they are 3367 replaced with octal escape sequences. 3368 3369 After the file name comes zero or more flags, which are `1', `2', 3370 `3', or `4'. If there are multiple flags, spaces separate them. Here 3371 is what the flags mean: 3372 3373 `1' 3374 This indicates the start of a new file. 3375 3376 `2' 3377 This indicates returning to a file (after having included another 3378 file). 3379 3380 `3' 3381 This indicates that the following text comes from a system header 3382 file, so certain warnings should be suppressed. 3383 3384 `4' 3385 This indicates that the following text should be treated as being 3386 wrapped in an implicit `extern "C"' block. 3387 3388 As an extension, the preprocessor accepts linemarkers in 3389 non-assembler input files. They are treated like the corresponding 3390 `#line' directive, (*note Line Control::), except that trailing flags 3391 are permitted, and are interpreted with the meanings described above. 3392 If multiple flags are given, they must be in ascending order. 3393 3394 Some directives may be duplicated in the output of the preprocessor. 3395 These are `#ident' (always), `#pragma' (only if the preprocessor does 3396 not handle the pragma itself), and `#define' and `#undef' (with certain 3397 debugging options). If this happens, the `#' of the directive will 3398 always be in the first column, and there will be no space between the 3399 `#' and the directive name. If macro expansion happens to generate 3400 tokens which might be mistaken for a duplicated directive, a space will 3401 be inserted between the `#' and the directive name. 3402 3403 3404 File: cpp.info, Node: Traditional Mode, Next: Implementation Details, Prev: Preprocessor Output, Up: Top 3405 3406 10 Traditional Mode 3407 ******************* 3408 3409 Traditional (pre-standard) C preprocessing is rather different from the 3410 preprocessing specified by the standard. When GCC is given the 3411 `-traditional-cpp' option, it attempts to emulate a traditional 3412 preprocessor. 3413 3414 GCC versions 3.2 and later only support traditional mode semantics in 3415 the preprocessor, and not in the compiler front ends. This chapter 3416 outlines the traditional preprocessor semantics we implemented. 3417 3418 The implementation does not correspond precisely to the behavior of 3419 earlier versions of GCC, nor to any true traditional preprocessor. 3420 After all, inconsistencies among traditional implementations were a 3421 major motivation for C standardization. However, we intend that it 3422 should be compatible with true traditional preprocessors in all ways 3423 that actually matter. 3424 3425 * Menu: 3426 3427 * Traditional lexical analysis:: 3428 * Traditional macros:: 3429 * Traditional miscellany:: 3430 * Traditional warnings:: 3431 3432 3433 File: cpp.info, Node: Traditional lexical analysis, Next: Traditional macros, Up: Traditional Mode 3434 3435 10.1 Traditional lexical analysis 3436 ================================= 3437 3438 The traditional preprocessor does not decompose its input into tokens 3439 the same way a standards-conforming preprocessor does. The input is 3440 simply treated as a stream of text with minimal internal form. 3441 3442 This implementation does not treat trigraphs (*note trigraphs::) 3443 specially since they were an invention of the standards committee. It 3444 handles arbitrarily-positioned escaped newlines properly and splices 3445 the lines as you would expect; many traditional preprocessors did not 3446 do this. 3447 3448 The form of horizontal whitespace in the input file is preserved in 3449 the output. In particular, hard tabs remain hard tabs. This can be 3450 useful if, for example, you are preprocessing a Makefile. 3451 3452 Traditional CPP only recognizes C-style block comments, and treats 3453 the `/*' sequence as introducing a comment only if it lies outside 3454 quoted text. Quoted text is introduced by the usual single and double 3455 quotes, and also by an initial `<' in a `#include' directive. 3456 3457 Traditionally, comments are completely removed and are not replaced 3458 with a space. Since a traditional compiler does its own tokenization 3459 of the output of the preprocessor, this means that comments can 3460 effectively be used as token paste operators. However, comments behave 3461 like separators for text handled by the preprocessor itself, since it 3462 doesn't re-lex its input. For example, in 3463 3464 #if foo/**/bar 3465 3466 `foo' and `bar' are distinct identifiers and expanded separately if 3467 they happen to be macros. In other words, this directive is equivalent 3468 to 3469 3470 #if foo bar 3471 3472 rather than 3473 3474 #if foobar 3475 3476 Generally speaking, in traditional mode an opening quote need not 3477 have a matching closing quote. In particular, a macro may be defined 3478 with replacement text that contains an unmatched quote. Of course, if 3479 you attempt to compile preprocessed output containing an unmatched quote 3480 you will get a syntax error. 3481 3482 However, all preprocessing directives other than `#define' require 3483 matching quotes. For example: 3484 3485 #define m This macro's fine and has an unmatched quote 3486 "/* This is not a comment. */ 3487 /* This is a comment. The following #include directive 3488 is ill-formed. */ 3489 #include <stdio.h 3490 3491 Just as for the ISO preprocessor, what would be a closing quote can 3492 be escaped with a backslash to prevent the quoted text from closing. 3493 3494 3495 File: cpp.info, Node: Traditional macros, Next: Traditional miscellany, Prev: Traditional lexical analysis, Up: Traditional Mode 3496 3497 10.2 Traditional macros 3498 ======================= 3499 3500 The major difference between traditional and ISO macros is that the 3501 former expand to text rather than to a token sequence. CPP removes all 3502 leading and trailing horizontal whitespace from a macro's replacement 3503 text before storing it, but preserves the form of internal whitespace. 3504 3505 One consequence is that it is legitimate for the replacement text to 3506 contain an unmatched quote (*note Traditional lexical analysis::). An 3507 unclosed string or character constant continues into the text following 3508 the macro call. Similarly, the text at the end of a macro's expansion 3509 can run together with the text after the macro invocation to produce a 3510 single token. 3511 3512 Normally comments are removed from the replacement text after the 3513 macro is expanded, but if the `-CC' option is passed on the command 3514 line comments are preserved. (In fact, the current implementation 3515 removes comments even before saving the macro replacement text, but it 3516 careful to do it in such a way that the observed effect is identical 3517 even in the function-like macro case.) 3518 3519 The ISO stringification operator `#' and token paste operator `##' 3520 have no special meaning. As explained later, an effect similar to 3521 these operators can be obtained in a different way. Macro names that 3522 are embedded in quotes, either from the main file or after macro 3523 replacement, do not expand. 3524 3525 CPP replaces an unquoted object-like macro name with its replacement 3526 text, and then rescans it for further macros to replace. Unlike 3527 standard macro expansion, traditional macro expansion has no provision 3528 to prevent recursion. If an object-like macro appears unquoted in its 3529 replacement text, it will be replaced again during the rescan pass, and 3530 so on _ad infinitum_. GCC detects when it is expanding recursive 3531 macros, emits an error message, and continues after the offending macro 3532 invocation. 3533 3534 #define PLUS + 3535 #define INC(x) PLUS+x 3536 INC(foo); 3537 ==> ++foo; 3538 3539 Function-like macros are similar in form but quite different in 3540 behavior to their ISO counterparts. Their arguments are contained 3541 within parentheses, are comma-separated, and can cross physical lines. 3542 Commas within nested parentheses are not treated as argument 3543 separators. Similarly, a quote in an argument cannot be left unclosed; 3544 a following comma or parenthesis that comes before the closing quote is 3545 treated like any other character. There is no facility for handling 3546 variadic macros. 3547 3548 This implementation removes all comments from macro arguments, unless 3549 the `-C' option is given. The form of all other horizontal whitespace 3550 in arguments is preserved, including leading and trailing whitespace. 3551 In particular 3552 3553 f( ) 3554 3555 is treated as an invocation of the macro `f' with a single argument 3556 consisting of a single space. If you want to invoke a function-like 3557 macro that takes no arguments, you must not leave any whitespace 3558 between the parentheses. 3559 3560 If a macro argument crosses a new line, the new line is replaced with 3561 a space when forming the argument. If the previous line contained an 3562 unterminated quote, the following line inherits the quoted state. 3563 3564 Traditional preprocessors replace parameters in the replacement text 3565 with their arguments regardless of whether the parameters are within 3566 quotes or not. This provides a way to stringize arguments. For example 3567 3568 #define str(x) "x" 3569 str(/* A comment */some text ) 3570 ==> "some text " 3571 3572 Note that the comment is removed, but that the trailing space is 3573 preserved. Here is an example of using a comment to effect token 3574 pasting. 3575 3576 #define suffix(x) foo_/**/x 3577 suffix(bar) 3578 ==> foo_bar 3579 3580 3581 File: cpp.info, Node: Traditional miscellany, Next: Traditional warnings, Prev: Traditional macros, Up: Traditional Mode 3582 3583 10.3 Traditional miscellany 3584 =========================== 3585 3586 Here are some things to be aware of when using the traditional 3587 preprocessor. 3588 3589 * Preprocessing directives are recognized only when their leading 3590 `#' appears in the first column. There can be no whitespace 3591 between the beginning of the line and the `#', but whitespace can 3592 follow the `#'. 3593 3594 * A true traditional C preprocessor does not recognize `#error' or 3595 `#pragma', and may not recognize `#elif'. CPP supports all the 3596 directives in traditional mode that it supports in ISO mode, 3597 including extensions, with the exception that the effects of 3598 `#pragma GCC poison' are undefined. 3599 3600 * __STDC__ is not defined. 3601 3602 * If you use digraphs the behavior is undefined. 3603 3604 * If a line that looks like a directive appears within macro 3605 arguments, the behavior is undefined. 3606 3607 3608 3609 File: cpp.info, Node: Traditional warnings, Prev: Traditional miscellany, Up: Traditional Mode 3610 3611 10.4 Traditional warnings 3612 ========================= 3613 3614 You can request warnings about features that did not exist, or worked 3615 differently, in traditional C with the `-Wtraditional' option. GCC 3616 does not warn about features of ISO C which you must use when you are 3617 using a conforming compiler, such as the `#' and `##' operators. 3618 3619 Presently `-Wtraditional' warns about: 3620 3621 * Macro parameters that appear within string literals in the macro 3622 body. In traditional C macro replacement takes place within 3623 string literals, but does not in ISO C. 3624 3625 * In traditional C, some preprocessor directives did not exist. 3626 Traditional preprocessors would only consider a line to be a 3627 directive if the `#' appeared in column 1 on the line. Therefore 3628 `-Wtraditional' warns about directives that traditional C 3629 understands but would ignore because the `#' does not appear as the 3630 first character on the line. It also suggests you hide directives 3631 like `#pragma' not understood by traditional C by indenting them. 3632 Some traditional implementations would not recognize `#elif', so it 3633 suggests avoiding it altogether. 3634 3635 * A function-like macro that appears without an argument list. In 3636 some traditional preprocessors this was an error. In ISO C it 3637 merely means that the macro is not expanded. 