1 This is doc/cpp.info, produced by makeinfo version 4.11 from 2 /home/jingyu/projects/gcc/android-toolchainsrc/build/../gcc/gcc-4.4.0/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 Free 6 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 1.2 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 Free 139 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 1.2 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=c89' or `-std=c99' options, depending on which 201 version of the standard you want. To get all the mandatory 202 diagnostics, you must also use `-pedantic'. *Note Invocation::. 203 204 This manual describes the behavior of the ISO preprocessor. To 205 minimize gratuitous differences, where the ISO preprocessor's behavior 206 does not conflict with traditional semantics, the traditional 207 preprocessor should behave the same way. The various differences that 208 do exist are detailed in the section *note Traditional Mode::. 209 210 For clarity, unless noted otherwise, references to `CPP' in this 211 manual refer to GNU CPP. 212 213 * Menu: 214 215 * Character sets:: 216 * Initial processing:: 217 * Tokenization:: 218 * The preprocessing language:: 219 220 221 File: cpp.info, Node: Character sets, Next: Initial processing, Up: Overview 222 223 1.1 Character sets 224 ================== 225 226 Source code character set processing in C and related languages is 227 rather complicated. The C standard discusses two character sets, but 228 there are really at least four. 229 230 The files input to CPP might be in any character set at all. CPP's 231 very first action, before it even looks for line boundaries, is to 232 convert the file into the character set it uses for internal 233 processing. That set is what the C standard calls the "source" 234 character set. It must be isomorphic with ISO 10646, also known as 235 Unicode. CPP uses the UTF-8 encoding of Unicode. 236 237 The character sets of the input files are specified using the 238 `-finput-charset=' option. 239 240 All preprocessing work (the subject of the rest of this manual) is 241 carried out in the source character set. If you request textual output 242 from the preprocessor with the `-E' option, it will be in UTF-8. 243 244 After preprocessing is complete, string and character constants are 245 converted again, into the "execution" character set. This character 246 set is under control of the user; the default is UTF-8, matching the 247 source character set. Wide string and character constants have their 248 own character set, which is not called out specifically in the 249 standard. Again, it is under control of the user. The default is 250 UTF-16 or UTF-32, whichever fits in the target's `wchar_t' type, in the 251 target machine's byte order.(1) Octal and hexadecimal escape sequences 252 do not undergo conversion; '\x12' has the value 0x12 regardless of the 253 currently selected execution character set. All other escapes are 254 replaced by the character in the source character set that they 255 represent, then converted to the execution character set, just like 256 unescaped characters. 257 258 Unless the experimental `-fextended-identifiers' option is used, GCC 259 does not permit the use of characters outside the ASCII range, nor `\u' 260 and `\U' escapes, in identifiers. Even with that option, characters 261 outside the ASCII range can only be specified with the `\u' and `\U' 262 escapes, not used directly in identifiers. 263 264 ---------- Footnotes ---------- 265 266 (1) UTF-16 does not meet the requirements of the C standard for a 267 wide character set, but the choice of 16-bit `wchar_t' is enshrined in 268 some system ABIs so we cannot fix this. 269 270 271 File: cpp.info, Node: Initial processing, Next: Tokenization, Prev: Character sets, Up: Overview 272 273 1.2 Initial processing 274 ====================== 275 276 The preprocessor performs a series of textual transformations on its 277 input. These happen before all other processing. Conceptually, they 278 happen in a rigid order, and the entire file is run through each 279 transformation before the next one begins. CPP actually does them all 280 at once, for performance reasons. These transformations correspond 281 roughly to the first three "phases of translation" described in the C 282 standard. 283 284 1. The input file is read into memory and broken into lines. 285 286 Different systems use different conventions to indicate the end of 287 a line. GCC accepts the ASCII control sequences `LF', `CR LF' and 288 `CR' as end-of-line markers. These are the canonical sequences 289 used by Unix, DOS and VMS, and the classic Mac OS (before OSX) 290 respectively. You may therefore safely copy source code written 291 on any of those systems to a different one and use it without 292 conversion. (GCC may lose track of the current line number if a 293 file doesn't consistently use one convention, as sometimes happens 294 when it is edited on computers with different conventions that 295 share a network file system.) 296 297 If the last line of any input file lacks an end-of-line marker, 298 the end of the file is considered to implicitly supply one. The C 299 standard says that this condition provokes undefined behavior, so 300 GCC will emit a warning message. 301 302 2. If trigraphs are enabled, they are replaced by their corresponding 303 single characters. By default GCC ignores trigraphs, but if you 304 request a strictly conforming mode with the `-std' option, or you 305 specify the `-trigraphs' option, then it converts them. 306 307 These are nine three-character sequences, all starting with `??', 308 that are defined by ISO C to stand for single characters. They 309 permit obsolete systems that lack some of C's punctuation to use 310 C. For example, `??/' stands for `\', so '??/n' is a character 311 constant for a newline. 312 313 Trigraphs are not popular and many compilers implement them 314 incorrectly. Portable code should not rely on trigraphs being 315 either converted or ignored. With `-Wtrigraphs' GCC will warn you 316 when a trigraph may change the meaning of your program if it were 317 converted. *Note Wtrigraphs::. 318 319 In a string constant, you can prevent a sequence of question marks 320 from being confused with a trigraph by inserting a backslash 321 between the question marks, or by separating the string literal at 322 the trigraph and making use of string literal concatenation. 323 "(??\?)" is the string `(???)', not `(?]'. Traditional C 324 compilers do not recognize these idioms. 325 326 The nine trigraphs and their replacements are 327 328 Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- 329 Replacement: [ ] { } # \ ^ | ~ 330 331 3. Continued lines are merged into one long line. 332 333 A continued line is a line which ends with a backslash, `\'. The 334 backslash is removed and the following line is joined with the 335 current one. No space is inserted, so you may split a line 336 anywhere, even in the middle of a word. (It is generally more 337 readable to split lines only at white space.) 338 339 The trailing backslash on a continued line is commonly referred to 340 as a "backslash-newline". 341 342 If there is white space between a backslash and the end of a line, 343 that is still a continued line. However, as this is usually the 344 result of an editing mistake, and many compilers will not accept 345 it as a continued line, GCC will warn you about it. 346 347 4. All comments are replaced with single spaces. 348 349 There are two kinds of comments. "Block comments" begin with `/*' 350 and continue until the next `*/'. Block comments do not nest: 351 352 /* this is /* one comment */ text outside comment 353 354 "Line comments" begin with `//' and continue to the end of the 355 current line. Line comments do not nest either, but it does not 356 matter, because they would end in the same place anyway. 357 358 // this is // one comment 359 text outside comment 360 361 It is safe to put line comments inside block comments, or vice versa. 362 363 /* block comment 364 // contains line comment 365 yet more comment 366 */ outside comment 367 368 // line comment /* contains block comment */ 369 370 But beware of commenting out one end of a block comment with a line 371 comment. 372 373 // l.c. /* block comment begins 374 oops! this isn't a comment anymore */ 375 376 Comments are not recognized within string literals. "/* blah */" is 377 the string constant `/* blah */', not an empty string. 378 379 Line comments are not in the 1989 edition of the C standard, but they 380 are recognized by GCC as an extension. In C++ and in the 1999 edition 381 of the C standard, they are an official part of the language. 382 383 Since these transformations happen before all other processing, you 384 can split a line mechanically with backslash-newline anywhere. You can 385 comment out the end of a line. You can continue a line comment onto the 386 next line with backslash-newline. You can even split `/*', `*/', and 387 `//' onto multiple lines with backslash-newline. For example: 388 389 /\ 390 * 391 */ # /* 392 */ defi\ 393 ne FO\ 394 O 10\ 395 20 396 397 is equivalent to `#define FOO 1020'. All these tricks are extremely 398 confusing and should not be used in code intended to be readable. 399 400 There is no way to prevent a backslash at the end of a line from 401 being interpreted as a backslash-newline. This cannot affect any 402 correct program, however. 403 404 405 File: cpp.info, Node: Tokenization, Next: The preprocessing language, Prev: Initial processing, Up: Overview 406 407 1.3 Tokenization 408 ================ 409 410 After the textual transformations are finished, the input file is 411 converted into a sequence of "preprocessing tokens". These mostly 412 correspond to the syntactic tokens used by the C compiler, but there are 413 a few differences. White space separates tokens; it is not itself a 414 token of any kind. Tokens do not have to be separated by white space, 415 but it is often necessary to avoid ambiguities. 416 417 When faced with a sequence of characters that has more than one 418 possible tokenization, the preprocessor is greedy. It always makes 419 each token, starting from the left, as big as possible before moving on 420 to the next token. For instance, `a+++++b' is interpreted as 421 `a ++ ++ + b', not as `a ++ + ++ b', even though the latter 422 tokenization could be part of a valid C program and the former could 423 not. 424 425 Once the input file is broken into tokens, the token boundaries never 426 change, except when the `##' preprocessing operator is used to paste 427 tokens together. *Note Concatenation::. For example, 428 429 #define foo() bar 430 foo()baz 431 ==> bar baz 432 _not_ 433 ==> barbaz 434 435 The compiler does not re-tokenize the preprocessor's output. Each 436 preprocessing token becomes one compiler token. 437 438 Preprocessing tokens fall into five broad classes: identifiers, 439 preprocessing numbers, string literals, punctuators, and other. An 440 "identifier" is the same as an identifier in C: any sequence of 441 letters, digits, or underscores, which begins with a letter or 442 underscore. Keywords of C have no significance to the preprocessor; 443 they are ordinary identifiers. You can define a macro whose name is a 444 keyword, for instance. The only identifier which can be considered a 445 preprocessing keyword is `defined'. *Note Defined::. 446 447 This is mostly true of other languages which use the C preprocessor. 448 However, a few of the keywords of C++ are significant even in the 449 preprocessor. *Note C++ Named Operators::. 450 451 In the 1999 C standard, identifiers may contain letters which are not 452 part of the "basic source character set", at the implementation's 453 discretion (such as accented Latin letters, Greek letters, or Chinese 454 ideograms). This may be done with an extended character set, or the 455 `\u' and `\U' escape sequences. The implementation of this feature in 456 GCC is experimental; such characters are only accepted in the `\u' and 457 `\U' forms and only if `-fextended-identifiers' is used. 458 459 As an extension, GCC treats `$' as a letter. This is for 460 compatibility with some systems, such as VMS, where `$' is commonly 461 used in system-defined function and object names. `$' is not a letter 462 in strictly conforming mode, or if you specify the `-$' option. *Note 463 Invocation::. 464 465 A "preprocessing number" has a rather bizarre definition. The 466 category includes all the normal integer and floating point constants 467 one expects of C, but also a number of other things one might not 468 initially recognize as a number. Formally, preprocessing numbers begin 469 with an optional period, a required decimal digit, and then continue 470 with any sequence of letters, digits, underscores, periods, and 471 exponents. Exponents are the two-character sequences `e+', `e-', `E+', 472 `E-', `p+', `p-', `P+', and `P-'. (The exponents that begin with `p' 473 or `P' are new to C99. They are used for hexadecimal floating-point 474 constants.) 475 476 The purpose of this unusual definition is to isolate the preprocessor 477 from the full complexity of numeric constants. It does not have to 478 distinguish between lexically valid and invalid floating-point numbers, 479 which is complicated. The definition also permits you to split an 480 identifier at any position and get exactly two tokens, which can then be 481 pasted back together with the `##' operator. 482 483 It's possible for preprocessing numbers to cause programs to be 484 misinterpreted. For example, `0xE+12' is a preprocessing number which 485 does not translate to any valid numeric constant, therefore a syntax 486 error. It does not mean `0xE + 12', which is what you might have 487 intended. 488 489 "String literals" are string constants, character constants, and 490 header file names (the argument of `#include').(1) String constants 491 and character constants are straightforward: "..." or '...'. In either 492 case embedded quotes should be escaped with a backslash: '\'' is the 493 character constant for `''. There is no limit on the length of a 494 character constant, but the value of a character constant that contains 495 more than one character is implementation-defined. *Note 496 Implementation Details::. 497 498 Header file names either look like string constants, "...", or are 499 written with angle brackets instead, <...>. In either case, backslash 500 is an ordinary character. There is no way to escape the closing quote 501 or angle bracket. The preprocessor looks for the header file in 502 different places depending on which form you use. *Note Include 503 Operation::. 504 505 No string literal may extend past the end of a line. Older versions 506 of GCC accepted multi-line string constants. You may use continued 507 lines instead, or string constant concatenation. *Note Differences 508 from previous versions::. 