3638 3639 * The unary plus operator. This did not exist in traditional C. 3640 3641 * The `U' and `LL' integer constant suffixes, which were not 3642 available in traditional C. (Traditional C does support the `L' 3643 suffix for simple long integer constants.) You are not warned 3644 about uses of these suffixes in macros defined in system headers. 3645 For instance, `UINT_MAX' may well be defined as `4294967295U', but 3646 you will not be warned if you use `UINT_MAX'. 3647 3648 You can usually avoid the warning, and the related warning about 3649 constants which are so large that they are unsigned, by writing the 3650 integer constant in question in hexadecimal, with no U suffix. 3651 Take care, though, because this gives the wrong result in exotic 3652 cases. 3653 3654 3655 File: cpp.info, Node: Implementation Details, Next: Invocation, Prev: Traditional Mode, Up: Top 3656 3657 11 Implementation Details 3658 ************************* 3659 3660 Here we document details of how the preprocessor's implementation 3661 affects its user-visible behavior. You should try to avoid undue 3662 reliance on behavior described here, as it is possible that it will 3663 change subtly in future implementations. 3664 3665 Also documented here are obsolete features and changes from previous 3666 versions of CPP. 3667 3668 * Menu: 3669 3670 * Implementation-defined behavior:: 3671 * Implementation limits:: 3672 * Obsolete Features:: 3673 * Differences from previous versions:: 3674 3675 3676 File: cpp.info, Node: Implementation-defined behavior, Next: Implementation limits, Up: Implementation Details 3677 3678 11.1 Implementation-defined behavior 3679 ==================================== 3680 3681 This is how CPP behaves in all the cases which the C standard describes 3682 as "implementation-defined". This term means that the implementation 3683 is free to do what it likes, but must document its choice and stick to 3684 it. 3685 3686 * The mapping of physical source file multi-byte characters to the 3687 execution character set. 3688 3689 The input character set can be specified using the 3690 `-finput-charset' option, while the execution character set may be 3691 controlled using the `-fexec-charset' and `-fwide-exec-charset' 3692 options. 3693 3694 * Identifier characters. The C and C++ standards allow identifiers 3695 to be composed of `_' and the alphanumeric characters. C++ and 3696 C99 also allow universal character names, and C99 further permits 3697 implementation-defined characters. GCC currently only permits 3698 universal character names if `-fextended-identifiers' is used, 3699 because the implementation of universal character names in 3700 identifiers is experimental. 3701 3702 GCC allows the `$' character in identifiers as an extension for 3703 most targets. This is true regardless of the `std=' switch, since 3704 this extension cannot conflict with standards-conforming programs. 3705 When preprocessing assembler, however, dollars are not identifier 3706 characters by default. 3707 3708 Currently the targets that by default do not permit `$' are AVR, 3709 IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 3710 operating system. 3711 3712 You can override the default with `-fdollars-in-identifiers' or 3713 `fno-dollars-in-identifiers'. *Note fdollars-in-identifiers::. 3714 3715 * Non-empty sequences of whitespace characters. 3716 3717 In textual output, each whitespace sequence is collapsed to a 3718 single space. For aesthetic reasons, the first token on each 3719 non-directive line of output is preceded with sufficient spaces 3720 that it appears in the same column as it did in the original 3721 source file. 3722 3723 * The numeric value of character constants in preprocessor 3724 expressions. 3725 3726 The preprocessor and compiler interpret character constants in the 3727 same way; i.e. escape sequences such as `\a' are given the values 3728 they would have on the target machine. 3729 3730 The compiler evaluates a multi-character character constant a 3731 character at a time, shifting the previous value left by the 3732 number of bits per target character, and then or-ing in the 3733 bit-pattern of the new character truncated to the width of a 3734 target character. The final bit-pattern is given type `int', and 3735 is therefore signed, regardless of whether single characters are 3736 signed or not (a slight change from versions 3.1 and earlier of 3737 GCC). If there are more characters in the constant than would fit 3738 in the target `int' the compiler issues a warning, and the excess 3739 leading characters are ignored. 3740 3741 For example, `'ab'' for a target with an 8-bit `char' would be 3742 interpreted as 3743 `(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and 3744 `'\234a'' as 3745 `(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'. 3746 3747 * Source file inclusion. 3748 3749 For a discussion on how the preprocessor locates header files, 3750 *note Include Operation::. 3751 3752 * Interpretation of the filename resulting from a macro-expanded 3753 `#include' directive. 3754 3755 *Note Computed Includes::. 3756 3757 * Treatment of a `#pragma' directive that after macro-expansion 3758 results in a standard pragma. 3759 3760 No macro expansion occurs on any `#pragma' directive line, so the 3761 question does not arise. 3762 3763 Note that GCC does not yet implement any of the standard pragmas. 3764 3765 3766 3767 File: cpp.info, Node: Implementation limits, Next: Obsolete Features, Prev: Implementation-defined behavior, Up: Implementation Details 3768 3769 11.2 Implementation limits 3770 ========================== 3771 3772 CPP has a small number of internal limits. This section lists the 3773 limits which the C standard requires to be no lower than some minimum, 3774 and all the others known. It is intended that there should be as few 3775 limits as possible. If you encounter an undocumented or inconvenient 3776 limit, please report that as a bug. *Note Reporting Bugs: (gcc)Bugs. 3777 3778 Where we say something is limited "only by available memory", that 3779 means that internal data structures impose no intrinsic limit, and space 3780 is allocated with `malloc' or equivalent. The actual limit will 3781 therefore depend on many things, such as the size of other things 3782 allocated by the compiler at the same time, the amount of memory 3783 consumed by other processes on the same computer, etc. 3784 3785 * Nesting levels of `#include' files. 3786 3787 We impose an arbitrary limit of 200 levels, to avoid runaway 3788 recursion. The standard requires at least 15 levels. 3789 3790 * Nesting levels of conditional inclusion. 3791 3792 The C standard mandates this be at least 63. CPP is limited only 3793 by available memory. 3794 3795 * Levels of parenthesized expressions within a full expression. 3796 3797 The C standard requires this to be at least 63. In preprocessor 3798 conditional expressions, it is limited only by available memory. 3799 3800 * Significant initial characters in an identifier or macro name. 3801 3802 The preprocessor treats all characters as significant. The C 3803 standard requires only that the first 63 be significant. 3804 3805 * Number of macros simultaneously defined in a single translation 3806 unit. 3807 3808 The standard requires at least 4095 be possible. CPP is limited 3809 only by available memory. 3810 3811 * Number of parameters in a macro definition and arguments in a 3812 macro call. 3813 3814 We allow `USHRT_MAX', which is no smaller than 65,535. The minimum 3815 required by the standard is 127. 3816 3817 * Number of characters on a logical source line. 3818 3819 The C standard requires a minimum of 4096 be permitted. CPP places 3820 no limits on this, but you may get incorrect column numbers 3821 reported in diagnostics for lines longer than 65,535 characters. 3822 3823 * Maximum size of a source file. 3824 3825 The standard does not specify any lower limit on the maximum size 3826 of a source file. GNU cpp maps files into memory, so it is 3827 limited by the available address space. This is generally at 3828 least two gigabytes. Depending on the operating system, the size 3829 of physical memory may or may not be a limitation. 3830 3831 3832 3833 File: cpp.info, Node: Obsolete Features, Next: Differences from previous versions, Prev: Implementation limits, Up: Implementation Details 3834 3835 11.3 Obsolete Features 3836 ====================== 3837 3838 CPP has some features which are present mainly for compatibility with 3839 older programs. We discourage their use in new code. In some cases, 3840 we plan to remove the feature in a future version of GCC. 3841 3842 11.3.1 Assertions 3843 ----------------- 3844 3845 "Assertions" are a deprecated alternative to macros in writing 3846 conditionals to test what sort of computer or system the compiled 3847 program will run on. Assertions are usually predefined, but you can 3848 define them with preprocessing directives or command-line options. 3849 3850 Assertions were intended to provide a more systematic way to describe 3851 the compiler's target system and we added them for compatibility with 3852 existing compilers. In practice they are just as unpredictable as the 3853 system-specific predefined macros. In addition, they are not part of 3854 any standard, and only a few compilers support them. Therefore, the 3855 use of assertions is *less* portable than the use of system-specific 3856 predefined macros. We recommend you do not use them at all. 3857 3858 An assertion looks like this: 3859 3860 #PREDICATE (ANSWER) 3861 3862 PREDICATE must be a single identifier. ANSWER can be any sequence of 3863 tokens; all characters are significant except for leading and trailing 3864 whitespace, and differences in internal whitespace sequences are 3865 ignored. (This is similar to the rules governing macro redefinition.) 3866 Thus, `(x + y)' is different from `(x+y)' but equivalent to 3867 `( x + y )'. Parentheses do not nest inside an answer. 3868 3869 To test an assertion, you write it in an `#if'. For example, this 3870 conditional succeeds if either `vax' or `ns16000' has been asserted as 3871 an answer for `machine'. 3872 3873 #if #machine (vax) || #machine (ns16000) 3874 3875 You can test whether _any_ answer is asserted for a predicate by 3876 omitting the answer in the conditional: 3877 3878 #if #machine 3879 3880 Assertions are made with the `#assert' directive. Its sole argument 3881 is the assertion to make, without the leading `#' that identifies 3882 assertions in conditionals. 3883 3884 #assert PREDICATE (ANSWER) 3885 3886 You may make several assertions with the same predicate and different 3887 answers. Subsequent assertions do not override previous ones for the 3888 same predicate. All the answers for any given predicate are 3889 simultaneously true. 3890 3891 Assertions can be canceled with the `#unassert' directive. It has 3892 the same syntax as `#assert'. In that form it cancels only the answer 3893 which was specified on the `#unassert' line; other answers for that 3894 predicate remain true. You can cancel an entire predicate by leaving 3895 out the answer: 3896 3897 #unassert PREDICATE 3898 3899 In either form, if no such assertion has been made, `#unassert' has no 3900 effect. 3901 3902 You can also make or cancel assertions using command line options. 3903 *Note Invocation::. 3904 3905 3906 File: cpp.info, Node: Differences from previous versions, Prev: Obsolete Features, Up: Implementation Details 3907 3908 11.4 Differences from previous versions 3909 ======================================= 3910 3911 This section details behavior which has changed from previous versions 3912 of CPP. We do not plan to change it again in the near future, but we 3913 do not promise not to, either. 3914 3915 The "previous versions" discussed here are 2.95 and before. The 3916 behavior of GCC 3.0 is mostly the same as the behavior of the widely 3917 used 2.96 and 2.97 development snapshots. Where there are differences, 3918 they generally represent bugs in the snapshots. 3919 3920 * -I- deprecated 3921 3922 This option has been deprecated in 4.0. `-iquote' is meant to 3923 replace the need for this option. 3924 3925 * Order of evaluation of `#' and `##' operators 3926 3927 The standard does not specify the order of evaluation of a chain of 3928 `##' operators, nor whether `#' is evaluated before, after, or at 3929 the same time as `##'. You should therefore not write any code 3930 which depends on any specific ordering. It is possible to 3931 guarantee an ordering, if you need one, by suitable use of nested 3932 macros. 3933 3934 An example of where this might matter is pasting the arguments `1', 3935 `e' and `-2'. This would be fine for left-to-right pasting, but 3936 right-to-left pasting would produce an invalid token `e-2'. 3937 3938 GCC 3.0 evaluates `#' and `##' at the same time and strictly left 3939 to right. Older versions evaluated all `#' operators first, then 3940 all `##' operators, in an unreliable order. 3941 3942 * The form of whitespace between tokens in preprocessor output 3943 3944 *Note Preprocessor Output::, for the current textual format. This 3945 is also the format used by stringification. Normally, the 3946 preprocessor communicates tokens directly to the compiler's 3947 parser, and whitespace does not come up at all. 3948 3949 Older versions of GCC preserved all whitespace provided by the 3950 user and inserted lots more whitespace of their own, because they 3951 could not accurately predict when extra spaces were needed to 3952 prevent accidental token pasting. 3953 3954 * Optional argument when invoking rest argument macros 3955 3956 As an extension, GCC permits you to omit the variable arguments 3957 entirely when you use a variable argument macro. This is 3958 forbidden by the 1999 C standard, and will provoke a pedantic 3959 warning with GCC 3.0. Previous versions accepted it silently. 3960 3961 * `##' swallowing preceding text in rest argument macros 3962 3963 Formerly, in a macro expansion, if `##' appeared before a variable 3964 arguments parameter, and the set of tokens specified for that 3965 argument in the macro invocation was empty, previous versions of 3966 CPP would back up and remove the preceding sequence of 3967 non-whitespace characters (*not* the preceding token). This 3968 extension is in direct conflict with the 1999 C standard and has 3969 been drastically pared back. 3970 3971 In the current version of the preprocessor, if `##' appears between 3972 a comma and a variable arguments parameter, and the variable 3973 argument is omitted entirely, the comma will be removed from the 3974 expansion. If the variable argument is empty, or the token before 3975 `##' is not a comma, then `##' behaves as a normal token paste. 3976 3977 * `#line' and `#include' 3978 3979 The `#line' directive used to change GCC's notion of the 3980 "directory containing the current file", used by `#include' with a 3981 double-quoted header file name. In 3.0 and later, it does not. 3982 *Note Line Control::, for further explanation. 3983 3984 * Syntax of `#line' 3985 3986 In GCC 2.95 and previous, the string constant argument to `#line' 3987 was treated the same way as the argument to `#include': backslash 3988 escapes were not honored, and the string ended at the second `"'. 3989 This is not compliant with the C standard. In GCC 3.0, an attempt 3990 was made to correct the behavior, so that the string was treated 3991 as a real string constant, but it turned out to be buggy. In 3.1, 3992 the bugs have been fixed. (We are not fixing the bugs in 3.0 3993 because they affect relatively few people and the fix is quite 3994 invasive.) 3995 3996 3997 3998 File: cpp.info, Node: Invocation, Next: Environment Variables, Prev: Implementation Details, Up: Top 3999 4000 12 Invocation 4001 ************* 4002 4003 Most often when you use the C preprocessor you will not have to invoke 4004 it explicitly: the C compiler will do so automatically. However, the 4005 preprocessor is sometimes useful on its own. All the options listed 4006 here are also acceptable to the C compiler and have the same meaning, 4007 except that the C compiler has different rules for specifying the output 4008 file. 4009 4010 _Note:_ Whether you use the preprocessor by way of `gcc' or `cpp', 4011 the "compiler driver" is run first. This program's purpose is to 4012 translate your command into invocations of the programs that do the 4013 actual work. Their command line interfaces are similar but not 4014 identical to the documented interface, and may change without notice. 4015 4016 The C preprocessor expects two file names as arguments, INFILE and 4017 OUTFILE. The preprocessor reads INFILE together with any other files 4018 it specifies with `#include'. All the output generated by the combined 4019 input files is written in OUTFILE. 4020 4021 Either INFILE or OUTFILE may be `-', which as INFILE means to read 4022 from standard input and as OUTFILE means to write to standard output. 4023 Also, if either file is omitted, it means the same as if `-' had been 4024 specified for that file. 4025 4026 Unless otherwise noted, or the option ends in `=', all options which 4027 take an argument may have that argument appear either immediately after 4028 the option, or with a space between option and argument: `-Ifoo' and 4029 `-I foo' have the same effect. 4030 4031 Many options have multi-letter names; therefore multiple 4032 single-letter options may _not_ be grouped: `-dM' is very different from 4033 `-d -M'. 4034 4035 `-D NAME' 4036 Predefine NAME as a macro, with definition `1'. 4037 4038 `-D NAME=DEFINITION' 4039 The contents of DEFINITION are tokenized and processed as if they 4040 appeared during translation phase three in a `#define' directive. 4041 In particular, the definition will be truncated by embedded 4042 newline characters. 4043 4044 If you are invoking the preprocessor from a shell or shell-like 4045 program you may need to use the shell's quoting syntax to protect 4046 characters such as spaces that have a meaning in the shell syntax. 4047 4048 If you wish to define a function-like macro on the command line, 4049 write its argument list with surrounding parentheses before the 4050 equals sign (if any). Parentheses are meaningful to most shells, 4051 so you will need to quote the option. With `sh' and `csh', 4052 `-D'NAME(ARGS...)=DEFINITION'' works. 4053 4054 `-D' and `-U' options are processed in the order they are given on 4055 the command line. All `-imacros FILE' and `-include FILE' options 4056 are processed after all `-D' and `-U' options. 4057 4058 `-U NAME' 4059 Cancel any previous definition of NAME, either built in or 4060 provided with a `-D' option. 4061 4062 `-undef' 4063 Do not predefine any system-specific or GCC-specific macros. The 4064 standard predefined macros remain defined. *Note Standard 4065 Predefined Macros::. 4066 4067 `-I DIR' 4068 Add the directory DIR to the list of directories to be searched 4069 for header files. *Note Search Path::. Directories named by `-I' 4070 are searched before the standard system include directories. If 4071 the directory DIR is a standard system include directory, the 4072 option is ignored to ensure that the default search order for 4073 system directories and the special treatment of system headers are 4074 not defeated (*note System Headers::) . If DIR begins with `=', 4075 then the `=' will be replaced by the sysroot prefix; see 4076 `--sysroot' and `-isysroot'. 4077 4078 `-o FILE' 4079 Write output to FILE. This is the same as specifying FILE as the 4080 second non-option argument to `cpp'. `gcc' has a different 4081 interpretation of a second non-option argument, so you must use 4082 `-o' to specify the output file. 4083 4084 `-Wall' 4085 Turns on all optional warnings which are desirable for normal code. 4086 At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a 4087 warning about integer promotion causing a change of sign in `#if' 4088 expressions. Note that many of the preprocessor's warnings are on 4089 by default and have no options to control them. 4090 4091 `-Wcomment' 4092 `-Wcomments' 4093 Warn whenever a comment-start sequence `/*' appears in a `/*' 4094 comment, or whenever a backslash-newline appears in a `//' comment. 4095 (Both forms have the same effect.) 4096 4097 `-Wtrigraphs' 4098 Most trigraphs in comments cannot affect the meaning of the 4099 program. However, a trigraph that would form an escaped newline 4100 (`??/' at the end of a line) can, by changing where the comment 4101 begins or ends. Therefore, only trigraphs that would form escaped 4102 newlines produce warnings inside a comment. 4103 4104 This option is implied by `-Wall'. If `-Wall' is not given, this 4105 option is still enabled unless trigraphs are enabled. To get 4106 trigraph conversion without warnings, but get the other `-Wall' 4107 warnings, use `-trigraphs -Wall -Wno-trigraphs'. 4108 4109 `-Wtraditional' 4110 Warn about certain constructs that behave differently in 4111 traditional and ISO C. Also warn about ISO C constructs that have 4112 no traditional C equivalent, and problematic constructs which 4113 should be avoided. *Note Traditional Mode::. 4114 4115 `-Wundef' 4116 Warn whenever an identifier which is not a macro is encountered in 4117 an `#if' directive, outside of `defined'. Such identifiers are 4118 replaced with zero. 4119 4120 `-Wunused-macros' 4121 Warn about macros defined in the main file that are unused. A 4122 macro is "used" if it is expanded or tested for existence at least 4123 once. The preprocessor will also warn if the macro has not been 4124 used at the time it is redefined or undefined. 4125 4126 Built-in macros, macros defined on the command line, and macros 4127 defined in include files are not warned about. 4128 4129 _Note:_ If a macro is actually used, but only used in skipped 4130 conditional blocks, then CPP will report it as unused. To avoid 4131 the warning in such a case, you might improve the scope of the 4132 macro's definition by, for example, moving it into the first 4133 skipped block. Alternatively, you could provide a dummy use with 4134 something like: 4135 4136 #if defined the_macro_causing_the_warning 4137 #endif 4138 4139 `-Wendif-labels' 4140 Warn whenever an `#else' or an `#endif' are followed by text. 4141 This usually happens in code of the form 4142 4143 #if FOO 4144 ... 4145 #else FOO 4146 ... 4147 #endif FOO 4148 4149 The second and third `FOO' should be in comments, but often are not 4150 in older programs. This warning is on by default. 4151 4152 `-Werror' 4153 Make all warnings into hard errors. Source code which triggers 4154 warnings will be rejected. 4155 4156 `-Wsystem-headers' 4157 Issue warnings for code in system headers. These are normally 4158 unhelpful in finding bugs in your own code, therefore suppressed. 4159 If you are responsible for the system library, you may want to see 4160 them. 4161 4162 `-w' 4163 Suppress all warnings, including those which GNU CPP issues by 4164 default. 4165 4166 `-pedantic' 4167 Issue all the mandatory diagnostics listed in the C standard. 4168 Some of them are left out by default, since they trigger 4169 frequently on harmless code. 4170 4171 `-pedantic-errors' 4172 Issue all the mandatory diagnostics, and make all mandatory 4173 diagnostics into errors. This includes mandatory diagnostics that 4174 GCC issues without `-pedantic' but treats as warnings. 4175 4176 `-M' 4177 Instead of outputting the result of preprocessing, output a rule 4178 suitable for `make' describing the dependencies of the main source 4179 file. The preprocessor outputs one `make' rule containing the 4180 object file name for that source file, a colon, and the names of 4181 all the included files, including those coming from `-include' or 4182 `-imacros' command line options. 4183 4184 Unless specified explicitly (with `-MT' or `-MQ'), the object file 4185 name consists of the name of the source file with any suffix 4186 replaced with object file suffix and with any leading directory 4187 parts removed. If there are many included files then the rule is 4188 split into several lines using `\'-newline. The rule has no 4189 commands. 