509 510 "Punctuators" are all the usual bits of punctuation which are 511 meaningful to C and C++. All but three of the punctuation characters in 512 ASCII are C punctuators. The exceptions are `@', `$', and ``'. In 513 addition, all the two- and three-character operators are punctuators. 514 There are also six "digraphs", which the C++ standard calls 515 "alternative tokens", which are merely alternate ways to spell other 516 punctuators. This is a second attempt to work around missing 517 punctuation in obsolete systems. It has no negative side effects, 518 unlike trigraphs, but does not cover as much ground. The digraphs and 519 their corresponding normal punctuators are: 520 521 Digraph: <% %> <: :> %: %:%: 522 Punctuator: { } [ ] # ## 523 524 Any other single character is considered "other". It is passed on to 525 the preprocessor's output unmolested. The C compiler will almost 526 certainly reject source code containing "other" tokens. In ASCII, the 527 only other characters are `@', `$', ``', and control characters other 528 than NUL (all bits zero). (Note that `$' is normally considered a 529 letter.) All characters with the high bit set (numeric range 530 0x7F-0xFF) are also "other" in the present implementation. This will 531 change when proper support for international character sets is added to 532 GCC. 533 534 NUL is a special case because of the high probability that its 535 appearance is accidental, and because it may be invisible to the user 536 (many terminals do not display NUL at all). Within comments, NULs are 537 silently ignored, just as any other character would be. In running 538 text, NUL is considered white space. For example, these two directives 539 have the same meaning. 540 541 #define X^@1 542 #define X 1 543 544 (where `^@' is ASCII NUL). Within string or character constants, NULs 545 are preserved. In the latter two cases the preprocessor emits a 546 warning message. 547 548 ---------- Footnotes ---------- 549 550 (1) The C standard uses the term "string literal" to refer only to 551 what we are calling "string constants". 552 553 554 File: cpp.info, Node: The preprocessing language, Prev: Tokenization, Up: Overview 555 556 1.4 The preprocessing language 557 ============================== 558 559 After tokenization, the stream of tokens may simply be passed straight 560 to the compiler's parser. However, if it contains any operations in the 561 "preprocessing language", it will be transformed first. This stage 562 corresponds roughly to the standard's "translation phase 4" and is what 563 most people think of as the preprocessor's job. 564 565 The preprocessing language consists of "directives" to be executed 566 and "macros" to be expanded. Its primary capabilities are: 567 568 * Inclusion of header files. These are files of declarations that 569 can be substituted into your program. 570 571 * Macro expansion. You can define "macros", which are abbreviations 572 for arbitrary fragments of C code. The preprocessor will replace 573 the macros with their definitions throughout the program. Some 574 macros are automatically defined for you. 575 576 * Conditional compilation. You can include or exclude parts of the 577 program according to various conditions. 578 579 * Line control. If you use a program to combine or rearrange source 580 files into an intermediate file which is then compiled, you can 581 use line control to inform the compiler where each source line 582 originally came from. 583 584 * Diagnostics. You can detect problems at compile time and issue 585 errors or warnings. 586 587 There are a few more, less useful, features. 588 589 Except for expansion of predefined macros, all these operations are 590 triggered with "preprocessing directives". Preprocessing directives 591 are lines in your program that start with `#'. Whitespace is allowed 592 before and after the `#'. The `#' is followed by an identifier, the 593 "directive name". It specifies the operation to perform. Directives 594 are commonly referred to as `#NAME' where NAME is the directive name. 595 For example, `#define' is the directive that defines a macro. 596 597 The `#' which begins a directive cannot come from a macro expansion. 598 Also, the directive name is not macro expanded. Thus, if `foo' is 599 defined as a macro expanding to `define', that does not make `#foo' a 600 valid preprocessing directive. 601 602 The set of valid directive names is fixed. Programs cannot define 603 new preprocessing directives. 604 605 Some directives require arguments; these make up the rest of the 606 directive line and must be separated from the directive name by 607 whitespace. For example, `#define' must be followed by a macro name 608 and the intended expansion of the macro. 609 610 A preprocessing directive cannot cover more than one line. The line 611 may, however, be continued with backslash-newline, or by a block comment 612 which extends past the end of the line. In either case, when the 613 directive is processed, the continuations have already been merged with 614 the first line to make one long line. 615 616 617 File: cpp.info, Node: Header Files, Next: Macros, Prev: Overview, Up: Top 618 619 2 Header Files 620 ************** 621 622 A header file is a file containing C declarations and macro definitions 623 (*note Macros::) to be shared between several source files. You request 624 the use of a header file in your program by "including" it, with the C 625 preprocessing directive `#include'. 626 627 Header files serve two purposes. 628 629 * System header files declare the interfaces to parts of the 630 operating system. You include them in your program to supply the 631 definitions and declarations you need to invoke system calls and 632 libraries. 633 634 * Your own header files contain declarations for interfaces between 635 the source files of your program. Each time you have a group of 636 related declarations and macro definitions all or most of which 637 are needed in several different source files, it is a good idea to 638 create a header file for them. 639 640 Including a header file produces the same results as copying the 641 header file into each source file that needs it. Such copying would be 642 time-consuming and error-prone. With a header file, the related 643 declarations appear in only one place. If they need to be changed, they 644 can be changed in one place, and programs that include the header file 645 will automatically use the new version when next recompiled. The header 646 file eliminates the labor of finding and changing all the copies as well 647 as the risk that a failure to find one copy will result in 648 inconsistencies within a program. 649 650 In C, the usual convention is to give header files names that end 651 with `.h'. It is most portable to use only letters, digits, dashes, and 652 underscores in header file names, and at most one dot. 653 654 * Menu: 655 656 * Include Syntax:: 657 * Include Operation:: 658 * Search Path:: 659 * Once-Only Headers:: 660 * Alternatives to Wrapper #ifndef:: 661 * Computed Includes:: 662 * Wrapper Headers:: 663 * System Headers:: 664 665 666 File: cpp.info, Node: Include Syntax, Next: Include Operation, Up: Header Files 667 668 2.1 Include Syntax 669 ================== 670 671 Both user and system header files are included using the preprocessing 672 directive `#include'. It has two variants: 673 674 `#include <FILE>' 675 This variant is used for system header files. It searches for a 676 file named FILE in a standard list of system directories. You can 677 prepend directories to this list with the `-I' option (*note 678 Invocation::). 679 680 `#include "FILE"' 681 This variant is used for header files of your own program. It 682 searches for a file named FILE first in the directory containing 683 the current file, then in the quote directories and then the same 684 directories used for `<FILE>'. You can prepend directories to the 685 list of quote directories with the `-iquote' option. 686 687 The argument of `#include', whether delimited with quote marks or 688 angle brackets, behaves like a string constant in that comments are not 689 recognized, and macro names are not expanded. Thus, `#include <x/*y>' 690 specifies inclusion of a system header file named `x/*y'. 691 692 However, if backslashes occur within FILE, they are considered 693 ordinary text characters, not escape characters. None of the character 694 escape sequences appropriate to string constants in C are processed. 695 Thus, `#include "x\n\\y"' specifies a filename containing three 696 backslashes. (Some systems interpret `\' as a pathname separator. All 697 of these also interpret `/' the same way. It is most portable to use 698 only `/'.) 699 700 It is an error if there is anything (other than comments) on the line 701 after the file name. 702 703 704 File: cpp.info, Node: Include Operation, Next: Search Path, Prev: Include Syntax, Up: Header Files 705 706 2.2 Include Operation 707 ===================== 708 709 The `#include' directive works by directing the C preprocessor to scan 710 the specified file as input before continuing with the rest of the 711 current file. The output from the preprocessor contains the output 712 already generated, followed by the output resulting from the included 713 file, followed by the output that comes from the text after the 714 `#include' directive. For example, if you have a header file 715 `header.h' as follows, 716 717 char *test (void); 718 719 and a main program called `program.c' that uses the header file, like 720 this, 721 722 int x; 723 #include "header.h" 724 725 int 726 main (void) 727 { 728 puts (test ()); 729 } 730 731 the compiler will see the same token stream as it would if `program.c' 732 read 733 734 int x; 735 char *test (void); 736 737 int 738 main (void) 739 { 740 puts (test ()); 741 } 742 743 Included files are not limited to declarations and macro definitions; 744 those are merely the typical uses. Any fragment of a C program can be 745 included from another file. The include file could even contain the 746 beginning of a statement that is concluded in the containing file, or 747 the end of a statement that was started in the including file. However, 748 an included file must consist of complete tokens. Comments and string 749 literals which have not been closed by the end of an included file are 750 invalid. For error recovery, they are considered to end at the end of 751 the file. 752 753 To avoid confusion, it is best if header files contain only complete 754 syntactic units--function declarations or definitions, type 755 declarations, etc. 756 757 The line following the `#include' directive is always treated as a 758 separate line by the C preprocessor, even if the included file lacks a 759 final newline. 760 761 762 File: cpp.info, Node: Search Path, Next: Once-Only Headers, Prev: Include Operation, Up: Header Files 763 764 2.3 Search Path 765 =============== 766 767 GCC looks in several different places for headers. On a normal Unix 768 system, if you do not instruct it otherwise, it will look for headers 769 requested with `#include <FILE>' in: 770 771 /usr/local/include 772 LIBDIR/gcc/TARGET/VERSION/include 773 /usr/TARGET/include 774 /usr/include 775 776 For C++ programs, it will also look in `/usr/include/g++-v3', first. 777 In the above, TARGET is the canonical name of the system GCC was 778 configured to compile code for; often but not always the same as the 779 canonical name of the system it runs on. VERSION is the version of GCC 780 in use. 781 782 You can add to this list with the `-IDIR' command line option. All 783 the directories named by `-I' are searched, in left-to-right order, 784 _before_ the default directories. The only exception is when `dir' is 785 already searched by default. In this case, the option is ignored and 786 the search order for system directories remains unchanged. 787 788 Duplicate directories are removed from the quote and bracket search 789 chains before the two chains are merged to make the final search chain. 790 Thus, it is possible for a directory to occur twice in the final search 791 chain if it was specified in both the quote and bracket chains. 792 793 You can prevent GCC from searching any of the default directories 794 with the `-nostdinc' option. This is useful when you are compiling an 795 operating system kernel or some other program that does not use the 796 standard C library facilities, or the standard C library itself. `-I' 797 options are not ignored as described above when `-nostdinc' is in 798 effect. 799 800 GCC looks for headers requested with `#include "FILE"' first in the 801 directory containing the current file, then in the directories as 802 specified by `-iquote' options, then in the same places it would have 803 looked for a header requested with angle brackets. For example, if 804 `/usr/include/sys/stat.h' contains `#include "types.h"', GCC looks for 805 `types.h' first in `/usr/include/sys', then in its usual search path. 806 807 `#line' (*note Line Control::) does not change GCC's idea of the 808 directory containing the current file. 809 810 You may put `-I-' at any point in your list of `-I' options. This 811 has two effects. First, directories appearing before the `-I-' in the 812 list are searched only for headers requested with quote marks. 813 Directories after `-I-' are searched for all headers. Second, the 814 directory containing the current file is not searched for anything, 815 unless it happens to be one of the directories named by an `-I' switch. 816 `-I-' is deprecated, `-iquote' should be used instead. 817 818 `-I. -I-' is not the same as no `-I' options at all, and does not 819 cause the same behavior for `<>' includes that `""' includes get with 820 no special options. `-I.' searches the compiler's current working 821 directory for header files. That may or may not be the same as the 822 directory containing the current file. 823 824 If you need to look for headers in a directory named `-', write 825 `-I./-'. 826 827 There are several more ways to adjust the header search path. They 828 are generally less useful. *Note Invocation::. 829 830 831 File: cpp.info, Node: Once-Only Headers, Next: Alternatives to Wrapper #ifndef, Prev: Search Path, Up: Header Files 832 833 2.4 Once-Only Headers 834 ===================== 835 836 If a header file happens to be included twice, the compiler will process 837 its contents twice. This is very likely to cause an error, e.g. when 838 the compiler sees the same structure definition twice. Even if it does 839 not, it will certainly waste time. 840 841 The standard way to prevent this is to enclose the entire real 842 contents of the file in a conditional, like this: 843 844 /* File foo. */ 845 #ifndef FILE_FOO_SEEN 846 #define FILE_FOO_SEEN 847 848 THE ENTIRE FILE 849 850 #endif /* !FILE_FOO_SEEN */ 851 852 This construct is commonly known as a "wrapper #ifndef". When the 853 header is included again, the conditional will be false, because 854 `FILE_FOO_SEEN' is defined. The preprocessor will skip over the entire 855 contents of the file, and the compiler will not see it twice. 856 857 CPP optimizes even further. It remembers when a header file has a 858 wrapper `#ifndef'. If a subsequent `#include' specifies that header, 859 and the macro in the `#ifndef' is still defined, it does not bother to 860 rescan the file at all. 861 862 You can put comments outside the wrapper. They will not interfere 863 with this optimization. 864 865 The macro `FILE_FOO_SEEN' is called the "controlling macro" or 866 "guard macro". In a user header file, the macro name should not begin 867 with `_'. In a system header file, it should begin with `__' to avoid 868 conflicts with user programs. In any kind of header file, the macro 869 name should contain the name of the file and some additional text, to 870 avoid conflicts with other header files. 871 872 873 File: cpp.info, Node: Alternatives to Wrapper #ifndef, Next: Computed Includes, Prev: Once-Only Headers, Up: Header Files 874 875 2.5 Alternatives to Wrapper #ifndef 876 =================================== 877 878 CPP supports two more ways of indicating that a header file should be 879 read only once. Neither one is as portable as a wrapper `#ifndef' and 880 we recommend you do not use them in new programs, with the caveat that 881 `#import' is standard practice in Objective-C. 882 883 CPP supports a variant of `#include' called `#import' which includes 884 a file, but does so at most once. If you use `#import' instead of 885 `#include', then you don't need the conditionals inside the header file 886 to prevent multiple inclusion of the contents. `#import' is standard 887 in Objective-C, but is considered a deprecated extension in C and C++. 888 889 `#import' is not a well designed feature. It requires the users of 890 a header file to know that it should only be included once. It is much 891 better for the header file's implementor to write the file so that users 892 don't need to know this. Using a wrapper `#ifndef' accomplishes this 893 goal. 894 895 In the present implementation, a single use of `#import' will 896 prevent the file from ever being read again, by either `#import' or 897 `#include'. You should not rely on this; do not use both `#import' and 898 `#include' to refer to the same header file. 899 900 Another way to prevent a header file from being included more than 901 once is with the `#pragma once' directive. If `#pragma once' is seen 902 when scanning a header file, that file will never be read again, no 903 matter what. 904 905 `#pragma once' does not have the problems that `#import' does, but 906 it is not recognized by all preprocessors, so you cannot rely on it in 907 a portable program. 908 909 910 File: cpp.info, Node: Computed Includes, Next: Wrapper Headers, Prev: Alternatives to Wrapper #ifndef, Up: Header Files 911 912 2.6 Computed Includes 913 ===================== 914 915 Sometimes it is necessary to select one of several different header 916 files to be included into your program. They might specify 917 configuration parameters to be used on different sorts of operating 918 systems, for instance. You could do this with a series of conditionals, 919 920 #if SYSTEM_1 921 # include "system_1.h" 922 #elif SYSTEM_2 923 # include "system_2.h" 924 #elif SYSTEM_3 925 ... 926 #endif 927 928 That rapidly becomes tedious. Instead, the preprocessor offers the 929 ability to use a macro for the header name. This is called a "computed 930 include". Instead of writing a header name as the direct argument of 931 `#include', you simply put a macro name there instead: 932 933 #define SYSTEM_H "system_1.h" 934 ... 935 #include SYSTEM_H 936 937 `SYSTEM_H' will be expanded, and the preprocessor will look for 938 `system_1.h' as if the `#include' had been written that way originally. 939 `SYSTEM_H' could be defined by your Makefile with a `-D' option. 940 941 You must be careful when you define the macro. `#define' saves 942 tokens, not text. The preprocessor has no way of knowing that the macro 943 will be used as the argument of `#include', so it generates ordinary 944 tokens, not a header name. This is unlikely to cause problems if you 945 use double-quote includes, which are close enough to string constants. 946 If you use angle brackets, however, you may have trouble. 947 948 The syntax of a computed include is actually a bit more general than 949 the above. If the first non-whitespace character after `#include' is 950 not `"' or `<', then the entire line is macro-expanded like running 951 text would be. 952 953 If the line expands to a single string constant, the contents of that 954 string constant are the file to be included. CPP does not re-examine 955 the string for embedded quotes, but neither does it process backslash 956 escapes in the string. Therefore 957 958 #define HEADER "a\"b" 959 #include HEADER 960 961 looks for a file named `a\"b'. CPP searches for the file according to 962 the rules for double-quoted includes. 963 964 If the line expands to a token stream beginning with a `<' token and 965 including a `>' token, then the tokens between the `<' and the first 966 `>' are combined to form the filename to be included. Any whitespace 967 between tokens is reduced to a single space; then any space after the 968 initial `<' is retained, but a trailing space before the closing `>' is 969 ignored. CPP searches for the file according to the rules for 970 angle-bracket includes. 971 972 In either case, if there are any tokens on the line after the file 973 name, an error occurs and the directive is not processed. It is also 974 an error if the result of expansion does not match either of the two 975 expected forms. 976 977 These rules are implementation-defined behavior according to the C 978 standard. To minimize the risk of different compilers interpreting your 979 computed includes differently, we recommend you use only a single 980 object-like macro which expands to a string constant. This will also 981 minimize confusion for people reading your program. 982 983 984 File: cpp.info, Node: Wrapper Headers, Next: System Headers, Prev: Computed Includes, Up: Header Files 985 986 2.7 Wrapper Headers 987 =================== 988 989 Sometimes it is necessary to adjust the contents of a system-provided 990 header file without editing it directly. GCC's `fixincludes' operation 991 does this, for example. One way to do that would be to create a new 992 header file with the same name and insert it in the search path before 993 the original header. That works fine as long as you're willing to 994 replace the old header entirely. But what if you want to refer to the 995 old header from the new one? 996 997 You cannot simply include the old header with `#include'. That will 998 start from the beginning, and find your new header again. If your 999 header is not protected from multiple inclusion (*note Once-Only 1000 Headers::), it will recurse infinitely and cause a fatal error. 1001 1002 You could include the old header with an absolute pathname: 1003 #include "/usr/include/old-header.h" 1004 This works, but is not clean; should the system headers ever move, 1005 you would have to edit the new headers to match. 1006 1007 There is no way to solve this problem within the C standard, but you 1008 can use the GNU extension `#include_next'. It means, "Include the 1009 _next_ file with this name". This directive works like `#include' 1010 except in searching for the specified file: it starts searching the 1011 list of header file directories _after_ the directory in which the 1012 current file was found. 1013 1014 Suppose you specify `-I /usr/local/include', and the list of 1015 directories to search also includes `/usr/include'; and suppose both 1016 directories contain `signal.h'. Ordinary `#include <signal.h>' finds 1017 the file under `/usr/local/include'. If that file contains 1018 `#include_next <signal.h>', it starts searching after that directory, 1019 and finds the file in `/usr/include'. 1020 1021 `#include_next' does not distinguish between `<FILE>' and `"FILE"' 1022 inclusion, nor does it check that the file you specify has the same 1023 name as the current file. It simply looks for the file named, starting 1024 with the directory in the search path after the one where the current 1025 file was found. 1026 1027 The use of `#include_next' can lead to great confusion. We 1028 recommend it be used only when there is no other alternative. In 1029 particular, it should not be used in the headers belonging to a specific 1030 program; it should be used only to make global corrections along the 1031 lines of `fixincludes'. 1032 1033 1034 File: cpp.info, Node: System Headers, Prev: Wrapper Headers, Up: Header Files 1035 1036 2.8 System Headers 1037 ================== 1038 1039 The header files declaring interfaces to the operating system and 1040 runtime libraries often cannot be written in strictly conforming C. 1041 Therefore, GCC gives code found in "system headers" special treatment. 1042 All warnings, other than those generated by `#warning' (*note 1043 Diagnostics::), are suppressed while GCC is processing a system header. 1044 Macros defined in a system header are immune to a few warnings wherever 1045 they are expanded. This immunity is granted on an ad-hoc basis, when 1046 we find that a warning generates lots of false positives because of 1047 code in macros defined in system headers. 1048 1049 Normally, only the headers found in specific directories are 1050 considered system headers. These directories are determined when GCC 1051 is compiled. There are, however, two ways to make normal headers into 1052 system headers. 1053 1054 The `-isystem' command line option adds its argument to the list of 1055 directories to search for headers, just like `-I'. Any headers found 1056 in that directory will be considered system headers. 1057 1058 All directories named by `-isystem' are searched _after_ all 1059 directories named by `-I', no matter what their order was on the 1060 command line. If the same directory is named by both `-I' and 1061 `-isystem', the `-I' option is ignored. GCC provides an informative 1062 message when this occurs if `-v' is used. 1063 1064 There is also a directive, `#pragma GCC system_header', which tells 1065 GCC to consider the rest of the current include file a system header, 1066 no matter where it was found. Code that comes before the `#pragma' in 1067 the file will not be affected. `#pragma GCC system_header' has no 1068 effect in the primary source file. 1069 1070 On very old systems, some of the pre-defined system header 1071 directories get even more special treatment. GNU C++ considers code in 1072 headers found in those directories to be surrounded by an `extern "C"' 1073 block. There is no way to request this behavior with a `#pragma', or 1074 from the command line. 1075 1076 1077 File: cpp.info, Node: Macros, Next: Conditionals, Prev: Header Files, Up: Top 1078 1079 3 Macros 1080 ******** 1081 1082 A "macro" is a fragment of code which has been given a name. Whenever 1083 the name is used, it is replaced by the contents of the macro. There 1084 are two kinds of macros. They differ mostly in what they look like 1085 when they are used. "Object-like" macros resemble data objects when 1086 used, "function-like" macros resemble function calls. 1087 1088 You may define any valid identifier as a macro, even if it is a C 1089 keyword. The preprocessor does not know anything about keywords. This 1090 can be useful if you wish to hide a keyword such as `const' from an 1091 older compiler that does not understand it. However, the preprocessor 1092 operator `defined' (*note Defined::) can never be defined as a macro, 1093 and C++'s named operators (*note C++ Named Operators::) cannot be 1094 macros when you are compiling C++. 1095 1096 * Menu: 1097 1098 * Object-like Macros:: 1099 * Function-like Macros:: 1100 * Macro Arguments:: 1101 * Stringification:: 1102 * Concatenation:: 1103 * Variadic Macros:: 1104 * Predefined Macros:: 1105 * Undefining and Redefining Macros:: 1106 * Directives Within Macro Arguments:: 1107 * Macro Pitfalls:: 1108 1109 1110 File: cpp.info, Node: Object-like Macros, Next: Function-like Macros, Up: Macros 1111 1112 3.1 Object-like Macros 1113 ====================== 1114 1115 An "object-like macro" is a simple identifier which will be replaced by 1116 a code fragment. It is called object-like because it looks like a data 1117 object in code that uses it. They are most commonly used to give 1118 symbolic names to numeric constants. 1119 1120 You create macros with the `#define' directive. `#define' is 1121 followed by the name of the macro and then the token sequence it should 1122 be an abbreviation for, which is variously referred to as the macro's 1123 "body", "expansion" or "replacement list". For example, 1124 1125 #define BUFFER_SIZE 1024 1126 1127 defines a macro named `BUFFER_SIZE' as an abbreviation for the token 1128 `1024'. If somewhere after this `#define' directive there comes a C 1129 statement of the form 1130 1131 foo = (char *) malloc (BUFFER_SIZE); 1132 1133 then the C preprocessor will recognize and "expand" the macro 1134 `BUFFER_SIZE'. The C compiler will see the same tokens as it would if 1135 you had written 1136 1137 foo = (char *) malloc (1024); 1138 1139 By convention, macro names are written in uppercase. Programs are 1140 easier to read when it is possible to tell at a glance which names are 1141 macros. 1142 1143 The macro's body ends at the end of the `#define' line. You may 1144 continue the definition onto multiple lines, if necessary, using 1145 backslash-newline. When the macro is expanded, however, it will all 1146 come out on one line. For example, 1147 1148 #define NUMBERS 1, \ 1149 2, \ 1150 3 1151 int x[] = { NUMBERS }; 1152 ==> int x[] = { 1, 2, 3 }; 1153 1154 The most common visible consequence of this is surprising line numbers 1155 in error messages. 1156 1157 There is no restriction on what can go in a macro body provided it 1158 decomposes into valid preprocessing tokens. Parentheses need not 1159 balance, and the body need not resemble valid C code. (If it does not, 1160 you may get error messages from the C compiler when you use the macro.) 