4190 4191 This option does not suppress the preprocessor's debug output, 4192 such as `-dM'. To avoid mixing such debug output with the 4193 dependency rules you should explicitly specify the dependency 4194 output file with `-MF', or use an environment variable like 4195 `DEPENDENCIES_OUTPUT' (*note Environment Variables::). Debug 4196 output will still be sent to the regular output stream as normal. 4197 4198 Passing `-M' to the driver implies `-E', and suppresses warnings 4199 with an implicit `-w'. 4200 4201 `-MM' 4202 Like `-M' but do not mention header files that are found in system 4203 header directories, nor header files that are included, directly 4204 or indirectly, from such a header. 4205 4206 This implies that the choice of angle brackets or double quotes in 4207 an `#include' directive does not in itself determine whether that 4208 header will appear in `-MM' dependency output. This is a slight 4209 change in semantics from GCC versions 3.0 and earlier. 4210 4211 `-MF FILE' 4212 When used with `-M' or `-MM', specifies a file to write the 4213 dependencies to. If no `-MF' switch is given the preprocessor 4214 sends the rules to the same place it would have sent preprocessed 4215 output. 4216 4217 When used with the driver options `-MD' or `-MMD', `-MF' overrides 4218 the default dependency output file. 4219 4220 `-MG' 4221 In conjunction with an option such as `-M' requesting dependency 4222 generation, `-MG' assumes missing header files are generated files 4223 and adds them to the dependency list without raising an error. 4224 The dependency filename is taken directly from the `#include' 4225 directive without prepending any path. `-MG' also suppresses 4226 preprocessed output, as a missing header file renders this useless. 4227 4228 This feature is used in automatic updating of makefiles. 4229 4230 `-MP' 4231 This option instructs CPP to add a phony target for each dependency 4232 other than the main file, causing each to depend on nothing. These 4233 dummy rules work around errors `make' gives if you remove header 4234 files without updating the `Makefile' to match. 4235 4236 This is typical output: 4237 4238 test.o: test.c test.h 4239 4240 test.h: 4241 4242 `-MT TARGET' 4243 Change the target of the rule emitted by dependency generation. By 4244 default CPP takes the name of the main input file, deletes any 4245 directory components and any file suffix such as `.c', and appends 4246 the platform's usual object suffix. The result is the target. 4247 4248 An `-MT' option will set the target to be exactly the string you 4249 specify. If you want multiple targets, you can specify them as a 4250 single argument to `-MT', or use multiple `-MT' options. 4251 4252 For example, `-MT '$(objpfx)foo.o'' might give 4253 4254 $(objpfx)foo.o: foo.c 4255 4256 `-MQ TARGET' 4257 Same as `-MT', but it quotes any characters which are special to 4258 Make. `-MQ '$(objpfx)foo.o'' gives 4259 4260 $$(objpfx)foo.o: foo.c 4261 4262 The default target is automatically quoted, as if it were given 4263 with `-MQ'. 4264 4265 `-MD' 4266 `-MD' is equivalent to `-M -MF FILE', except that `-E' is not 4267 implied. The driver determines FILE based on whether an `-o' 4268 option is given. If it is, the driver uses its argument but with 4269 a suffix of `.d', otherwise it takes the name of the input file, 4270 removes any directory components and suffix, and applies a `.d' 4271 suffix. 4272 4273 If `-MD' is used in conjunction with `-E', any `-o' switch is 4274 understood to specify the dependency output file (*note -MF: 4275 dashMF.), but if used without `-E', each `-o' is understood to 4276 specify a target object file. 4277 4278 Since `-E' is not implied, `-MD' can be used to generate a 4279 dependency output file as a side-effect of the compilation process. 4280 4281 `-MMD' 4282 Like `-MD' except mention only user header files, not system 4283 header files. 4284 4285 `-x c' 4286 `-x c++' 4287 `-x objective-c' 4288 `-x assembler-with-cpp' 4289 Specify the source language: C, C++, Objective-C, or assembly. 4290 This has nothing to do with standards conformance or extensions; 4291 it merely selects which base syntax to expect. If you give none 4292 of these options, cpp will deduce the language from the extension 4293 of the source file: `.c', `.cc', `.m', or `.S'. Some other common 4294 extensions for C++ and assembly are also recognized. If cpp does 4295 not recognize the extension, it will treat the file as C; this is 4296 the most generic mode. 4297 4298 _Note:_ Previous versions of cpp accepted a `-lang' option which 4299 selected both the language and the standards conformance level. 4300 This option has been removed, because it conflicts with the `-l' 4301 option. 4302 4303 `-std=STANDARD' 4304 `-ansi' 4305 Specify the standard to which the code should conform. Currently 4306 CPP knows about C and C++ standards; others may be added in the 4307 future. 4308 4309 STANDARD may be one of: 4310 `c90' 4311 `c89' 4312 `iso9899:1990' 4313 The ISO C standard from 1990. `c90' is the customary 4314 shorthand for this version of the standard. 4315 4316 The `-ansi' option is equivalent to `-std=c90'. 4317 4318 `iso9899:199409' 4319 The 1990 C standard, as amended in 1994. 4320 4321 `iso9899:1999' 4322 `c99' 4323 `iso9899:199x' 4324 `c9x' 4325 The revised ISO C standard, published in December 1999. 4326 Before publication, this was known as C9X. 4327 4328 `c1x' 4329 The next version of the ISO C standard, still under 4330 development. 4331 4332 `gnu90' 4333 `gnu89' 4334 The 1990 C standard plus GNU extensions. This is the default. 4335 4336 `gnu99' 4337 `gnu9x' 4338 The 1999 C standard plus GNU extensions. 4339 4340 `gnu1x' 4341 The next version of the ISO C standard, still under 4342 development, plus GNU extensions. 4343 4344 `c++98' 4345 The 1998 ISO C++ standard plus amendments. 4346 4347 `gnu++98' 4348 The same as `-std=c++98' plus GNU extensions. This is the 4349 default for C++ code. 4350 4351 `-I-' 4352 Split the include path. Any directories specified with `-I' 4353 options before `-I-' are searched only for headers requested with 4354 `#include "FILE"'; they are not searched for `#include <FILE>'. 4355 If additional directories are specified with `-I' options after 4356 the `-I-', those directories are searched for all `#include' 4357 directives. 4358 4359 In addition, `-I-' inhibits the use of the directory of the current 4360 file directory as the first search directory for `#include "FILE"'. 4361 *Note Search Path::. This option has been deprecated. 4362 4363 `-nostdinc' 4364 Do not search the standard system directories for header files. 4365 Only the directories you have specified with `-I' options (and the 4366 directory of the current file, if appropriate) are searched. 4367 4368 `-nostdinc++' 4369 Do not search for header files in the C++-specific standard 4370 directories, but do still search the other standard directories. 4371 (This option is used when building the C++ library.) 4372 4373 `-include FILE' 4374 Process FILE as if `#include "file"' appeared as the first line of 4375 the primary source file. However, the first directory searched 4376 for FILE is the preprocessor's working directory _instead of_ the 4377 directory containing the main source file. If not found there, it 4378 is searched for in the remainder of the `#include "..."' search 4379 chain as normal. 4380 4381 If multiple `-include' options are given, the files are included 4382 in the order they appear on the command line. 4383 4384 `-imacros FILE' 4385 Exactly like `-include', except that any output produced by 4386 scanning FILE is thrown away. Macros it defines remain defined. 4387 This allows you to acquire all the macros from a header without 4388 also processing its declarations. 4389 4390 All files specified by `-imacros' are processed before all files 4391 specified by `-include'. 4392 4393 `-idirafter DIR' 4394 Search DIR for header files, but do it _after_ all directories 4395 specified with `-I' and the standard system directories have been 4396 exhausted. DIR is treated as a system include directory. If DIR 4397 begins with `=', then the `=' will be replaced by the sysroot 4398 prefix; see `--sysroot' and `-isysroot'. 4399 4400 `-iprefix PREFIX' 4401 Specify PREFIX as the prefix for subsequent `-iwithprefix' 4402 options. If the prefix represents a directory, you should include 4403 the final `/'. 4404 4405 `-iwithprefix DIR' 4406 `-iwithprefixbefore DIR' 4407 Append DIR to the prefix specified previously with `-iprefix', and 4408 add the resulting directory to the include search path. 4409 `-iwithprefixbefore' puts it in the same place `-I' would; 4410 `-iwithprefix' puts it where `-idirafter' would. 4411 4412 `-isysroot DIR' 4413 This option is like the `--sysroot' option, but applies only to 4414 header files (except for Darwin targets, where it applies to both 4415 header files and libraries). See the `--sysroot' option for more 4416 information. 4417 4418 `-imultilib DIR' 4419 Use DIR as a subdirectory of the directory containing 4420 target-specific C++ headers. 4421 4422 `-isystem DIR' 4423 Search DIR for header files, after all directories specified by 4424 `-I' but before the standard system directories. Mark it as a 4425 system directory, so that it gets the same special treatment as is 4426 applied to the standard system directories. *Note System 4427 Headers::. If DIR begins with `=', then the `=' will be replaced 4428 by the sysroot prefix; see `--sysroot' and `-isysroot'. 4429 4430 `-iquote DIR' 4431 Search DIR only for header files requested with `#include "FILE"'; 4432 they are not searched for `#include <FILE>', before all 4433 directories specified by `-I' and before the standard system 4434 directories. *Note Search Path::. If DIR begins with `=', then 4435 the `=' will be replaced by the sysroot prefix; see `--sysroot' 4436 and `-isysroot'. 4437 4438 `-fdirectives-only' 4439 When preprocessing, handle directives, but do not expand macros. 4440 4441 The option's behavior depends on the `-E' and `-fpreprocessed' 4442 options. 4443 4444 With `-E', preprocessing is limited to the handling of directives 4445 such as `#define', `#ifdef', and `#error'. Other preprocessor 4446 operations, such as macro expansion and trigraph conversion are 4447 not performed. In addition, the `-dD' option is implicitly 4448 enabled. 4449 4450 With `-fpreprocessed', predefinition of command line and most 4451 builtin macros is disabled. Macros such as `__LINE__', which are 4452 contextually dependent, are handled normally. This enables 4453 compilation of files previously preprocessed with `-E 4454 -fdirectives-only'. 4455 4456 With both `-E' and `-fpreprocessed', the rules for 4457 `-fpreprocessed' take precedence. This enables full preprocessing 4458 of files previously preprocessed with `-E -fdirectives-only'. 4459 4460 `-fdollars-in-identifiers' 4461 Accept `$' in identifiers. *Note Identifier characters::. 4462 4463 `-fextended-identifiers' 4464 Accept universal character names in identifiers. This option is 4465 experimental; in a future version of GCC, it will be enabled by 4466 default for C99 and C++. 4467 4468 `-fpreprocessed' 4469 Indicate to the preprocessor that the input file has already been 4470 preprocessed. This suppresses things like macro expansion, 4471 trigraph conversion, escaped newline splicing, and processing of 4472 most directives. The preprocessor still recognizes and removes 4473 comments, so that you can pass a file preprocessed with `-C' to 4474 the compiler without problems. In this mode the integrated 4475 preprocessor is little more than a tokenizer for the front ends. 4476 4477 `-fpreprocessed' is implicit if the input file has one of the 4478 extensions `.i', `.ii' or `.mi'. These are the extensions that 4479 GCC uses for preprocessed files created by `-save-temps'. 4480 4481 `-ftabstop=WIDTH' 4482 Set the distance between tab stops. This helps the preprocessor 4483 report correct column numbers in warnings or errors, even if tabs 4484 appear on the line. If the value is less than 1 or greater than 4485 100, the option is ignored. The default is 8. 