1161 1162 The C preprocessor scans your program sequentially. Macro 1163 definitions take effect at the place you write them. Therefore, the 1164 following input to the C preprocessor 1165 1166 foo = X; 1167 #define X 4 1168 bar = X; 1169 1170 produces 1171 1172 foo = X; 1173 bar = 4; 1174 1175 When the preprocessor expands a macro name, the macro's expansion 1176 replaces the macro invocation, then the expansion is examined for more 1177 macros to expand. For example, 1178 1179 #define TABLESIZE BUFSIZE 1180 #define BUFSIZE 1024 1181 TABLESIZE 1182 ==> BUFSIZE 1183 ==> 1024 1184 1185 `TABLESIZE' is expanded first to produce `BUFSIZE', then that macro is 1186 expanded to produce the final result, `1024'. 1187 1188 Notice that `BUFSIZE' was not defined when `TABLESIZE' was defined. 1189 The `#define' for `TABLESIZE' uses exactly the expansion you 1190 specify--in this case, `BUFSIZE'--and does not check to see whether it 1191 too contains macro names. Only when you _use_ `TABLESIZE' is the 1192 result of its expansion scanned for more macro names. 1193 1194 This makes a difference if you change the definition of `BUFSIZE' at 1195 some point in the source file. `TABLESIZE', defined as shown, will 1196 always expand using the definition of `BUFSIZE' that is currently in 1197 effect: 1198 1199 #define BUFSIZE 1020 1200 #define TABLESIZE BUFSIZE 1201 #undef BUFSIZE 1202 #define BUFSIZE 37 1203 1204 Now `TABLESIZE' expands (in two stages) to `37'. 1205 1206 If the expansion of a macro contains its own name, either directly or 1207 via intermediate macros, it is not expanded again when the expansion is 1208 examined for more macros. This prevents infinite recursion. *Note 1209 Self-Referential Macros::, for the precise details. 1210 1211 1212 File: cpp.info, Node: Function-like Macros, Next: Macro Arguments, Prev: Object-like Macros, Up: Macros 1213 1214 3.2 Function-like Macros 1215 ======================== 1216 1217 You can also define macros whose use looks like a function call. These 1218 are called "function-like macros". To define a function-like macro, 1219 you use the same `#define' directive, but you put a pair of parentheses 1220 immediately after the macro name. For example, 1221 1222 #define lang_init() c_init() 1223 lang_init() 1224 ==> c_init() 1225 1226 A function-like macro is only expanded if its name appears with a 1227 pair of parentheses after it. If you write just the name, it is left 1228 alone. This can be useful when you have a function and a macro of the 1229 same name, and you wish to use the function sometimes. 1230 1231 extern void foo(void); 1232 #define foo() /* optimized inline version */ 1233 ... 1234 foo(); 1235 funcptr = foo; 1236 1237 Here the call to `foo()' will use the macro, but the function 1238 pointer will get the address of the real function. If the macro were to 1239 be expanded, it would cause a syntax error. 1240 1241 If you put spaces between the macro name and the parentheses in the 1242 macro definition, that does not define a function-like macro, it defines 1243 an object-like macro whose expansion happens to begin with a pair of 1244 parentheses. 1245 1246 #define lang_init () c_init() 1247 lang_init() 1248 ==> () c_init()() 1249 1250 The first two pairs of parentheses in this expansion come from the 1251 macro. The third is the pair that was originally after the macro 1252 invocation. Since `lang_init' is an object-like macro, it does not 1253 consume those parentheses. 1254 1255 1256 File: cpp.info, Node: Macro Arguments, Next: Stringification, Prev: Function-like Macros, Up: Macros 1257 1258 3.3 Macro Arguments 1259 =================== 1260 1261 Function-like macros can take "arguments", just like true functions. 1262 To define a macro that uses arguments, you insert "parameters" between 1263 the pair of parentheses in the macro definition that make the macro 1264 function-like. The parameters must be valid C identifiers, separated 1265 by commas and optionally whitespace. 1266 1267 To invoke a macro that takes arguments, you write the name of the 1268 macro followed by a list of "actual arguments" in parentheses, separated 1269 by commas. The invocation of the macro need not be restricted to a 1270 single logical line--it can cross as many lines in the source file as 1271 you wish. The number of arguments you give must match the number of 1272 parameters in the macro definition. When the macro is expanded, each 1273 use of a parameter in its body is replaced by the tokens of the 1274 corresponding argument. (You need not use all of the parameters in the 1275 macro body.) 1276 1277 As an example, here is a macro that computes the minimum of two 1278 numeric values, as it is defined in many C programs, and some uses. 1279 1280 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1281 x = min(a, b); ==> x = ((a) < (b) ? (a) : (b)); 1282 y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2)); 1283 z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1284 1285 (In this small example you can already see several of the dangers of 1286 macro arguments. *Note Macro Pitfalls::, for detailed explanations.) 1287 1288 Leading and trailing whitespace in each argument is dropped, and all 1289 whitespace between the tokens of an argument is reduced to a single 1290 space. Parentheses within each argument must balance; a comma within 1291 such parentheses does not end the argument. However, there is no 1292 requirement for square brackets or braces to balance, and they do not 1293 prevent a comma from separating arguments. Thus, 1294 1295 macro (array[x = y, x + 1]) 1296 1297 passes two arguments to `macro': `array[x = y' and `x + 1]'. If you 1298 want to supply `array[x = y, x + 1]' as an argument, you can write it 1299 as `array[(x = y, x + 1)]', which is equivalent C code. 1300 1301 All arguments to a macro are completely macro-expanded before they 1302 are substituted into the macro body. After substitution, the complete 1303 text is scanned again for macros to expand, including the arguments. 1304 This rule may seem strange, but it is carefully designed so you need 1305 not worry about whether any function call is actually a macro 1306 invocation. You can run into trouble if you try to be too clever, 1307 though. *Note Argument Prescan::, for detailed discussion. 1308 1309 For example, `min (min (a, b), c)' is first expanded to 1310 1311 min (((a) < (b) ? (a) : (b)), (c)) 1312 1313 and then to 1314 1315 ((((a) < (b) ? (a) : (b))) < (c) 1316 ? (((a) < (b) ? (a) : (b))) 1317 : (c)) 1318 1319 (Line breaks shown here for clarity would not actually be generated.) 1320 1321 You can leave macro arguments empty; this is not an error to the 1322 preprocessor (but many macros will then expand to invalid code). You 1323 cannot leave out arguments entirely; if a macro takes two arguments, 1324 there must be exactly one comma at the top level of its argument list. 1325 Here are some silly examples using `min': 1326 1327 min(, b) ==> (( ) < (b) ? ( ) : (b)) 1328 min(a, ) ==> ((a ) < ( ) ? (a ) : ( )) 1329 min(,) ==> (( ) < ( ) ? ( ) : ( )) 1330 min((,),) ==> (((,)) < ( ) ? ((,)) : ( )) 1331 1332 min() error--> macro "min" requires 2 arguments, but only 1 given 1333 min(,,) error--> macro "min" passed 3 arguments, but takes just 2 1334 1335 Whitespace is not a preprocessing token, so if a macro `foo' takes 1336 one argument, `foo ()' and `foo ( )' both supply it an empty argument. 1337 Previous GNU preprocessor implementations and documentation were 1338 incorrect on this point, insisting that a function-like macro that 1339 takes a single argument be passed a space if an empty argument was 1340 required. 1341 1342 Macro parameters appearing inside string literals are not replaced by 1343 their corresponding actual arguments. 1344 1345 #define foo(x) x, "x" 1346 foo(bar) ==> bar, "x" 1347 1348 1349 File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Macro Arguments, Up: Macros 1350 1351 3.4 Stringification 1352 =================== 1353 1354 Sometimes you may want to convert a macro argument into a string 1355 constant. Parameters are not replaced inside string constants, but you 1356 can use the `#' preprocessing operator instead. When a macro parameter 1357 is used with a leading `#', the preprocessor replaces it with the 1358 literal text of the actual argument, converted to a string constant. 1359 Unlike normal parameter replacement, the argument is not macro-expanded 1360 first. This is called "stringification". 1361 1362 There is no way to combine an argument with surrounding text and 1363 stringify it all together. Instead, you can write a series of adjacent 1364 string constants and stringified arguments. The preprocessor will 1365 replace the stringified arguments with string constants. The C 1366 compiler will then combine all the adjacent string constants into one 1367 long string. 1368 1369 Here is an example of a macro definition that uses stringification: 1370 1371 #define WARN_IF(EXP) \ 1372 do { if (EXP) \ 1373 fprintf (stderr, "Warning: " #EXP "\n"); } \ 1374 while (0) 1375 WARN_IF (x == 0); 1376 ==> do { if (x == 0) 1377 fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0); 1378 1379 The argument for `EXP' is substituted once, as-is, into the `if' 1380 statement, and once, stringified, into the argument to `fprintf'. If 1381 `x' were a macro, it would be expanded in the `if' statement, but not 1382 in the string. 1383 1384 The `do' and `while (0)' are a kludge to make it possible to write 1385 `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function 1386 would make C programmers want to do; see *note Swallowing the 1387 Semicolon::. 1388 1389 Stringification in C involves more than putting double-quote 1390 characters around the fragment. The preprocessor backslash-escapes the 1391 quotes surrounding embedded string constants, and all backslashes 1392 within string and character constants, in order to get a valid C string 1393 constant with the proper contents. Thus, stringifying `p = "foo\n";' 1394 results in "p = \"foo\\n\";". However, backslashes that are not inside 1395 string or character constants are not duplicated: `\n' by itself 1396 stringifies to "\n". 1397 1398 All leading and trailing whitespace in text being stringified is 1399 ignored. Any sequence of whitespace in the middle of the text is 1400 converted to a single space in the stringified result. Comments are 1401 replaced by whitespace long before stringification happens, so they 1402 never appear in stringified text. 1403 1404 There is no way to convert a macro argument into a character 1405 constant. 1406 1407 If you want to stringify the result of expansion of a macro argument, 1408 you have to use two levels of macros. 1409 1410 #define xstr(s) str(s) 1411 #define str(s) #s 1412 #define foo 4 1413 str (foo) 1414 ==> "foo" 1415 xstr (foo) 1416 ==> xstr (4) 1417 ==> str (4) 1418 ==> "4" 1419 1420 `s' is stringified when it is used in `str', so it is not 1421 macro-expanded first. But `s' is an ordinary argument to `xstr', so it 1422 is completely macro-expanded before `xstr' itself is expanded (*note 1423 Argument Prescan::). Therefore, by the time `str' gets to its 1424 argument, it has already been macro-expanded. 1425 1426 1427 File: cpp.info, Node: Concatenation, Next: Variadic Macros, Prev: Stringification, Up: Macros 1428 1429 3.5 Concatenation 1430 ================= 1431 1432 It is often useful to merge two tokens into one while expanding macros. 1433 This is called "token pasting" or "token concatenation". The `##' 1434 preprocessing operator performs token pasting. When a macro is 1435 expanded, the two tokens on either side of each `##' operator are 1436 combined into a single token, which then replaces the `##' and the two 1437 original tokens in the macro expansion. Usually both will be 1438 identifiers, or one will be an identifier and the other a preprocessing 1439 number. When pasted, they make a longer identifier. This isn't the 1440 only valid case. It is also possible to concatenate two numbers (or a 1441 number and a name, such as `1.5' and `e3') into a number. Also, 1442 multi-character operators such as `+=' can be formed by token pasting. 1443 1444 However, two tokens that don't together form a valid token cannot be 1445 pasted together. For example, you cannot concatenate `x' with `+' in 1446 either order. If you try, the preprocessor issues a warning and emits 1447 the two tokens. Whether it puts white space between the tokens is 1448 undefined. It is common to find unnecessary uses of `##' in complex 1449 macros. If you get this warning, it is likely that you can simply 1450 remove the `##'. 1451 1452 Both the tokens combined by `##' could come from the macro body, but 1453 you could just as well write them as one token in the first place. 1454 Token pasting is most useful when one or both of the tokens comes from a 1455 macro argument. If either of the tokens next to an `##' is a parameter 1456 name, it is replaced by its actual argument before `##' executes. As 1457 with stringification, the actual argument is not macro-expanded first. 1458 If the argument is empty, that `##' has no effect. 1459 1460 Keep in mind that the C preprocessor converts comments to whitespace 1461 before macros are even considered. Therefore, you cannot create a 1462 comment by concatenating `/' and `*'. You can put as much whitespace 1463 between `##' and its operands as you like, including comments, and you 1464 can put comments in arguments that will be concatenated. However, it 1465 is an error if `##' appears at either end of a macro body. 1466 1467 Consider a C program that interprets named commands. There probably 1468 needs to be a table of commands, perhaps an array of structures declared 1469 as follows: 1470 1471 struct command 1472 { 1473 char *name; 1474 void (*function) (void); 1475 }; 1476 1477 struct command commands[] = 1478 { 1479 { "quit", quit_command }, 1480 { "help", help_command }, 1481 ... 1482 }; 1483 1484 It would be cleaner not to have to give each command name twice, 1485 once in the string constant and once in the function name. A macro 1486 which takes the name of a command as an argument can make this 1487 unnecessary. The string constant can be created with stringification, 1488 and the function name by concatenating the argument with `_command'. 1489 Here is how it is done: 1490 1491 #define COMMAND(NAME) { #NAME, NAME ## _command } 1492 1493 struct command commands[] = 1494 { 1495 COMMAND (quit), 1496 COMMAND (help), 1497 ... 1498 }; 1499 1500 1501 File: cpp.info, Node: Variadic Macros, Next: Predefined Macros, Prev: Concatenation, Up: Macros 1502 1503 3.6 Variadic Macros 1504 =================== 1505 1506 A macro can be declared to accept a variable number of arguments much as 1507 a function can. The syntax for defining the macro is similar to that of 1508 a function. Here is an example: 1509 1510 #define eprintf(...) fprintf (stderr, __VA_ARGS__) 1511 1512 This kind of macro is called "variadic". When the macro is invoked, 1513 all the tokens in its argument list after the last named argument (this 1514 macro has none), including any commas, become the "variable argument". 1515 This sequence of tokens replaces the identifier `__VA_ARGS__' in the 1516 macro body wherever it appears. Thus, we have this expansion: 1517 1518 eprintf ("%s:%d: ", input_file, lineno) 1519 ==> fprintf (stderr, "%s:%d: ", input_file, lineno) 1520 1521 The variable argument is completely macro-expanded before it is 1522 inserted into the macro expansion, just like an ordinary argument. You 1523 may use the `#' and `##' operators to stringify the variable argument 1524 or to paste its leading or trailing token with another token. (But see 1525 below for an important special case for `##'.) 1526 1527 If your macro is complicated, you may want a more descriptive name 1528 for the variable argument than `__VA_ARGS__'. CPP permits this, as an 1529 extension. You may write an argument name immediately before the 1530 `...'; that name is used for the variable argument. The `eprintf' 1531 macro above could be written 1532 1533 #define eprintf(args...) fprintf (stderr, args) 1534 1535 using this extension. You cannot use `__VA_ARGS__' and this extension 1536 in the same macro. 1537 1538 You can have named arguments as well as variable arguments in a 1539 variadic macro. We could define `eprintf' like this, instead: 1540 1541 #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__) 1542 1543 This formulation looks more descriptive, but unfortunately it is less 1544 flexible: you must now supply at least one argument after the format 1545 string. In standard C, you cannot omit the comma separating the named 1546 argument from the variable arguments. Furthermore, if you leave the 1547 variable argument empty, you will get a syntax error, because there 1548 will be an extra comma after the format string. 1549 1550 eprintf("success!\n", ); 1551 ==> fprintf(stderr, "success!\n", ); 1552 1553 GNU CPP has a pair of extensions which deal with this problem. 1554 First, you are allowed to leave the variable argument out entirely: 1555 1556 eprintf ("success!\n") 1557 ==> fprintf(stderr, "success!\n", ); 1558 1559 Second, the `##' token paste operator has a special meaning when placed 1560 between a comma and a variable argument. If you write 1561 1562 #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__) 1563 1564 and the variable argument is left out when the `eprintf' macro is used, 1565 then the comma before the `##' will be deleted. This does _not_ happen 1566 if you pass an empty argument, nor does it happen if the token 1567 preceding `##' is anything other than a comma. 1568 1569 eprintf ("success!\n") 1570 ==> fprintf(stderr, "success!\n"); 1571 1572 The above explanation is ambiguous about the case where the only macro 1573 parameter is a variable arguments parameter, as it is meaningless to 1574 try to distinguish whether no argument at all is an empty argument or a 1575 missing argument. In this case the C99 standard is clear that the 1576 comma must remain, however the existing GCC extension used to swallow 1577 the comma. So CPP retains the comma when conforming to a specific C 1578 standard, and drops it otherwise. 1579 1580 C99 mandates that the only place the identifier `__VA_ARGS__' can 1581 appear is in the replacement list of a variadic macro. It may not be 1582 used as a macro name, macro argument name, or within a different type 1583 of macro. It may also be forbidden in open text; the standard is 1584 ambiguous. We recommend you avoid using it except for its defined 1585 purpose. 1586 1587 Variadic macros are a new feature in C99. GNU CPP has supported them 1588 for a long time, but only with a named variable argument (`args...', 1589 not `...' and `__VA_ARGS__'). If you are concerned with portability to 1590 previous versions of GCC, you should use only named variable arguments. 1591 On the other hand, if you are concerned with portability to other 1592 conforming implementations of C99, you should use only `__VA_ARGS__'. 1593 1594 Previous versions of CPP implemented the comma-deletion extension 1595 much more generally. We have restricted it in this release to minimize 1596 the differences from C99. To get the same effect with both this and 1597 previous versions of GCC, the token preceding the special `##' must be 1598 a comma, and there must be white space between that comma and whatever 1599 comes immediately before it: 1600 1601 #define eprintf(format, args...) fprintf (stderr, format , ##args) 1602 1603 *Note Differences from previous versions::, for the gory details. 1604 1605 1606 File: cpp.info, Node: Predefined Macros, Next: Undefining and Redefining Macros, Prev: Variadic Macros, Up: Macros 1607 1608 3.7 Predefined Macros 1609 ===================== 1610 1611 Several object-like macros are predefined; you use them without 1612 supplying their definitions. They fall into three classes: standard, 1613 common, and system-specific. 1614 1615 In C++, there is a fourth category, the named operators. They act 1616 like predefined macros, but you cannot undefine them. 1617 1618 * Menu: 1619 1620 * Standard Predefined Macros:: 1621 * Common Predefined Macros:: 1622 * System-specific Predefined Macros:: 1623 * C++ Named Operators:: 1624 1625 1626 File: cpp.info, Node: Standard Predefined Macros, Next: Common Predefined Macros, Up: Predefined Macros 1627 1628 3.7.1 Standard Predefined Macros 1629 -------------------------------- 1630 1631 The standard predefined macros are specified by the relevant language 1632 standards, so they are available with all compilers that implement 1633 those standards. Older compilers may not provide all of them. Their 1634 names all start with double underscores. 1635 1636 `__FILE__' 1637 This macro expands to the name of the current input file, in the 1638 form of a C string constant. This is the path by which the 1639 preprocessor opened the file, not the short name specified in 1640 `#include' or as the input file name argument. For example, 1641 `"/usr/local/include/myheader.h"' is a possible expansion of this 1642 macro. 1643 1644 `__LINE__' 1645 This macro expands to the current input line number, in the form 1646 of a decimal integer constant. While we call it a predefined 1647 macro, it's a pretty strange macro, since its "definition" changes 1648 with each new line of source code. 1649 1650 `__FILE__' and `__LINE__' are useful in generating an error message 1651 to report an inconsistency detected by the program; the message can 1652 state the source line at which the inconsistency was detected. For 1653 example, 1654 1655 fprintf (stderr, "Internal error: " 1656 "negative string length " 1657 "%d at %s, line %d.", 1658 length, __FILE__, __LINE__); 1659 1660 An `#include' directive changes the expansions of `__FILE__' and 1661 `__LINE__' to correspond to the included file. At the end of that 1662 file, when processing resumes on the input file that contained the 1663 `#include' directive, the expansions of `__FILE__' and `__LINE__' 1664 revert to the values they had before the `#include' (but `__LINE__' is 1665 then incremented by one as processing moves to the line after the 1666 `#include'). 1667 1668 A `#line' directive changes `__LINE__', and may change `__FILE__' as 1669 well. *Note Line Control::. 1670 1671 C99 introduces `__func__', and GCC has provided `__FUNCTION__' for a 1672 long time. Both of these are strings containing the name of the 1673 current function (there are slight semantic differences; see the GCC 1674 manual). Neither of them is a macro; the preprocessor does not know the 1675 name of the current function. They tend to be useful in conjunction 1676 with `__FILE__' and `__LINE__', though. 1677 1678 `__DATE__' 1679 This macro expands to a string constant that describes the date on 1680 which the preprocessor is being run. The string constant contains 1681 eleven characters and looks like `"Feb 12 1996"'. If the day of 1682 the month is less than 10, it is padded with a space on the left. 1683 1684 If GCC cannot determine the current date, it will emit a warning 1685 message (once per compilation) and `__DATE__' will expand to 1686 `"??? ?? ????"'. 1687 1688 `__TIME__' 1689 This macro expands to a string constant that describes the time at 1690 which the preprocessor is being run. The string constant contains 1691 eight characters and looks like `"23:59:01"'. 1692 1693 If GCC cannot determine the current time, it will emit a warning 1694 message (once per compilation) and `__TIME__' will expand to 1695 `"??:??:??"'. 1696 1697 `__STDC__' 1698 In normal operation, this macro expands to the constant 1, to 1699 signify that this compiler conforms to ISO Standard C. If GNU CPP 1700 is used with a compiler other than GCC, this is not necessarily 1701 true; however, the preprocessor always conforms to the standard 1702 unless the `-traditional-cpp' option is used. 1703 1704 This macro is not defined if the `-traditional-cpp' option is used. 1705 1706 On some hosts, the system compiler uses a different convention, 1707 where `__STDC__' is normally 0, but is 1 if the user specifies 1708 strict conformance to the C Standard. CPP follows the host 1709 convention when processing system header files, but when 1710 processing user files `__STDC__' is always 1. This has been 1711 reported to cause problems; for instance, some versions of Solaris 1712 provide X Windows headers that expect `__STDC__' to be either 1713 undefined or 1. *Note Invocation::. 1714 1715 `__STDC_VERSION__' 1716 This macro expands to the C Standard's version number, a long 1717 integer constant of the form `YYYYMML' where YYYY and MM are the 1718 year and month of the Standard version. This signifies which 1719 version of the C Standard the compiler conforms to. Like 1720 `__STDC__', this is not necessarily accurate for the entire 1721 implementation, unless GNU CPP is being used with GCC. 1722 1723 The value `199409L' signifies the 1989 C standard as amended in 1724 1994, which is the current default; the value `199901L' signifies 1725 the 1999 revision of the C standard. Support for the 1999 1726 revision is not yet complete. 1727 1728 This macro is not defined if the `-traditional-cpp' option is 1729 used, nor when compiling C++ or Objective-C. 1730 1731 `__STDC_HOSTED__' 1732 This macro is defined, with value 1, if the compiler's target is a 1733 "hosted environment". A hosted environment has the complete 1734 facilities of the standard C library available. 1735 1736 `__cplusplus' 1737 This macro is defined when the C++ compiler is in use. You can use 1738 `__cplusplus' to test whether a header is compiled by a C compiler 1739 or a C++ compiler. This macro is similar to `__STDC_VERSION__', in 1740 that it expands to a version number. A fully conforming 1741 implementation of the 1998 C++ standard will define this macro to 1742 `199711L'. The GNU C++ compiler is not yet fully conforming, so 1743 it uses `1' instead. It is hoped to complete the implementation 1744 of standard C++ in the near future. 1745 1746 `__OBJC__' 1747 This macro is defined, with value 1, when the Objective-C compiler 1748 is in use. You can use `__OBJC__' to test whether a header is 1749 compiled by a C compiler or a Objective-C compiler. 1750 1751 `__ASSEMBLER__' 1752 This macro is defined with value 1 when preprocessing assembly 1753 language. 1754 1755 1756 1757 File: cpp.info, Node: Common Predefined Macros, Next: System-specific Predefined Macros, Prev: Standard Predefined Macros, Up: Predefined Macros 1758 1759 3.7.2 Common Predefined Macros 1760 ------------------------------ 1761 1762 The common predefined macros are GNU C extensions. They are available 1763 with the same meanings regardless of the machine or operating system on 1764 which you are using GNU C or GNU Fortran. Their names all start with 1765 double underscores. 1766 1767 `__COUNTER__' 1768 This macro expands to sequential integral values starting from 0. 1769 In conjunction with the `##' operator, this provides a convenient 1770 means to generate unique identifiers. Care must be taken to 1771 ensure that `__COUNTER__' is not expanded prior to inclusion of 1772 precompiled headers which use it. Otherwise, the precompiled 1773 headers will not be used. 1774 1775 `__GFORTRAN__' 1776 The GNU Fortran compiler defines this. 1777 1778 `__GNUC__' 1779 `__GNUC_MINOR__' 1780 `__GNUC_PATCHLEVEL__' 1781 These macros are defined by all GNU compilers that use the C 1782 preprocessor: C, C++, Objective-C and Fortran. Their values are 1783 the major version, minor version, and patch level of the compiler, 1784 as integer constants. For example, GCC 3.2.1 will define 1785 `__GNUC__' to 3, `__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__' 1786 to 1. These macros are also defined if you invoke the 1787 preprocessor directly. 1788 1789 `__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the 1790 widely-used development snapshots leading up to 3.0 (which identify 1791 themselves as GCC 2.96 or 2.97, depending on which snapshot you 1792 have). 1793 1794 If all you need to know is whether or not your program is being 1795 compiled by GCC, or a non-GCC compiler that claims to accept the 1796 GNU C dialects, you can simply test `__GNUC__'. If you need to 1797 write code which depends on a specific version, you must be more 1798 careful. Each time the minor version is increased, the patch 1799 level is reset to zero; each time the major version is increased 1800 (which happens rarely), the minor version and patch level are 1801 reset. If you wish to use the predefined macros directly in the 1802 conditional, you will need to write it like this: 1803 1804 /* Test for GCC > 3.2.0 */ 1805 #if __GNUC__ > 3 || \ 1806 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1807 (__GNUC_MINOR__ == 2 && \ 1808 __GNUC_PATCHLEVEL__ > 0)) 1809 1810 Another approach is to use the predefined macros to calculate a 1811 single number, then compare that against a threshold: 1812 1813 #define GCC_VERSION (__GNUC__ * 10000 \ 1814 + __GNUC_MINOR__ * 100 \ 1815 + __GNUC_PATCHLEVEL__) 1816 ... 1817 /* Test for GCC > 3.2.0 */ 1818 #if GCC_VERSION > 30200 1819 1820 Many people find this form easier to understand. 1821 1822 `__GNUG__' 1823 The GNU C++ compiler defines this. Testing it is equivalent to 1824 testing `(__GNUC__ && __cplusplus)'. 1825 1826 `__STRICT_ANSI__' 1827 GCC defines this macro if and only if the `-ansi' switch, or a 1828 `-std' switch specifying strict conformance to some version of ISO 1829 C, was specified when GCC was invoked. It is defined to `1'. 1830 This macro exists primarily to direct GNU libc's header files to 1831 restrict their definitions to the minimal set found in the 1989 C 1832 standard. 1833 1834 `__BASE_FILE__' 1835 This macro expands to the name of the main input file, in the form 1836 of a C string constant. This is the source file that was specified 1837 on the command line of the preprocessor or C compiler. 