4486 4487 `-fexec-charset=CHARSET' 4488 Set the execution character set, used for string and character 4489 constants. The default is UTF-8. CHARSET can be any encoding 4490 supported by the system's `iconv' library routine. 4491 4492 `-fwide-exec-charset=CHARSET' 4493 Set the wide execution character set, used for wide string and 4494 character constants. The default is UTF-32 or UTF-16, whichever 4495 corresponds to the width of `wchar_t'. As with `-fexec-charset', 4496 CHARSET can be any encoding supported by the system's `iconv' 4497 library routine; however, you will have problems with encodings 4498 that do not fit exactly in `wchar_t'. 4499 4500 `-finput-charset=CHARSET' 4501 Set the input character set, used for translation from the 4502 character set of the input file to the source character set used 4503 by GCC. If the locale does not specify, or GCC cannot get this 4504 information from the locale, the default is UTF-8. This can be 4505 overridden by either the locale or this command line option. 4506 Currently the command line option takes precedence if there's a 4507 conflict. CHARSET can be any encoding supported by the system's 4508 `iconv' library routine. 4509 4510 `-fworking-directory' 4511 Enable generation of linemarkers in the preprocessor output that 4512 will let the compiler know the current working directory at the 4513 time of preprocessing. When this option is enabled, the 4514 preprocessor will emit, after the initial linemarker, a second 4515 linemarker with the current working directory followed by two 4516 slashes. GCC will use this directory, when it's present in the 4517 preprocessed input, as the directory emitted as the current 4518 working directory in some debugging information formats. This 4519 option is implicitly enabled if debugging information is enabled, 4520 but this can be inhibited with the negated form 4521 `-fno-working-directory'. If the `-P' flag is present in the 4522 command line, this option has no effect, since no `#line' 4523 directives are emitted whatsoever. 4524 4525 `-fno-show-column' 4526 Do not print column numbers in diagnostics. This may be necessary 4527 if diagnostics are being scanned by a program that does not 4528 understand the column numbers, such as `dejagnu'. 4529 4530 `-A PREDICATE=ANSWER' 4531 Make an assertion with the predicate PREDICATE and answer ANSWER. 4532 This form is preferred to the older form `-A PREDICATE(ANSWER)', 4533 which is still supported, because it does not use shell special 4534 characters. *Note Obsolete Features::. 4535 4536 `-A -PREDICATE=ANSWER' 4537 Cancel an assertion with the predicate PREDICATE and answer ANSWER. 4538 4539 `-dCHARS' 4540 CHARS is a sequence of one or more of the following characters, 4541 and must not be preceded by a space. Other characters are 4542 interpreted by the compiler proper, or reserved for future 4543 versions of GCC, and so are silently ignored. If you specify 4544 characters whose behavior conflicts, the result is undefined. 4545 4546 `M' 4547 Instead of the normal output, generate a list of `#define' 4548 directives for all the macros defined during the execution of 4549 the preprocessor, including predefined macros. This gives 4550 you a way of finding out what is predefined in your version 4551 of the preprocessor. Assuming you have no file `foo.h', the 4552 command 4553 4554 touch foo.h; cpp -dM foo.h 4555 4556 will show all the predefined macros. 4557 4558 If you use `-dM' without the `-E' option, `-dM' is 4559 interpreted as a synonym for `-fdump-rtl-mach'. *Note 4560 Debugging Options: (gcc)Debugging Options. 4561 4562 `D' 4563 Like `M' except in two respects: it does _not_ include the 4564 predefined macros, and it outputs _both_ the `#define' 4565 directives and the result of preprocessing. Both kinds of 4566 output go to the standard output file. 4567 4568 `N' 4569 Like `D', but emit only the macro names, not their expansions. 4570 4571 `I' 4572 Output `#include' directives in addition to the result of 4573 preprocessing. 4574 4575 `U' 4576 Like `D' except that only macros that are expanded, or whose 4577 definedness is tested in preprocessor directives, are output; 4578 the output is delayed until the use or test of the macro; and 4579 `#undef' directives are also output for macros tested but 4580 undefined at the time. 4581 4582 `-P' 4583 Inhibit generation of linemarkers in the output from the 4584 preprocessor. This might be useful when running the preprocessor 4585 on something that is not C code, and will be sent to a program 4586 which might be confused by the linemarkers. *Note Preprocessor 4587 Output::. 4588 4589 `-C' 4590 Do not discard comments. All comments are passed through to the 4591 output file, except for comments in processed directives, which 4592 are deleted along with the directive. 4593 4594 You should be prepared for side effects when using `-C'; it causes 4595 the preprocessor to treat comments as tokens in their own right. 4596 For example, comments appearing at the start of what would be a 4597 directive line have the effect of turning that line into an 4598 ordinary source line, since the first token on the line is no 4599 longer a `#'. 4600 4601 `-CC' 4602 Do not discard comments, including during macro expansion. This is 4603 like `-C', except that comments contained within macros are also 4604 passed through to the output file where the macro is expanded. 4605 4606 In addition to the side-effects of the `-C' option, the `-CC' 4607 option causes all C++-style comments inside a macro to be 4608 converted to C-style comments. This is to prevent later use of 4609 that macro from inadvertently commenting out the remainder of the 4610 source line. 4611 4612 The `-CC' option is generally used to support lint comments. 4613 4614 `-traditional-cpp' 4615 Try to imitate the behavior of old-fashioned C preprocessors, as 4616 opposed to ISO C preprocessors. *Note Traditional Mode::. 4617 4618 `-trigraphs' 4619 Process trigraph sequences. *Note Initial processing::. 4620 4621 `-remap' 4622 Enable special code to work around file systems which only permit 4623 very short file names, such as MS-DOS. 4624 4625 `--help' 4626 `--target-help' 4627 Print text describing all the command line options instead of 4628 preprocessing anything. 4629 4630 `-v' 4631 Verbose mode. Print out GNU CPP's version number at the beginning 4632 of execution, and report the final form of the include path. 4633 4634 `-H' 4635 Print the name of each header file used, in addition to other 4636 normal activities. Each name is indented to show how deep in the 4637 `#include' stack it is. Precompiled header files are also 4638 printed, even if they are found to be invalid; an invalid 4639 precompiled header file is printed with `...x' and a valid one 4640 with `...!' . 4641 4642 `-version' 4643 `--version' 4644 Print out GNU CPP's version number. With one dash, proceed to 4645 preprocess as normal. With two dashes, exit immediately. 4646 4647 4648 File: cpp.info, Node: Environment Variables, Next: GNU Free Documentation License, Prev: Invocation, Up: Top 4649 4650 13 Environment Variables 4651 ************************ 4652 4653 This section describes the environment variables that affect how CPP 4654 operates. You can use them to specify directories or prefixes to use 4655 when searching for include files, or to control dependency output. 4656 4657 Note that you can also specify places to search using options such as 4658 `-I', and control dependency output with options like `-M' (*note 4659 Invocation::). These take precedence over environment variables, which 4660 in turn take precedence over the configuration of GCC. 4661 4662 `CPATH' 4663 `C_INCLUDE_PATH' 4664 `CPLUS_INCLUDE_PATH' 4665 `OBJC_INCLUDE_PATH' 4666 Each variable's value is a list of directories separated by a 4667 special character, much like `PATH', in which to look for header 4668 files. The special character, `PATH_SEPARATOR', is 4669 target-dependent and determined at GCC build time. For Microsoft 4670 Windows-based targets it is a semicolon, and for almost all other 4671 targets it is a colon. 4672 4673 `CPATH' specifies a list of directories to be searched as if 4674 specified with `-I', but after any paths given with `-I' options 4675 on the command line. This environment variable is used regardless 4676 of which language is being preprocessed. 4677 4678 The remaining environment variables apply only when preprocessing 4679 the particular language indicated. Each specifies a list of 4680 directories to be searched as if specified with `-isystem', but 4681 after any paths given with `-isystem' options on the command line. 4682 4683 In all these variables, an empty element instructs the compiler to 4684 search its current working directory. Empty elements can appear 4685 at the beginning or end of a path. For instance, if the value of 4686 `CPATH' is `:/special/include', that has the same effect as 4687 `-I. -I/special/include'. 4688 4689 See also *note Search Path::. 4690 4691 `DEPENDENCIES_OUTPUT' 4692 If this variable is set, its value specifies how to output 4693 dependencies for Make based on the non-system header files 4694 processed by the compiler. System header files are ignored in the 4695 dependency output. 4696 4697 The value of `DEPENDENCIES_OUTPUT' can be just a file name, in 4698 which case the Make rules are written to that file, guessing the 4699 target name from the source file name. Or the value can have the 4700 form `FILE TARGET', in which case the rules are written to file 4701 FILE using TARGET as the target name. 4702 4703 In other words, this environment variable is equivalent to 4704 combining the options `-MM' and `-MF' (*note Invocation::), with 4705 an optional `-MT' switch too. 4706 4707 `SUNPRO_DEPENDENCIES' 4708 This variable is the same as `DEPENDENCIES_OUTPUT' (see above), 4709 except that system header files are not ignored, so it implies 4710 `-M' rather than `-MM'. However, the dependence on the main input 4711 file is omitted. *Note Invocation::. 4712 4713 4714 File: cpp.info, Node: GNU Free Documentation License, Next: Index of Directives, Prev: Environment Variables, Up: Top 4715 4716 GNU Free Documentation License 4717 ****************************** 4718 4719 Version 1.3, 3 November 2008 4720 4721 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. 4722 `http://fsf.org/' 4723 4724 Everyone is permitted to copy and distribute verbatim copies 4725 of this license document, but changing it is not allowed. 4726 4727 0. PREAMBLE 4728 4729 The purpose of this License is to make a manual, textbook, or other 4730 functional and useful document "free" in the sense of freedom: to 4731 assure everyone the effective freedom to copy and redistribute it, 4732 with or without modifying it, either commercially or 4733 noncommercially. Secondarily, this License preserves for the 4734 author and publisher a way to get credit for their work, while not 4735 being considered responsible for modifications made by others. 4736 4737 This License is a kind of "copyleft", which means that derivative 4738 works of the document must themselves be free in the same sense. 4739 It complements the GNU General Public License, which is a copyleft 4740 license designed for free software. 4741 4742 We have designed this License in order to use it for manuals for 4743 free software, because free software needs free documentation: a 4744 free program should come with manuals providing the same freedoms 4745 that the software does. But this License is not limited to 4746 software manuals; it can be used for any textual work, regardless 4747 of subject matter or whether it is published as a printed book. 4748 We recommend this License principally for works whose purpose is 4749 instruction or reference. 