1838 1839 `__INCLUDE_LEVEL__' 1840 This macro expands to a decimal integer constant that represents 1841 the depth of nesting in include files. The value of this macro is 1842 incremented on every `#include' directive and decremented at the 1843 end of every included file. It starts out at 0, its value within 1844 the base file specified on the command line. 1845 1846 `__ELF__' 1847 This macro is defined if the target uses the ELF object format. 1848 1849 `__VERSION__' 1850 This macro expands to a string constant which describes the 1851 version of the compiler in use. You should not rely on its 1852 contents having any particular form, but it can be counted on to 1853 contain at least the release number. 1854 1855 `__OPTIMIZE__' 1856 `__OPTIMIZE_SIZE__' 1857 `__NO_INLINE__' 1858 These macros describe the compilation mode. `__OPTIMIZE__' is 1859 defined in all optimizing compilations. `__OPTIMIZE_SIZE__' is 1860 defined if the compiler is optimizing for size, not speed. 1861 `__NO_INLINE__' is defined if no functions will be inlined into 1862 their callers (when not optimizing, or when inlining has been 1863 specifically disabled by `-fno-inline'). 1864 1865 These macros cause certain GNU header files to provide optimized 1866 definitions, using macros or inline functions, of system library 1867 functions. You should not use these macros in any way unless you 1868 make sure that programs will execute with the same effect whether 1869 or not they are defined. If they are defined, their value is 1. 1870 1871 `__GNUC_GNU_INLINE__' 1872 GCC defines this macro if functions declared `inline' will be 1873 handled in GCC's traditional gnu89 mode. Object files will contain 1874 externally visible definitions of all functions declared `inline' 1875 without `extern' or `static'. They will not contain any 1876 definitions of any functions declared `extern inline'. 1877 1878 `__GNUC_STDC_INLINE__' 1879 GCC defines this macro if functions declared `inline' will be 1880 handled according to the ISO C99 standard. Object files will 1881 contain externally visible definitions of all functions declared 1882 `extern inline'. They will not contain definitions of any 1883 functions declared `inline' without `extern'. 1884 1885 If this macro is defined, GCC supports the `gnu_inline' function 1886 attribute as a way to always get the gnu89 behavior. Support for 1887 this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3. If neither 1888 macro is defined, an older version of GCC is being used: `inline' 1889 functions will be compiled in gnu89 mode, and the `gnu_inline' 1890 function attribute will not be recognized. 1891 1892 `__CHAR_UNSIGNED__' 1893 GCC defines this macro if and only if the data type `char' is 1894 unsigned on the target machine. It exists to cause the standard 1895 header file `limits.h' to work correctly. You should not use this 1896 macro yourself; instead, refer to the standard macros defined in 1897 `limits.h'. 1898 1899 `__WCHAR_UNSIGNED__' 1900 Like `__CHAR_UNSIGNED__', this macro is defined if and only if the 1901 data type `wchar_t' is unsigned and the front-end is in C++ mode. 1902 1903 `__REGISTER_PREFIX__' 1904 This macro expands to a single token (not a string constant) which 1905 is the prefix applied to CPU register names in assembly language 1906 for this target. You can use it to write assembly that is usable 1907 in multiple environments. For example, in the `m68k-aout' 1908 environment it expands to nothing, but in the `m68k-coff' 1909 environment it expands to a single `%'. 1910 1911 `__USER_LABEL_PREFIX__' 1912 This macro expands to a single token which is the prefix applied to 1913 user labels (symbols visible to C code) in assembly. For example, 1914 in the `m68k-aout' environment it expands to an `_', but in the 1915 `m68k-coff' environment it expands to nothing. 1916 1917 This macro will have the correct definition even if 1918 `-f(no-)underscores' is in use, but it will not be correct if 1919 target-specific options that adjust this prefix are used (e.g. the 1920 OSF/rose `-mno-underscores' option). 1921 1922 `__SIZE_TYPE__' 1923 `__PTRDIFF_TYPE__' 1924 `__WCHAR_TYPE__' 1925 `__WINT_TYPE__' 1926 `__INTMAX_TYPE__' 1927 `__UINTMAX_TYPE__' 1928 These macros are defined to the correct underlying types for the 1929 `size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', and 1930 `uintmax_t' typedefs, respectively. They exist to make the 1931 standard header files `stddef.h' and `wchar.h' work correctly. 1932 You should not use these macros directly; instead, include the 1933 appropriate headers and use the typedefs. 1934 1935 `__CHAR_BIT__' 1936 Defined to the number of bits used in the representation of the 1937 `char' data type. It exists to make the standard header given 1938 numerical limits work correctly. You should not use this macro 1939 directly; instead, include the appropriate headers. 1940 1941 `__SCHAR_MAX__' 1942 `__WCHAR_MAX__' 1943 `__SHRT_MAX__' 1944 `__INT_MAX__' 1945 `__LONG_MAX__' 1946 `__LONG_LONG_MAX__' 1947 `__INTMAX_MAX__' 1948 Defined to the maximum value of the `signed char', `wchar_t', 1949 `signed short', `signed int', `signed long', `signed long long', 1950 and `intmax_t' types respectively. They exist to make the 1951 standard header given numerical limits work correctly. You should 1952 not use these macros directly; instead, include the appropriate 1953 headers. 1954 1955 `__SIZEOF_INT__' 1956 `__SIZEOF_LONG__' 1957 `__SIZEOF_LONG_LONG__' 1958 `__SIZEOF_SHORT__' 1959 `__SIZEOF_POINTER__' 1960 `__SIZEOF_FLOAT__' 1961 `__SIZEOF_DOUBLE__' 1962 `__SIZEOF_LONG_DOUBLE__' 1963 `__SIZEOF_SIZE_T__' 1964 `__SIZEOF_WCHAR_T__' 1965 `__SIZEOF_WINT_T__' 1966 `__SIZEOF_PTRDIFF_T__' 1967 Defined to the number of bytes of the C standard data types: `int', 1968 `long', `long long', `short', `void *', `float', `double', `long 1969 double', `size_t', `wchar_t', `wint_t' and `ptrdiff_t'. 1970 1971 `__DEPRECATED' 1972 This macro is defined, with value 1, when compiling a C++ source 1973 file with warnings about deprecated constructs enabled. These 1974 warnings are enabled by default, but can be disabled with 1975 `-Wno-deprecated'. 1976 1977 `__EXCEPTIONS' 1978 This macro is defined, with value 1, when compiling a C++ source 1979 file with exceptions enabled. If `-fno-exceptions' is used when 1980 compiling the file, then this macro is not defined. 1981 1982 `__GXX_RTTI' 1983 This macro is defined, with value 1, when compiling a C++ source 1984 file with runtime type identification enabled. If `-fno-rtti' is 1985 used when compiling the file, then this macro is not defined. 1986 1987 `__USING_SJLJ_EXCEPTIONS__' 1988 This macro is defined, with value 1, if the compiler uses the old 1989 mechanism based on `setjmp' and `longjmp' for exception handling. 1990 1991 `__GXX_EXPERIMENTAL_CXX0X__' 1992 This macro is defined when compiling a C++ source file with the 1993 option `-std=c++0x' or `-std=gnu++0x'. It indicates that some 1994 features likely to be included in C++0x are available. Note that 1995 these features are experimental, and may change or be removed in 1996 future versions of GCC. 1997 1998 `__GXX_WEAK__' 1999 This macro is defined when compiling a C++ source file. It has the 2000 value 1 if the compiler will use weak symbols, COMDAT sections, or 2001 other similar techniques to collapse symbols with "vague linkage" 2002 that are defined in multiple translation units. If the compiler 2003 will not collapse such symbols, this macro is defined with value 2004 0. In general, user code should not need to make use of this 2005 macro; the purpose of this macro is to ease implementation of the 2006 C++ runtime library provided with G++. 2007 2008 `__NEXT_RUNTIME__' 2009 This macro is defined, with value 1, if (and only if) the NeXT 2010 runtime (as in `-fnext-runtime') is in use for Objective-C. If 2011 the GNU runtime is used, this macro is not defined, so that you 2012 can use this macro to determine which runtime (NeXT or GNU) is 2013 being used. 2014 2015 `__LP64__' 2016 `_LP64' 2017 These macros are defined, with value 1, if (and only if) the 2018 compilation is for a target where `long int' and pointer both use 2019 64-bits and `int' uses 32-bit. 2020 2021 `__SSP__' 2022 This macro is defined, with value 1, when `-fstack-protector' is in 2023 use. 2024 2025 `__SSP_ALL__' 2026 This macro is defined, with value 2, when `-fstack-protector-all' 2027 is in use. 2028 2029 `__TIMESTAMP__' 2030 This macro expands to a string constant that describes the date 2031 and time of the last modification of the current source file. The 2032 string constant contains abbreviated day of the week, month, day 2033 of the month, time in hh:mm:ss form, year and looks like 2034 `"Sun Sep 16 01:03:52 1973"'. If the day of the month is less 2035 than 10, it is padded with a space on the left. 2036 2037 If GCC cannot determine the current date, it will emit a warning 2038 message (once per compilation) and `__TIMESTAMP__' will expand to 2039 `"??? ??? ?? ??:??:?? ????"'. 2040 2041 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1' 2042 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2' 2043 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4' 2044 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8' 2045 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16' 2046 These macros are defined when the target processor supports atomic 2047 compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in 2048 length, respectively. 2049 2050 `__GCC_HAVE_DWARF2_CFI_ASM' 2051 This macro is defined when the compiler is emitting Dwarf2 CFI 2052 directives to the assembler. When this is defined, it is possible 2053 to emit those same directives in inline assembly. 2054 2055 2056 File: cpp.info, Node: System-specific Predefined Macros, Next: C++ Named Operators, Prev: Common Predefined Macros, Up: Predefined Macros 2057 2058 3.7.3 System-specific Predefined Macros 2059 --------------------------------------- 2060 2061 The C preprocessor normally predefines several macros that indicate what 2062 type of system and machine is in use. They are obviously different on 2063 each target supported by GCC. This manual, being for all systems and 2064 machines, cannot tell you what their names are, but you can use `cpp 2065 -dM' to see them all. *Note Invocation::. All system-specific 2066 predefined macros expand to the constant 1, so you can test them with 2067 either `#ifdef' or `#if'. 2068 2069 The C standard requires that all system-specific macros be part of 2070 the "reserved namespace". All names which begin with two underscores, 2071 or an underscore and a capital letter, are reserved for the compiler and 2072 library to use as they wish. However, historically system-specific 2073 macros have had names with no special prefix; for instance, it is common 2074 to find `unix' defined on Unix systems. For all such macros, GCC 2075 provides a parallel macro with two underscores added at the beginning 2076 and the end. If `unix' is defined, `__unix__' will be defined too. 2077 There will never be more than two underscores; the parallel of `_mips' 2078 is `__mips__'. 2079 2080 When the `-ansi' option, or any `-std' option that requests strict 2081 conformance, is given to the compiler, all the system-specific 2082 predefined macros outside the reserved namespace are suppressed. The 2083 parallel macros, inside the reserved namespace, remain defined. 2084 2085 We are slowly phasing out all predefined macros which are outside the 2086 reserved namespace. You should never use them in new programs, and we 2087 encourage you to correct older code to use the parallel macros whenever 2088 you find it. We don't recommend you use the system-specific macros that 2089 are in the reserved namespace, either. It is better in the long run to 2090 check specifically for features you need, using a tool such as 2091 `autoconf'. 2092 2093 2094 File: cpp.info, Node: C++ Named Operators, Prev: System-specific Predefined Macros, Up: Predefined Macros 2095 2096 3.7.4 C++ Named Operators 2097 ------------------------- 2098 2099 In C++, there are eleven keywords which are simply alternate spellings 2100 of operators normally written with punctuation. These keywords are 2101 treated as such even in the preprocessor. They function as operators in 2102 `#if', and they cannot be defined as macros or poisoned. In C, you can 2103 request that those keywords take their C++ meaning by including 2104 `iso646.h'. That header defines each one as a normal object-like macro 2105 expanding to the appropriate punctuator. 2106 2107 These are the named operators and their corresponding punctuators: 2108 2109 Named Operator Punctuator 2110 `and' `&&' 2111 `and_eq' `&=' 2112 `bitand' `&' 2113 `bitor' `|' 2114 `compl' `~' 2115 `not' `!' 2116 `not_eq' `!=' 2117 `or' `||' 2118 `or_eq' `|=' 2119 `xor' `^' 2120 `xor_eq' `^=' 2121 2122 2123 File: cpp.info, Node: Undefining and Redefining Macros, Next: Directives Within Macro Arguments, Prev: Predefined Macros, Up: Macros 2124 2125 3.8 Undefining and Redefining Macros 2126 ==================================== 2127 2128 If a macro ceases to be useful, it may be "undefined" with the `#undef' 2129 directive. `#undef' takes a single argument, the name of the macro to 2130 undefine. You use the bare macro name, even if the macro is 2131 function-like. It is an error if anything appears on the line after 2132 the macro name. `#undef' has no effect if the name is not a macro. 2133 2134 #define FOO 4 2135 x = FOO; ==> x = 4; 2136 #undef FOO 2137 x = FOO; ==> x = FOO; 2138 2139 Once a macro has been undefined, that identifier may be "redefined" 2140 as a macro by a subsequent `#define' directive. The new definition 2141 need not have any resemblance to the old definition. 2142 2143 However, if an identifier which is currently a macro is redefined, 2144 then the new definition must be "effectively the same" as the old one. 2145 Two macro definitions are effectively the same if: 2146 * Both are the same type of macro (object- or function-like). 2147 2148 * All the tokens of the replacement list are the same. 2149 2150 * If there are any parameters, they are the same. 2151 2152 * Whitespace appears in the same places in both. It need not be 2153 exactly the same amount of whitespace, though. Remember that 2154 comments count as whitespace. 2155 2156 These definitions are effectively the same: 2157 #define FOUR (2 + 2) 2158 #define FOUR (2 + 2) 2159 #define FOUR (2 /* two */ + 2) 2160 but these are not: 2161 #define FOUR (2 + 2) 2162 #define FOUR ( 2+2 ) 2163 #define FOUR (2 * 2) 2164 #define FOUR(score,and,seven,years,ago) (2 + 2) 2165 2166 If a macro is redefined with a definition that is not effectively the 2167 same as the old one, the preprocessor issues a warning and changes the 2168 macro to use the new definition. If the new definition is effectively 2169 the same, the redefinition is silently ignored. This allows, for 2170 instance, two different headers to define a common macro. The 2171 preprocessor will only complain if the definitions do not match. 