4750 4751 1. APPLICABILITY AND DEFINITIONS 4752 4753 This License applies to any manual or other work, in any medium, 4754 that contains a notice placed by the copyright holder saying it 4755 can be distributed under the terms of this License. Such a notice 4756 grants a world-wide, royalty-free license, unlimited in duration, 4757 to use that work under the conditions stated herein. The 4758 "Document", below, refers to any such manual or work. Any member 4759 of the public is a licensee, and is addressed as "you". You 4760 accept the license if you copy, modify or distribute the work in a 4761 way requiring permission under copyright law. 4762 4763 A "Modified Version" of the Document means any work containing the 4764 Document or a portion of it, either copied verbatim, or with 4765 modifications and/or translated into another language. 4766 4767 A "Secondary Section" is a named appendix or a front-matter section 4768 of the Document that deals exclusively with the relationship of the 4769 publishers or authors of the Document to the Document's overall 4770 subject (or to related matters) and contains nothing that could 4771 fall directly within that overall subject. (Thus, if the Document 4772 is in part a textbook of mathematics, a Secondary Section may not 4773 explain any mathematics.) The relationship could be a matter of 4774 historical connection with the subject or with related matters, or 4775 of legal, commercial, philosophical, ethical or political position 4776 regarding them. 4777 4778 The "Invariant Sections" are certain Secondary Sections whose 4779 titles are designated, as being those of Invariant Sections, in 4780 the notice that says that the Document is released under this 4781 License. If a section does not fit the above definition of 4782 Secondary then it is not allowed to be designated as Invariant. 4783 The Document may contain zero Invariant Sections. If the Document 4784 does not identify any Invariant Sections then there are none. 4785 4786 The "Cover Texts" are certain short passages of text that are 4787 listed, as Front-Cover Texts or Back-Cover Texts, in the notice 4788 that says that the Document is released under this License. A 4789 Front-Cover Text may be at most 5 words, and a Back-Cover Text may 4790 be at most 25 words. 4791 4792 A "Transparent" copy of the Document means a machine-readable copy, 4793 represented in a format whose specification is available to the 4794 general public, that is suitable for revising the document 4795 straightforwardly with generic text editors or (for images 4796 composed of pixels) generic paint programs or (for drawings) some 4797 widely available drawing editor, and that is suitable for input to 4798 text formatters or for automatic translation to a variety of 4799 formats suitable for input to text formatters. A copy made in an 4800 otherwise Transparent file format whose markup, or absence of 4801 markup, has been arranged to thwart or discourage subsequent 4802 modification by readers is not Transparent. An image format is 4803 not Transparent if used for any substantial amount of text. A 4804 copy that is not "Transparent" is called "Opaque". 4805 4806 Examples of suitable formats for Transparent copies include plain 4807 ASCII without markup, Texinfo input format, LaTeX input format, 4808 SGML or XML using a publicly available DTD, and 4809 standard-conforming simple HTML, PostScript or PDF designed for 4810 human modification. Examples of transparent image formats include 4811 PNG, XCF and JPG. Opaque formats include proprietary formats that 4812 can be read and edited only by proprietary word processors, SGML or 4813 XML for which the DTD and/or processing tools are not generally 4814 available, and the machine-generated HTML, PostScript or PDF 4815 produced by some word processors for output purposes only. 4816 4817 The "Title Page" means, for a printed book, the title page itself, 4818 plus such following pages as are needed to hold, legibly, the 4819 material this License requires to appear in the title page. For 4820 works in formats which do not have any title page as such, "Title 4821 Page" means the text near the most prominent appearance of the 4822 work's title, preceding the beginning of the body of the text. 4823 4824 The "publisher" means any person or entity that distributes copies 4825 of the Document to the public. 4826 4827 A section "Entitled XYZ" means a named subunit of the Document 4828 whose title either is precisely XYZ or contains XYZ in parentheses 4829 following text that translates XYZ in another language. (Here XYZ 4830 stands for a specific section name mentioned below, such as 4831 "Acknowledgements", "Dedications", "Endorsements", or "History".) 4832 To "Preserve the Title" of such a section when you modify the 4833 Document means that it remains a section "Entitled XYZ" according 4834 to this definition. 4835 4836 The Document may include Warranty Disclaimers next to the notice 4837 which states that this License applies to the Document. These 4838 Warranty Disclaimers are considered to be included by reference in 4839 this License, but only as regards disclaiming warranties: any other 4840 implication that these Warranty Disclaimers may have is void and 4841 has no effect on the meaning of this License. 4842 4843 2. VERBATIM COPYING 4844 4845 You may copy and distribute the Document in any medium, either 4846 commercially or noncommercially, provided that this License, the 4847 copyright notices, and the license notice saying this License 4848 applies to the Document are reproduced in all copies, and that you 4849 add no other conditions whatsoever to those of this License. You 4850 may not use technical measures to obstruct or control the reading 4851 or further copying of the copies you make or distribute. However, 4852 you may accept compensation in exchange for copies. If you 4853 distribute a large enough number of copies you must also follow 4854 the conditions in section 3. 4855 4856 You may also lend copies, under the same conditions stated above, 4857 and you may publicly display copies. 4858 4859 3. COPYING IN QUANTITY 4860 4861 If you publish printed copies (or copies in media that commonly 4862 have printed covers) of the Document, numbering more than 100, and 4863 the Document's license notice requires Cover Texts, you must 4864 enclose the copies in covers that carry, clearly and legibly, all 4865 these Cover Texts: Front-Cover Texts on the front cover, and 4866 Back-Cover Texts on the back cover. Both covers must also clearly 4867 and legibly identify you as the publisher of these copies. The 4868 front cover must present the full title with all words of the 4869 title equally prominent and visible. You may add other material 4870 on the covers in addition. Copying with changes limited to the 4871 covers, as long as they preserve the title of the Document and 4872 satisfy these conditions, can be treated as verbatim copying in 4873 other respects. 4874 4875 If the required texts for either cover are too voluminous to fit 4876 legibly, you should put the first ones listed (as many as fit 4877 reasonably) on the actual cover, and continue the rest onto 4878 adjacent pages. 4879 4880 If you publish or distribute Opaque copies of the Document 4881 numbering more than 100, you must either include a 4882 machine-readable Transparent copy along with each Opaque copy, or 4883 state in or with each Opaque copy a computer-network location from 4884 which the general network-using public has access to download 4885 using public-standard network protocols a complete Transparent 4886 copy of the Document, free of added material. If you use the 4887 latter option, you must take reasonably prudent steps, when you 4888 begin distribution of Opaque copies in quantity, to ensure that 4889 this Transparent copy will remain thus accessible at the stated 4890 location until at least one year after the last time you 4891 distribute an Opaque copy (directly or through your agents or 4892 retailers) of that edition to the public. 4893 4894 It is requested, but not required, that you contact the authors of 4895 the Document well before redistributing any large number of 4896 copies, to give them a chance to provide you with an updated 4897 version of the Document. 4898 4899 4. MODIFICATIONS 4900 4901 You may copy and distribute a Modified Version of the Document 4902 under the conditions of sections 2 and 3 above, provided that you 4903 release the Modified Version under precisely this License, with 4904 the Modified Version filling the role of the Document, thus 4905 licensing distribution and modification of the Modified Version to 4906 whoever possesses a copy of it. In addition, you must do these 4907 things in the Modified Version: 4908 4909 A. Use in the Title Page (and on the covers, if any) a title 4910 distinct from that of the Document, and from those of 4911 previous versions (which should, if there were any, be listed 4912 in the History section of the Document). You may use the 4913 same title as a previous version if the original publisher of 4914 that version gives permission. 4915 4916 B. List on the Title Page, as authors, one or more persons or 4917 entities responsible for authorship of the modifications in 4918 the Modified Version, together with at least five of the 4919 principal authors of the Document (all of its principal 4920 authors, if it has fewer than five), unless they release you 4921 from this requirement. 4922 4923 C. State on the Title page the name of the publisher of the 4924 Modified Version, as the publisher. 4925 4926 D. Preserve all the copyright notices of the Document. 4927 4928 E. Add an appropriate copyright notice for your modifications 4929 adjacent to the other copyright notices. 4930 4931 F. Include, immediately after the copyright notices, a license 4932 notice giving the public permission to use the Modified 4933 Version under the terms of this License, in the form shown in 4934 the Addendum below. 4935 4936 G. Preserve in that license notice the full lists of Invariant 4937 Sections and required Cover Texts given in the Document's 4938 license notice. 4939 4940 H. Include an unaltered copy of this License. 4941 4942 I. Preserve the section Entitled "History", Preserve its Title, 4943 and add to it an item stating at least the title, year, new 4944 authors, and publisher of the Modified Version as given on 4945 the Title Page. If there is no section Entitled "History" in 4946 the Document, create one stating the title, year, authors, 4947 and publisher of the Document as given on its Title Page, 4948 then add an item describing the Modified Version as stated in 4949 the previous sentence. 4950 4951 J. Preserve the network location, if any, given in the Document 4952 for public access to a Transparent copy of the Document, and 4953 likewise the network locations given in the Document for 4954 previous versions it was based on. These may be placed in 4955 the "History" section. You may omit a network location for a 4956 work that was published at least four years before the 4957 Document itself, or if the original publisher of the version 4958 it refers to gives permission. 4959 4960 K. For any section Entitled "Acknowledgements" or "Dedications", 4961 Preserve the Title of the section, and preserve in the 4962 section all the substance and tone of each of the contributor 4963 acknowledgements and/or dedications given therein. 4964 4965 L. Preserve all the Invariant Sections of the Document, 4966 unaltered in their text and in their titles. Section numbers 4967 or the equivalent are not considered part of the section 4968 titles. 4969 4970 M. Delete any section Entitled "Endorsements". Such a section 4971 may not be included in the Modified Version. 4972 4973 N. Do not retitle any existing section to be Entitled 4974 "Endorsements" or to conflict in title with any Invariant 4975 Section. 4976 4977 O. Preserve any Warranty Disclaimers. 4978 4979 If the Modified Version includes new front-matter sections or 4980 appendices that qualify as Secondary Sections and contain no 4981 material copied from the Document, you may at your option 4982 designate some or all of these sections as invariant. To do this, 4983 add their titles to the list of Invariant Sections in the Modified 4984 Version's license notice. These titles must be distinct from any 4985 other section titles. 4986 4987 You may add a section Entitled "Endorsements", provided it contains 4988 nothing but endorsements of your Modified Version by various 4989 parties--for example, statements of peer review or that the text 4990 has been approved by an organization as the authoritative 4991 definition of a standard. 4992 4993 You may add a passage of up to five words as a Front-Cover Text, 4994 and a passage of up to 25 words as a Back-Cover Text, to the end 4995 of the list of Cover Texts in the Modified Version. Only one 4996 passage of Front-Cover Text and one of Back-Cover Text may be 4997 added by (or through arrangements made by) any one entity. If the 4998 Document already includes a cover text for the same cover, 4999 previously added by you or by arrangement made by the same entity 5000 you are acting on behalf of, you may not add another; but you may 5001 replace the old one, on explicit permission from the previous 5002 publisher that added the old one. 5003 5004 The author(s) and publisher(s) of the Document do not by this 5005 License give permission to use their names for publicity for or to 5006 assert or imply endorsement of any Modified Version. 5007 5008 5. COMBINING DOCUMENTS 5009 5010 You may combine the Document with other documents released under 5011 this License, under the terms defined in section 4 above for 5012 modified versions, provided that you include in the combination 5013 all of the Invariant Sections of all of the original documents, 5014 unmodified, and list them all as Invariant Sections of your 5015 combined work in its license notice, and that you preserve all 5016 their Warranty Disclaimers. 5017 5018 The combined work need only contain one copy of this License, and 5019 multiple identical Invariant Sections may be replaced with a single 5020 copy. If there are multiple Invariant Sections with the same name 5021 but different contents, make the title of each such section unique 5022 by adding at the end of it, in parentheses, the name of the 5023 original author or publisher of that section if known, or else a 5024 unique number. Make the same adjustment to the section titles in 5025 the list of Invariant Sections in the license notice of the 5026 combined work. 5027 5028 In the combination, you must combine any sections Entitled 5029 "History" in the various original documents, forming one section 5030 Entitled "History"; likewise combine any sections Entitled 5031 "Acknowledgements", and any sections Entitled "Dedications". You 5032 must delete all sections Entitled "Endorsements." 5033 5034 6. COLLECTIONS OF DOCUMENTS 5035 5036 You may make a collection consisting of the Document and other 5037 documents released under this License, and replace the individual 5038 copies of this License in the various documents with a single copy 5039 that is included in the collection, provided that you follow the 5040 rules of this License for verbatim copying of each of the 5041 documents in all other respects. 5042 5043 You may extract a single document from such a collection, and 5044 distribute it individually under this License, provided you insert 5045 a copy of this License into the extracted document, and follow 5046 this License in all other respects regarding verbatim copying of 5047 that document. 5048 5049 7. AGGREGATION WITH INDEPENDENT WORKS 5050 5051 A compilation of the Document or its derivatives with other 5052 separate and independent documents or works, in or on a volume of 5053 a storage or distribution medium, is called an "aggregate" if the 5054 copyright resulting from the compilation is not used to limit the 5055 legal rights of the compilation's users beyond what the individual 5056 works permit. When the Document is included in an aggregate, this 5057 License does not apply to the other works in the aggregate which 5058 are not themselves derivative works of the Document. 5059 5060 If the Cover Text requirement of section 3 is applicable to these 5061 copies of the Document, then if the Document is less than one half 5062 of the entire aggregate, the Document's Cover Texts may be placed 5063 on covers that bracket the Document within the aggregate, or the 5064 electronic equivalent of covers if the Document is in electronic 5065 form. Otherwise they must appear on printed covers that bracket 5066 the whole aggregate. 5067 5068 8. TRANSLATION 5069 5070 Translation is considered a kind of modification, so you may 5071 distribute translations of the Document under the terms of section 5072 4. Replacing Invariant Sections with translations requires special 5073 permission from their copyright holders, but you may include 5074 translations of some or all Invariant Sections in addition to the 5075 original versions of these Invariant Sections. You may include a 5076 translation of this License, and all the license notices in the 5077 Document, and any Warranty Disclaimers, provided that you also 5078 include the original English version of this License and the 5079 original versions of those notices and disclaimers. In case of a 5080 disagreement between the translation and the original version of 5081 this License or a notice or disclaimer, the original version will 5082 prevail. 5083 5084 If a section in the Document is Entitled "Acknowledgements", 5085 "Dedications", or "History", the requirement (section 4) to 5086 Preserve its Title (section 1) will typically require changing the 5087 actual title. 5088 5089 9. TERMINATION 5090 5091 You may not copy, modify, sublicense, or distribute the Document 5092 except as expressly provided under this License. Any attempt 5093 otherwise to copy, modify, sublicense, or distribute it is void, 5094 and will automatically terminate your rights under this License. 5095 5096 However, if you cease all violation of this License, then your 5097 license from a particular copyright holder is reinstated (a) 5098 provisionally, unless and until the copyright holder explicitly 5099 and finally terminates your license, and (b) permanently, if the 5100 copyright holder fails to notify you of the violation by some 5101 reasonable means prior to 60 days after the cessation. 5102 5103 Moreover, your license from a particular copyright holder is 5104 reinstated permanently if the copyright holder notifies you of the 5105 violation by some reasonable means, this is the first time you have 5106 received notice of violation of this License (for any work) from 5107 that copyright holder, and you cure the violation prior to 30 days 5108 after your receipt of the notice. 5109 5110 Termination of your rights under this section does not terminate 5111 the licenses of parties who have received copies or rights from 5112 you under this License. If your rights have been terminated and 5113 not permanently reinstated, receipt of a copy of some or all of 5114 the same material does not give you any rights to use it. 5115 5116 10. FUTURE REVISIONS OF THIS LICENSE 5117 5118 The Free Software Foundation may publish new, revised versions of 5119 the GNU Free Documentation License from time to time. Such new 5120 versions will be similar in spirit to the present version, but may 5121 differ in detail to address new problems or concerns. See 5122 `http://www.gnu.org/copyleft/'. 5123 5124 Each version of the License is given a distinguishing version 5125 number. If the Document specifies that a particular numbered 5126 version of this License "or any later version" applies to it, you 5127 have the option of following the terms and conditions either of 5128 that specified version or of any later version that has been 5129 published (not as a draft) by the Free Software Foundation. If 5130 the Document does not specify a version number of this License, 5131 you may choose any version ever published (not as a draft) by the 5132 Free Software Foundation. If the Document specifies that a proxy 5133 can decide which future versions of this License can be used, that 5134 proxy's public statement of acceptance of a version permanently 5135 authorizes you to choose that version for the Document. 5136 5137 11. RELICENSING 5138 5139 "Massive Multiauthor Collaboration Site" (or "MMC Site") means any 5140 World Wide Web server that publishes copyrightable works and also 5141 provides prominent facilities for anybody to edit those works. A 5142 public wiki that anybody can edit is an example of such a server. 5143 A "Massive Multiauthor Collaboration" (or "MMC") contained in the 5144 site means any set of copyrightable works thus published on the MMC 5145 site. 5146 5147 "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 5148 license published by Creative Commons Corporation, a not-for-profit 5149 corporation with a principal place of business in San Francisco, 5150 California, as well as future copyleft versions of that license 5151 published by that same organization. 5152 5153 "Incorporate" means to publish or republish a Document, in whole or 5154 in part, as part of another Document. 5155 5156 An MMC is "eligible for relicensing" if it is licensed under this 5157 License, and if all works that were first published under this 5158 License somewhere other than this MMC, and subsequently 5159 incorporated in whole or in part into the MMC, (1) had no cover 5160 texts or invariant sections, and (2) were thus incorporated prior 5161 to November 1, 2008. 5162 5163 The operator of an MMC Site may republish an MMC contained in the 5164 site under CC-BY-SA on the same site at any time before August 1, 5165 2009, provided the MMC is eligible for relicensing. 5166 5167 5168 ADDENDUM: How to use this License for your documents 5169 ==================================================== 5170 5171 To use this License in a document you have written, include a copy of 5172 the License in the document and put the following copyright and license 5173 notices just after the title page: 5174 5175 Copyright (C) YEAR YOUR NAME. 5176 Permission is granted to copy, distribute and/or modify this document 5177 under the terms of the GNU Free Documentation License, Version 1.3 5178 or any later version published by the Free Software Foundation; 5179 with no Invariant Sections, no Front-Cover Texts, and no Back-Cover 5180 Texts. A copy of the license is included in the section entitled ``GNU 5181 Free Documentation License''. 5182 5183 If you have Invariant Sections, Front-Cover Texts and Back-Cover 5184 Texts, replace the "with...Texts." line with this: 5185 5186 with the Invariant Sections being LIST THEIR TITLES, with 5187 the Front-Cover Texts being LIST, and with the Back-Cover Texts 5188 being LIST. 5189 5190 If you have Invariant Sections without Cover Texts, or some other 5191 combination of the three, merge those two alternatives to suit the 5192 situation. 5193 5194 If your document contains nontrivial examples of program code, we 5195 recommend releasing these examples in parallel under your choice of 5196 free software license, such as the GNU General Public License, to 5197 permit their use in free software. 5198 5199 5200 File: cpp.info, Node: Index of Directives, Next: Option Index, Prev: GNU Free Documentation License, Up: Top 5201 5202 Index of Directives 5203 ******************* 5204 5205 [index] 5206 * Menu: 5207 5208 * #assert: Obsolete Features. (line 48) 5209 * #define: Object-like Macros. (line 11) 5210 * #elif: Elif. (line 6) 5211 * #else: Else. (line 6) 5212 * #endif: Ifdef. (line 6) 5213 * #error: Diagnostics. (line 6) 5214 * #ident: Other Directives. (line 6) 5215 * #if: Conditional Syntax. (line 6) 5216 * #ifdef: Ifdef. (line 6) 5217 * #ifndef: Ifdef. (line 40) 5218 * #import: Alternatives to Wrapper #ifndef. 5219 (line 11) 5220 * #include: Include Syntax. (line 6) 5221 * #include_next: Wrapper Headers. (line 6) 5222 * #line: Line Control. (line 20) 5223 * #pragma GCC dependency: Pragmas. (line 55) 5224 * #pragma GCC poison: Pragmas. (line 67) 5225 * #pragma GCC system_header <1>: Pragmas. (line 94) 5226 * #pragma GCC system_header: System Headers. (line 31) 5227 * #sccs: Other Directives. (line 6) 5228 * #unassert: Obsolete Features. (line 59) 5229 * #undef: Undefining and Redefining Macros. 