2172 2173 2174 File: cpp.info, Node: Directives Within Macro Arguments, Next: Macro Pitfalls, Prev: Undefining and Redefining Macros, Up: Macros 2175 2176 3.9 Directives Within Macro Arguments 2177 ===================================== 2178 2179 Occasionally it is convenient to use preprocessor directives within the 2180 arguments of a macro. The C and C++ standards declare that behavior in 2181 these cases is undefined. 2182 2183 Versions of CPP prior to 3.2 would reject such constructs with an 2184 error message. This was the only syntactic difference between normal 2185 functions and function-like macros, so it seemed attractive to remove 2186 this limitation, and people would often be surprised that they could 2187 not use macros in this way. Moreover, sometimes people would use 2188 conditional compilation in the argument list to a normal library 2189 function like `printf', only to find that after a library upgrade 2190 `printf' had changed to be a function-like macro, and their code would 2191 no longer compile. So from version 3.2 we changed CPP to successfully 2192 process arbitrary directives within macro arguments in exactly the same 2193 way as it would have processed the directive were the function-like 2194 macro invocation not present. 2195 2196 If, within a macro invocation, that macro is redefined, then the new 2197 definition takes effect in time for argument pre-expansion, but the 2198 original definition is still used for argument replacement. Here is a 2199 pathological example: 2200 2201 #define f(x) x x 2202 f (1 2203 #undef f 2204 #define f 2 2205 f) 2206 2207 which expands to 2208 2209 1 2 1 2 2210 2211 with the semantics described above. 2212 2213 2214 File: cpp.info, Node: Macro Pitfalls, Prev: Directives Within Macro Arguments, Up: Macros 2215 2216 3.10 Macro Pitfalls 2217 =================== 2218 2219 In this section we describe some special rules that apply to macros and 2220 macro expansion, and point out certain cases in which the rules have 2221 counter-intuitive consequences that you must watch out for. 2222 2223 * Menu: 2224 2225 * Misnesting:: 2226 * Operator Precedence Problems:: 2227 * Swallowing the Semicolon:: 2228 * Duplication of Side Effects:: 2229 * Self-Referential Macros:: 2230 * Argument Prescan:: 2231 * Newlines in Arguments:: 2232 2233 2234 File: cpp.info, Node: Misnesting, Next: Operator Precedence Problems, Up: Macro Pitfalls 2235 2236 3.10.1 Misnesting 2237 ----------------- 2238 2239 When a macro is called with arguments, the arguments are substituted 2240 into the macro body and the result is checked, together with the rest of 2241 the input file, for more macro calls. It is possible to piece together 2242 a macro call coming partially from the macro body and partially from the 2243 arguments. For example, 2244 2245 #define twice(x) (2*(x)) 2246 #define call_with_1(x) x(1) 2247 call_with_1 (twice) 2248 ==> twice(1) 2249 ==> (2*(1)) 2250 2251 Macro definitions do not have to have balanced parentheses. By 2252 writing an unbalanced open parenthesis in a macro body, it is possible 2253 to create a macro call that begins inside the macro body but ends 2254 outside of it. For example, 2255 2256 #define strange(file) fprintf (file, "%s %d", 2257 ... 2258 strange(stderr) p, 35) 2259 ==> fprintf (stderr, "%s %d", p, 35) 2260 2261 The ability to piece together a macro call can be useful, but the 2262 use of unbalanced open parentheses in a macro body is just confusing, 2263 and should be avoided. 2264 2265 2266 File: cpp.info, Node: Operator Precedence Problems, Next: Swallowing the Semicolon, Prev: Misnesting, Up: Macro Pitfalls 2267 2268 3.10.2 Operator Precedence Problems 2269 ----------------------------------- 2270 2271 You may have noticed that in most of the macro definition examples shown 2272 above, each occurrence of a macro argument name had parentheses around 2273 it. In addition, another pair of parentheses usually surround the 2274 entire macro definition. Here is why it is best to write macros that 2275 way. 2276 2277 Suppose you define a macro as follows, 2278 2279 #define ceil_div(x, y) (x + y - 1) / y 2280 2281 whose purpose is to divide, rounding up. (One use for this operation is 2282 to compute how many `int' objects are needed to hold a certain number 2283 of `char' objects.) Then suppose it is used as follows: 2284 2285 a = ceil_div (b & c, sizeof (int)); 2286 ==> a = (b & c + sizeof (int) - 1) / sizeof (int); 2287 2288 This does not do what is intended. The operator-precedence rules of C 2289 make it equivalent to this: 2290 2291 a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2292 2293 What we want is this: 2294 2295 a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2296 2297 Defining the macro as 2298 2299 #define ceil_div(x, y) ((x) + (y) - 1) / (y) 2300 2301 provides the desired result. 2302 2303 Unintended grouping can result in another way. Consider `sizeof 2304 ceil_div(1, 2)'. That has the appearance of a C expression that would 2305 compute the size of the type of `ceil_div (1, 2)', but in fact it means 2306 something very different. Here is what it expands to: 2307 2308 sizeof ((1) + (2) - 1) / (2) 2309 2310 This would take the size of an integer and divide it by two. The 2311 precedence rules have put the division outside the `sizeof' when it was 2312 intended to be inside. 2313 2314 Parentheses around the entire macro definition prevent such problems. 2315 Here, then, is the recommended way to define `ceil_div': 2316 2317 #define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2318 2319 2320 File: cpp.info, Node: Swallowing the Semicolon, Next: Duplication of Side Effects, Prev: Operator Precedence Problems, Up: Macro Pitfalls 2321 2322 3.10.3 Swallowing the Semicolon 2323 ------------------------------- 2324 2325 Often it is desirable to define a macro that expands into a compound 2326 statement. Consider, for example, the following macro, that advances a 2327 pointer (the argument `p' says where to find it) across whitespace 2328 characters: 2329 2330 #define SKIP_SPACES(p, limit) \ 2331 { char *lim = (limit); \ 2332 while (p < lim) { \ 2333 if (*p++ != ' ') { \ 2334 p--; break; }}} 2335 2336 Here backslash-newline is used to split the macro definition, which must 2337 be a single logical line, so that it resembles the way such code would 2338 be laid out if not part of a macro definition. 2339 2340 A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly 2341 speaking, the call expands to a compound statement, which is a complete 2342 statement with no need for a semicolon to end it. However, since it 2343 looks like a function call, it minimizes confusion if you can use it 2344 like a function call, writing a semicolon afterward, as in `SKIP_SPACES 2345 (p, lim);' 2346 2347 This can cause trouble before `else' statements, because the 2348 semicolon is actually a null statement. Suppose you write 2349 2350 if (*p != 0) 2351 SKIP_SPACES (p, lim); 2352 else ... 2353 2354 The presence of two statements--the compound statement and a null 2355 statement--in between the `if' condition and the `else' makes invalid C 2356 code. 2357 2358 The definition of the macro `SKIP_SPACES' can be altered to solve 2359 this problem, using a `do ... while' statement. Here is how: 2360 2361 #define SKIP_SPACES(p, limit) \ 2362 do { char *lim = (limit); \ 2363 while (p < lim) { \ 2364 if (*p++ != ' ') { \ 2365 p--; break; }}} \ 2366 while (0) 2367 2368 Now `SKIP_SPACES (p, lim);' expands into 2369 2370 do {...} while (0); 2371 2372 which is one statement. The loop executes exactly once; most compilers 2373 generate no extra code for it. 2374 2375 2376 File: cpp.info, Node: Duplication of Side Effects, Next: Self-Referential Macros, Prev: Swallowing the Semicolon, Up: Macro Pitfalls 2377 2378 3.10.4 Duplication of Side Effects 2379 ---------------------------------- 2380 2381 Many C programs define a macro `min', for "minimum", like this: 2382 2383 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2384 2385 When you use this macro with an argument containing a side effect, 2386 as shown here, 2387 2388 next = min (x + y, foo (z)); 2389 2390 it expands as follows: 2391 2392 next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2393 2394 where `x + y' has been substituted for `X' and `foo (z)' for `Y'. 2395 2396 The function `foo' is used only once in the statement as it appears 2397 in the program, but the expression `foo (z)' has been substituted twice 2398 into the macro expansion. As a result, `foo' might be called two times 2399 when the statement is executed. If it has side effects or if it takes 2400 a long time to compute, the results might not be what you intended. We 2401 say that `min' is an "unsafe" macro. 2402 2403 The best solution to this problem is to define `min' in a way that 2404 computes the value of `foo (z)' only once. The C language offers no 2405 standard way to do this, but it can be done with GNU extensions as 2406 follows: 2407 2408 #define min(X, Y) \ 2409 ({ typeof (X) x_ = (X); \ 2410 typeof (Y) y_ = (Y); \ 2411 (x_ < y_) ? x_ : y_; }) 2412 2413 The `({ ... })' notation produces a compound statement that acts as 2414 an expression. Its value is the value of its last statement. This 2415 permits us to define local variables and assign each argument to one. 2416 The local variables have underscores after their names to reduce the 2417 risk of conflict with an identifier of wider scope (it is impossible to 2418 avoid this entirely). Now each argument is evaluated exactly once. 2419 2420 If you do not wish to use GNU C extensions, the only solution is to 2421 be careful when _using_ the macro `min'. For example, you can 2422 calculate the value of `foo (z)', save it in a variable, and use that 2423 variable in `min': 2424 2425 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2426 ... 2427 { 2428 int tem = foo (z); 2429 next = min (x + y, tem); 2430 } 2431 2432 (where we assume that `foo' returns type `int'). 2433 2434 2435 File: cpp.info, Node: Self-Referential Macros, Next: Argument Prescan, Prev: Duplication of Side Effects, Up: Macro Pitfalls 2436 2437 3.10.5 Self-Referential Macros 2438 ------------------------------ 2439 2440 A "self-referential" macro is one whose name appears in its definition. 2441 Recall that all macro definitions are rescanned for more macros to 2442 replace. If the self-reference were considered a use of the macro, it 2443 would produce an infinitely large expansion. To prevent this, the 2444 self-reference is not considered a macro call. It is passed into the 2445 preprocessor output unchanged. Consider an example: 2446 2447 #define foo (4 + foo) 2448 2449 where `foo' is also a variable in your program. 2450 2451 Following the ordinary rules, each reference to `foo' will expand 2452 into `(4 + foo)'; then this will be rescanned and will expand into `(4 2453 + (4 + foo))'; and so on until the computer runs out of memory. 2454 2455 The self-reference rule cuts this process short after one step, at 2456 `(4 + foo)'. Therefore, this macro definition has the possibly useful 2457 effect of causing the program to add 4 to the value of `foo' wherever 2458 `foo' is referred to. 2459 2460 In most cases, it is a bad idea to take advantage of this feature. A 2461 person reading the program who sees that `foo' is a variable will not 2462 expect that it is a macro as well. The reader will come across the 2463 identifier `foo' in the program and think its value should be that of 2464 the variable `foo', whereas in fact the value is four greater. 2465 2466 One common, useful use of self-reference is to create a macro which 2467 expands to itself. If you write 2468 2469 #define EPERM EPERM 2470 2471 then the macro `EPERM' expands to `EPERM'. Effectively, it is left 2472 alone by the preprocessor whenever it's used in running text. You can 2473 tell that it's a macro with `#ifdef'. You might do this if you want to 2474 define numeric constants with an `enum', but have `#ifdef' be true for 2475 each constant. 2476 2477 If a macro `x' expands to use a macro `y', and the expansion of `y' 2478 refers to the macro `x', that is an "indirect self-reference" of `x'. 2479 `x' is not expanded in this case either. Thus, if we have 2480 2481 #define x (4 + y) 2482 #define y (2 * x) 2483 2484 then `x' and `y' expand as follows: 2485 2486 x ==> (4 + y) 2487 ==> (4 + (2 * x)) 2488 2489 y ==> (2 * x) 2490 ==> (2 * (4 + y)) 2491 2492 Each macro is expanded when it appears in the definition of the other 2493 macro, but not when it indirectly appears in its own definition. 2494 2495 2496 File: cpp.info, Node: Argument Prescan, Next: Newlines in Arguments, Prev: Self-Referential Macros, Up: Macro Pitfalls 2497 2498 3.10.6 Argument Prescan 2499 ----------------------- 2500 2501 Macro arguments are completely macro-expanded before they are 2502 substituted into a macro body, unless they are stringified or pasted 2503 with other tokens. After substitution, the entire macro body, including 2504 the substituted arguments, is scanned again for macros to be expanded. 2505 The result is that the arguments are scanned _twice_ to expand macro 2506 calls in them. 2507 2508 Most of the time, this has no effect. If the argument contained any 2509 macro calls, they are expanded during the first scan. The result 2510 therefore contains no macro calls, so the second scan does not change 2511 it. If the argument were substituted as given, with no prescan, the 2512 single remaining scan would find the same macro calls and produce the 2513 same results. 2514 2515 You might expect the double scan to change the results when a 2516 self-referential macro is used in an argument of another macro (*note 2517 Self-Referential Macros::): the self-referential macro would be 2518 expanded once in the first scan, and a second time in the second scan. 2519 However, this is not what happens. The self-references that do not 2520 expand in the first scan are marked so that they will not expand in the 2521 second scan either. 2522 2523 You might wonder, "Why mention the prescan, if it makes no 2524 difference? And why not skip it and make the preprocessor faster?" 2525 The answer is that the prescan does make a difference in three special 2526 cases: 2527 2528 * Nested calls to a macro. 2529 2530 We say that "nested" calls to a macro occur when a macro's argument 2531 contains a call to that very macro. For example, if `f' is a macro 2532 that expects one argument, `f (f (1))' is a nested pair of calls to 2533 `f'. The desired expansion is made by expanding `f (1)' and 2534 substituting that into the definition of `f'. The prescan causes 2535 the expected result to happen. Without the prescan, `f (1)' itself 2536 would be substituted as an argument, and the inner use of `f' would 2537 appear during the main scan as an indirect self-reference and 2538 would not be expanded. 