5230 (line 6) 5231 * #warning: Diagnostics. (line 27) 5232 5233 5234 File: cpp.info, Node: Option Index, Next: Concept Index, Prev: Index of Directives, Up: Top 5235 5236 Option Index 5237 ************ 5238 5239 CPP's command line options and environment variables are indexed here 5240 without any initial `-' or `--'. 5241 5242 [index] 5243 * Menu: 5244 5245 * A: Invocation. (line 534) 5246 * ansi: Invocation. (line 308) 5247 * C: Invocation. (line 593) 5248 * C_INCLUDE_PATH: Environment Variables. 5249 (line 16) 5250 * CPATH: Environment Variables. 5251 (line 15) 5252 * CPLUS_INCLUDE_PATH: Environment Variables. 5253 (line 17) 5254 * D: Invocation. (line 39) 5255 * dD: Invocation. (line 566) 5256 * DEPENDENCIES_OUTPUT: Environment Variables. 5257 (line 44) 5258 * dI: Invocation. (line 575) 5259 * dM: Invocation. (line 550) 5260 * dN: Invocation. (line 572) 5261 * dU: Invocation. (line 579) 5262 * fdirectives-only: Invocation. (line 442) 5263 * fdollars-in-identifiers: Invocation. (line 464) 5264 * fexec-charset: Invocation. (line 491) 5265 * fextended-identifiers: Invocation. (line 467) 5266 * finput-charset: Invocation. (line 504) 5267 * fno-show-column: Invocation. (line 529) 5268 * fno-working-directory: Invocation. (line 514) 5269 * fpreprocessed: Invocation. (line 472) 5270 * ftabstop: Invocation. (line 485) 5271 * fwide-exec-charset: Invocation. (line 496) 5272 * fworking-directory: Invocation. (line 514) 5273 * H: Invocation. (line 638) 5274 * help: Invocation. (line 630) 5275 * I: Invocation. (line 71) 5276 * I-: Invocation. (line 355) 5277 * idirafter: Invocation. (line 397) 5278 * imacros: Invocation. (line 388) 5279 * imultilib: Invocation. (line 422) 5280 * include: Invocation. (line 377) 5281 * iprefix: Invocation. (line 404) 5282 * iquote: Invocation. (line 434) 5283 * isysroot: Invocation. (line 416) 5284 * isystem: Invocation. (line 426) 5285 * iwithprefix: Invocation. (line 410) 5286 * iwithprefixbefore: Invocation. (line 410) 5287 * M: Invocation. (line 180) 5288 * MD: Invocation. (line 269) 5289 * MF: Invocation. (line 215) 5290 * MG: Invocation. (line 224) 5291 * MM: Invocation. (line 205) 5292 * MMD: Invocation. (line 285) 5293 * MP: Invocation. (line 234) 5294 * MQ: Invocation. (line 260) 5295 * MT: Invocation. (line 246) 5296 * nostdinc: Invocation. (line 367) 5297 * nostdinc++: Invocation. (line 372) 5298 * o: Invocation. (line 82) 5299 * OBJC_INCLUDE_PATH: Environment Variables. 5300 (line 18) 5301 * P: Invocation. (line 586) 5302 * pedantic: Invocation. (line 170) 5303 * pedantic-errors: Invocation. (line 175) 5304 * remap: Invocation. (line 625) 5305 * std=: Invocation. (line 308) 5306 * SUNPRO_DEPENDENCIES: Environment Variables. 5307 (line 60) 5308 * target-help: Invocation. (line 630) 5309 * traditional-cpp: Invocation. (line 618) 5310 * trigraphs: Invocation. (line 622) 5311 * U: Invocation. (line 62) 5312 * undef: Invocation. (line 66) 5313 * v: Invocation. (line 634) 5314 * version: Invocation. (line 647) 5315 * w: Invocation. (line 166) 5316 * Wall: Invocation. (line 88) 5317 * Wcomment: Invocation. (line 96) 5318 * Wcomments: Invocation. (line 96) 5319 * Wendif-labels: Invocation. (line 143) 5320 * Werror: Invocation. (line 156) 5321 * Wsystem-headers: Invocation. (line 160) 5322 * Wtraditional: Invocation. (line 113) 5323 * Wtrigraphs: Invocation. (line 101) 5324 * Wundef: Invocation. (line 119) 5325 * Wunused-macros: Invocation. (line 124) 5326 * x: Invocation. (line 292) 5327 5328 5329 File: cpp.info, Node: Concept Index, Prev: Option Index, Up: Top 5330 5331 Concept Index 5332 ************* 5333 5334 [index] 5335 * Menu: 5336 5337 * # operator: Stringification. (line 6) 5338 * ## operator: Concatenation. (line 6) 5339 * _Pragma: Pragmas. (line 25) 5340 * alternative tokens: Tokenization. (line 106) 5341 * arguments: Macro Arguments. (line 6) 5342 * arguments in macro definitions: Macro Arguments. (line 6) 5343 * assertions: Obsolete Features. (line 13) 5344 * assertions, canceling: Obsolete Features. (line 59) 5345 * backslash-newline: Initial processing. (line 61) 5346 * block comments: Initial processing. (line 77) 5347 * C++ named operators: C++ Named Operators. (line 6) 5348 * character constants: Tokenization. (line 85) 5349 * character set, execution: Invocation. (line 491) 5350 * character set, input: Invocation. (line 504) 5351 * character set, wide execution: Invocation. (line 496) 5352 * command line: Invocation. (line 6) 5353 * commenting out code: Deleted Code. (line 6) 5354 * comments: Initial processing. (line 77) 5355 * common predefined macros: Common Predefined Macros. 5356 (line 6) 5357 * computed includes: Computed Includes. (line 6) 5358 * concatenation: Concatenation. (line 6) 5359 * conditional group: Ifdef. (line 14) 5360 * conditionals: Conditionals. (line 6) 5361 * continued lines: Initial processing. (line 61) 5362 * controlling macro: Once-Only Headers. (line 35) 5363 * defined: Defined. (line 6) 5364 * dependencies for make as output: Environment Variables. 5365 (line 45) 5366 * dependencies, make: Invocation. (line 180) 5367 * diagnostic: Diagnostics. (line 6) 5368 * differences from previous versions: Differences from previous versions. 5369 (line 6) 5370 * digraphs: Tokenization. (line 106) 5371 * directive line: The preprocessing language. 5372 (line 6) 5373 * directive name: The preprocessing language. 5374 (line 6) 5375 * directives: The preprocessing language. 5376 (line 6) 5377 * empty macro arguments: Macro Arguments. (line 66) 5378 * environment variables: Environment Variables. 5379 (line 6) 5380 * expansion of arguments: Argument Prescan. (line 6) 5381 * FDL, GNU Free Documentation License: GNU Free Documentation License. 5382 (line 6) 5383 * function-like macros: Function-like Macros. 5384 (line 6) 5385 * grouping options: Invocation. (line 34) 5386 * guard macro: Once-Only Headers. (line 35) 5387 * header file: Header Files. (line 6) 5388 * header file names: Tokenization. (line 85) 5389 * identifiers: Tokenization. (line 34) 5390 * implementation limits: Implementation limits. 5391 (line 6) 5392 * implementation-defined behavior: Implementation-defined behavior. 5393 (line 6) 5394 * including just once: Once-Only Headers. (line 6) 5395 * invocation: Invocation. (line 6) 5396 * iso646.h: C++ Named Operators. (line 6) 5397 * line comments: Initial processing. (line 77) 5398 * line control: Line Control. (line 6) 5399 * line endings: Initial processing. (line 14) 5400 * linemarkers: Preprocessor Output. (line 28) 5401 * macro argument expansion: Argument Prescan. (line 6) 5402 * macro arguments and directives: Directives Within Macro Arguments. 5403 (line 6) 5404 * macros in include: Computed Includes. (line 6) 5405 * macros with arguments: Macro Arguments. (line 6) 5406 * macros with variable arguments: Variadic Macros. (line 6) 5407 * make: Invocation. (line 180) 5408 * manifest constants: Object-like Macros. (line 6) 5409 * named operators: C++ Named Operators. (line 6) 5410 * newlines in macro arguments: Newlines in Arguments. 5411 (line 6) 5412 * null directive: Other Directives. (line 15) 5413 * numbers: Tokenization. (line 61) 5414 * object-like macro: Object-like Macros. (line 6) 5415 * options: Invocation. (line 38) 5416 * options, grouping: Invocation. (line 34) 5417 * other tokens: Tokenization. (line 120) 5418 * output format: Preprocessor Output. (line 12) 5419 * overriding a header file: Wrapper Headers. (line 6) 5420 * parentheses in macro bodies: Operator Precedence Problems. 5421 (line 6) 5422 * pitfalls of macros: Macro Pitfalls. (line 6) 5423 * predefined macros: Predefined Macros. (line 6) 5424 * predefined macros, system-specific: System-specific Predefined Macros. 5425 (line 6) 5426 * predicates: Obsolete Features. (line 26) 5427 * preprocessing directives: The preprocessing language. 5428 (line 6) 5429 * preprocessing numbers: Tokenization. (line 61) 5430 * preprocessing tokens: Tokenization. (line 6) 5431 * prescan of macro arguments: Argument Prescan. (line 6) 5432 * problems with macros: Macro Pitfalls. (line 6) 5433 * punctuators: Tokenization. (line 106) 5434 * redefining macros: Undefining and Redefining Macros. 5435 (line 6) 5436 * repeated inclusion: Once-Only Headers. (line 6) 5437 * reporting errors: Diagnostics. (line 6) 5438 * reporting warnings: Diagnostics. (line 6) 5439 * reserved namespace: System-specific Predefined Macros. 5440 (line 6) 5441 * self-reference: Self-Referential Macros. 5442 (line 6) 5443 * semicolons (after macro calls): Swallowing the Semicolon. 5444 (line 6) 5445 * side effects (in macro arguments): Duplication of Side Effects. 5446 (line 6) 5447 * standard predefined macros.: Standard Predefined Macros. 5448 (line 6) 5449 * string constants: Tokenization. (line 85) 5450 * string literals: Tokenization. (line 85) 5451 * stringification: Stringification. (line 6) 5452 * symbolic constants: Object-like Macros. (line 6) 5453 * system header files <1>: System Headers. (line 6) 5454 * system header files: Header Files. (line 13) 5455 * system-specific predefined macros: System-specific Predefined Macros. 5456 (line 6) 5457 * testing predicates: Obsolete Features. (line 37) 5458 * token concatenation: Concatenation. (line 6) 5459 * token pasting: Concatenation. (line 6) 5460 * tokens: Tokenization. (line 6) 5461 * trigraphs: Initial processing. (line 32) 5462 * undefining macros: Undefining and Redefining Macros. 5463 (line 6) 5464 * unsafe macros: Duplication of Side Effects. 5465 (line 6) 5466 * variable number of arguments: Variadic Macros. (line 6) 5467 * variadic macros: Variadic Macros. (line 6) 5468 * wrapper #ifndef: Once-Only Headers. (line 6) 5469 * wrapper headers: Wrapper Headers. (line 6) 5470 5471 5472 5473 Tag Table: 5474 Node: Top1149 5475 Node: Overview3881 5476 Node: Character sets6714 5477 Ref: Character sets-Footnote-18897 5478 Node: Initial processing9078 5479 Ref: trigraphs10637 5480 Node: Tokenization14839 5481 Ref: Tokenization-Footnote-121975 5482 Node: The preprocessing language22086 5483 Node: Header Files24964 5484 Node: Include Syntax26880 5485 Node: Include Operation28517 5486 Node: Search Path30365 5487 Node: Once-Only Headers33555 5488 Node: Alternatives to Wrapper #ifndef35214 5489 Node: Computed Includes36957 5490 Node: Wrapper Headers40115 5491 Node: System Headers42541 5492 Node: Macros44591 5493 Node: Object-like Macros45732 5494 Node: Function-like Macros49322 5495 Node: Macro Arguments50938 5496 Node: Stringification55083 5497 Node: Concatenation58289 5498 Node: Variadic Macros61397 5499 Node: Predefined Macros66184 5500 Node: Standard Predefined Macros66772 5501 Node: Common Predefined Macros72709 5502 Node: System-specific Predefined Macros90212 5503 Node: C++ Named Operators92233 5504 Node: Undefining and Redefining Macros93197 5505 Node: Directives Within Macro Arguments95301 5506 Node: Macro Pitfalls96849 5507 Node: Misnesting97382 5508 Node: Operator Precedence Problems98494 5509 Node: Swallowing the Semicolon100360 5510 Node: Duplication of Side Effects102383 5511 Node: Self-Referential Macros104566 5512 Node: Argument Prescan106975 5513 Node: Newlines in Arguments110729 5514 Node: Conditionals111680 5515 Node: Conditional Uses113510 5516 Node: Conditional Syntax114868 5517 Node: Ifdef115188 5518 Node: If118349 5519 Node: Defined120653 5520 Node: Else121936 5521 Node: Elif122506 5522 Node: Deleted Code123795 5523 Node: Diagnostics125042 5524 Node: Line Control126659 5525 Node: Pragmas130463 5526 Node: Other Directives134780 5527 Node: Preprocessor Output135830 5528 Node: Traditional Mode139031 5529 Node: Traditional lexical analysis140089 5530 Node: Traditional macros142592 5531 Node: Traditional miscellany146394 5532 Node: Traditional warnings147391 5533 Node: Implementation Details149588 5534 Node: Implementation-defined behavior150209 5535 Ref: Identifier characters150961 5536 Node: Implementation limits154039 5537 Node: Obsolete Features156713 5538 Node: Differences from previous versions159601 5539 Node: Invocation163809 5540 Ref: Wtrigraphs168261 5541 Ref: dashMF173036 5542 Ref: fdollars-in-identifiers182747 5543 Node: Environment Variables190910 5544 Node: GNU Free Documentation License193876 5545 Node: Index of Directives219040 5546 Node: Option Index220974 5547 Node: Concept Index227158 5548 5549 End Tag Table 5550