2539 2540 * Macros that call other macros that stringify or concatenate. 2541 2542 If an argument is stringified or concatenated, the prescan does not 2543 occur. If you _want_ to expand a macro, then stringify or 2544 concatenate its expansion, you can do that by causing one macro to 2545 call another macro that does the stringification or concatenation. 2546 For instance, if you have 2547 2548 #define AFTERX(x) X_ ## x 2549 #define XAFTERX(x) AFTERX(x) 2550 #define TABLESIZE 1024 2551 #define BUFSIZE TABLESIZE 2552 2553 then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and 2554 `XAFTERX(BUFSIZE)' expands to `X_1024'. (Not to `X_TABLESIZE'. 2555 Prescan always does a complete expansion.) 2556 2557 * Macros used in arguments, whose expansions contain unshielded 2558 commas. 2559 2560 This can cause a macro expanded on the second scan to be called 2561 with the wrong number of arguments. Here is an example: 2562 2563 #define foo a,b 2564 #define bar(x) lose(x) 2565 #define lose(x) (1 + (x)) 2566 2567 We would like `bar(foo)' to turn into `(1 + (foo))', which would 2568 then turn into `(1 + (a,b))'. Instead, `bar(foo)' expands into 2569 `lose(a,b)', and you get an error because `lose' requires a single 2570 argument. In this case, the problem is easily solved by the same 2571 parentheses that ought to be used to prevent misnesting of 2572 arithmetic operations: 2573 2574 #define foo (a,b) 2575 or 2576 #define bar(x) lose((x)) 2577 2578 The extra pair of parentheses prevents the comma in `foo''s 2579 definition from being interpreted as an argument separator. 2580 2581 2582 2583 File: cpp.info, Node: Newlines in Arguments, Prev: Argument Prescan, Up: Macro Pitfalls 2584 2585 3.10.7 Newlines in Arguments 2586 ---------------------------- 2587 2588 The invocation of a function-like macro can extend over many logical 2589 lines. However, in the present implementation, the entire expansion 2590 comes out on one line. Thus line numbers emitted by the compiler or 2591 debugger refer to the line the invocation started on, which might be 2592 different to the line containing the argument causing the problem. 2593 2594 Here is an example illustrating this: 2595 2596 #define ignore_second_arg(a,b,c) a; c 2597 2598 ignore_second_arg (foo (), 2599 ignored (), 2600 syntax error); 2601 2602 The syntax error triggered by the tokens `syntax error' results in an 2603 error message citing line three--the line of ignore_second_arg-- even 2604 though the problematic code comes from line five. 2605 2606 We consider this a bug, and intend to fix it in the near future. 2607 2608 2609 File: cpp.info, Node: Conditionals, Next: Diagnostics, Prev: Macros, Up: Top 2610 2611 4 Conditionals 2612 ************** 2613 2614 A "conditional" is a directive that instructs the preprocessor to 2615 select whether or not to include a chunk of code in the final token 2616 stream passed to the compiler. Preprocessor conditionals can test 2617 arithmetic expressions, or whether a name is defined as a macro, or both 2618 simultaneously using the special `defined' operator. 2619 2620 A conditional in the C preprocessor resembles in some ways an `if' 2621 statement in C, but it is important to understand the difference between 2622 them. The condition in an `if' statement is tested during the 2623 execution of your program. Its purpose is to allow your program to 2624 behave differently from run to run, depending on the data it is 2625 operating on. The condition in a preprocessing conditional directive is 2626 tested when your program is compiled. Its purpose is to allow different 2627 code to be included in the program depending on the situation at the 2628 time of compilation. 2629 2630 However, the distinction is becoming less clear. Modern compilers 2631 often do test `if' statements when a program is compiled, if their 2632 conditions are known not to vary at run time, and eliminate code which 2633 can never be executed. If you can count on your compiler to do this, 2634 you may find that your program is more readable if you use `if' 2635 statements with constant conditions (perhaps determined by macros). Of 2636 course, you can only use this to exclude code, not type definitions or 2637 other preprocessing directives, and you can only do it if the code 2638 remains syntactically valid when it is not to be used. 2639 2640 GCC version 3 eliminates this kind of never-executed code even when 2641 not optimizing. Older versions did it only when optimizing. 2642 2643 * Menu: 2644 2645 * Conditional Uses:: 2646 * Conditional Syntax:: 2647 * Deleted Code:: 2648 2649 2650 File: cpp.info, Node: Conditional Uses, Next: Conditional Syntax, Up: Conditionals 2651 2652 4.1 Conditional Uses 2653 ==================== 2654 2655 There are three general reasons to use a conditional. 2656 2657 * A program may need to use different code depending on the machine 2658 or operating system it is to run on. In some cases the code for 2659 one operating system may be erroneous on another operating system; 2660 for example, it might refer to data types or constants that do not 2661 exist on the other system. When this happens, it is not enough to 2662 avoid executing the invalid code. Its mere presence will cause 2663 the compiler to reject the program. With a preprocessing 2664 conditional, the offending code can be effectively excised from 2665 the program when it is not valid. 2666 2667 * You may want to be able to compile the same source file into two 2668 different programs. One version might make frequent time-consuming 2669 consistency checks on its intermediate data, or print the values of 2670 those data for debugging, and the other not. 2671 2672 * A conditional whose condition is always false is one way to 2673 exclude code from the program but keep it as a sort of comment for 2674 future reference. 2675 2676 Simple programs that do not need system-specific logic or complex 2677 debugging hooks generally will not need to use preprocessing 2678 conditionals. 2679 2680 2681 File: cpp.info, Node: Conditional Syntax, Next: Deleted Code, Prev: Conditional Uses, Up: Conditionals 2682 2683 4.2 Conditional Syntax 2684 ====================== 2685 2686 A conditional in the C preprocessor begins with a "conditional 2687 directive": `#if', `#ifdef' or `#ifndef'. 2688 2689 * Menu: 2690 2691 * Ifdef:: 2692 * If:: 2693 * Defined:: 2694 * Else:: 2695 * Elif:: 2696 2697 2698 File: cpp.info, Node: Ifdef, Next: If, Up: Conditional Syntax 2699 2700 4.2.1 Ifdef 2701 ----------- 2702 2703 The simplest sort of conditional is 2704 2705 #ifdef MACRO 2706 2707 CONTROLLED TEXT 2708 2709 #endif /* MACRO */ 2710 2711 This block is called a "conditional group". CONTROLLED TEXT will be 2712 included in the output of the preprocessor if and only if MACRO is 2713 defined. We say that the conditional "succeeds" if MACRO is defined, 2714 "fails" if it is not. 2715 2716 The CONTROLLED TEXT inside of a conditional can include 2717 preprocessing directives. They are executed only if the conditional 2718 succeeds. You can nest conditional groups inside other conditional 2719 groups, but they must be completely nested. In other words, `#endif' 2720 always matches the nearest `#ifdef' (or `#ifndef', or `#if'). Also, 2721 you cannot start a conditional group in one file and end it in another. 2722 2723 Even if a conditional fails, the CONTROLLED TEXT inside it is still 2724 run through initial transformations and tokenization. Therefore, it 2725 must all be lexically valid C. Normally the only way this matters is 2726 that all comments and string literals inside a failing conditional group 2727 must still be properly ended. 2728 2729 The comment following the `#endif' is not required, but it is a good 2730 practice if there is a lot of CONTROLLED TEXT, because it helps people 2731 match the `#endif' to the corresponding `#ifdef'. Older programs 2732 sometimes put MACRO directly after the `#endif' without enclosing it in 2733 a comment. This is invalid code according to the C standard. CPP 2734 accepts it with a warning. It never affects which `#ifndef' the 2735 `#endif' matches. 2736 2737 Sometimes you wish to use some code if a macro is _not_ defined. 2738 You can do this by writing `#ifndef' instead of `#ifdef'. One common 2739 use of `#ifndef' is to include code only the first time a header file 2740 is included. *Note Once-Only Headers::. 2741 2742 Macro definitions can vary between compilations for several reasons. 2743 Here are some samples. 2744 2745 * Some macros are predefined on each kind of machine (*note 2746 System-specific Predefined Macros::). This allows you to provide 2747 code specially tuned for a particular machine. 2748 2749 * System header files define more macros, associated with the 2750 features they implement. You can test these macros with 2751 conditionals to avoid using a system feature on a machine where it 2752 is not implemented. 2753 2754 * Macros can be defined or undefined with the `-D' and `-U' command 2755 line options when you compile the program. You can arrange to 2756 compile the same source file into two different programs by 2757 choosing a macro name to specify which program you want, writing 2758 conditionals to test whether or how this macro is defined, and 2759 then controlling the state of the macro with command line options, 2760 perhaps set in the Makefile. *Note Invocation::. 2761 2762 * Your program might have a special header file (often called 2763 `config.h') that is adjusted when the program is compiled. It can 2764 define or not define macros depending on the features of the 2765 system and the desired capabilities of the program. The 2766 adjustment can be automated by a tool such as `autoconf', or done 2767 by hand. 2768 2769 2770 File: cpp.info, Node: If, Next: Defined, Prev: Ifdef, Up: Conditional Syntax 2771 2772 4.2.2 If 2773 -------- 2774 2775 The `#if' directive allows you to test the value of an arithmetic 2776 expression, rather than the mere existence of one macro. Its syntax is 2777 2778 #if EXPRESSION 2779 2780 CONTROLLED TEXT 2781 2782 #endif /* EXPRESSION */ 2783 2784 EXPRESSION is a C expression of integer type, subject to stringent 2785 restrictions. It may contain 2786 2787 * Integer constants. 2788 2789 * Character constants, which are interpreted as they would be in 2790 normal code. 2791 2792 * Arithmetic operators for addition, subtraction, multiplication, 2793 division, bitwise operations, shifts, comparisons, and logical 2794 operations (`&&' and `||'). The latter two obey the usual 2795 short-circuiting rules of standard C. 2796 2797 * Macros. All macros in the expression are expanded before actual 2798 computation of the expression's value begins. 2799 2800 * Uses of the `defined' operator, which lets you check whether macros 2801 are defined in the middle of an `#if'. 2802 2803 * Identifiers that are not macros, which are all considered to be the 2804 number zero. This allows you to write `#if MACRO' instead of 2805 `#ifdef MACRO', if you know that MACRO, when defined, will always 2806 have a nonzero value. Function-like macros used without their 2807 function call parentheses are also treated as zero. 2808 2809 In some contexts this shortcut is undesirable. The `-Wundef' 2810 option causes GCC to warn whenever it encounters an identifier 2811 which is not a macro in an `#if'. 2812 2813 The preprocessor does not know anything about types in the language. 2814 Therefore, `sizeof' operators are not recognized in `#if', and neither 2815 are `enum' constants. They will be taken as identifiers which are not 2816 macros, and replaced by zero. In the case of `sizeof', this is likely 2817 to cause the expression to be invalid. 2818 2819 The preprocessor calculates the value of EXPRESSION. It carries out 2820 all calculations in the widest integer type known to the compiler; on 2821 most machines supported by GCC this is 64 bits. This is not the same 2822 rule as the compiler uses to calculate the value of a constant 2823 expression, and may give different results in some cases. If the value 2824 comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is 2825 included; otherwise it is skipped. 2826 2827 2828 File: cpp.info, Node: Defined, Next: Else, Prev: If, Up: Conditional Syntax 2829 2830 4.2.3 Defined 2831 ------------- 2832 2833 The special operator `defined' is used in `#if' and `#elif' expressions 2834 to test whether a certain name is defined as a macro. `defined NAME' 2835 and `defined (NAME)' are both expressions whose value is 1 if NAME is 2836 defined as a macro at the current point in the program, and 0 2837 otherwise. Thus, `#if defined MACRO' is precisely equivalent to 2838 `#ifdef MACRO'. 2839 2840 `defined' is useful when you wish to test more than one macro for 2841 existence at once. For example, 2842 2843 #if defined (__vax__) || defined (__ns16000__) 2844 2845 would succeed if either of the names `__vax__' or `__ns16000__' is 2846 defined as a macro. 2847 2848 Conditionals written like this: 2849 2850 #if defined BUFSIZE && BUFSIZE >= 1024 2851 2852 can generally be simplified to just `#if BUFSIZE >= 1024', since if 2853 `BUFSIZE' is not defined, it will be interpreted as having the value 2854 zero. 2855 2856 If the `defined' operator appears as a result of a macro expansion, 2857 the C standard says the behavior is undefined. GNU cpp treats it as a 2858 genuine `defined' operator and evaluates it normally. It will warn 2859 wherever your code uses this feature if you use the command-line option 2860 `-pedantic', since other compilers may handle it differently. 2861 2862 2863 File: cpp.info, Node: Else, Next: Elif, Prev: Defined, Up: Conditional Syntax 2864 2865 4.2.4 Else 2866 ---------- 2867 2868 The `#else' directive can be added to a conditional to provide 2869 alternative text to be used if the condition fails. This is what it 2870 looks like: 2871 2872 #if EXPRESSION 2873 TEXT-IF-TRUE 2874 #else /* Not EXPRESSION */ 2875 TEXT-IF-FALSE 2876 #endif /* Not EXPRESSION */ 2877 2878 If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the 2879 TEXT-IF-FALSE is skipped. If EXPRESSION is zero, the opposite happens. 2880 2881 You can use `#else' with `#ifdef' and `#ifndef', too. 2882 2883 2884 File: cpp.info, Node: Elif, Prev: Else, Up: Conditional Syntax 2885 2886 4.2.5 Elif 2887 ---------- 2888 2889 One common case of nested conditionals is used to check for more than 2890 two possible alternatives. For example, you might have 2891 2892 #if X == 1 2893 ... 2894 #else /* X != 1 */ 2895 #if X == 2 2896 ... 2897 #else /* X != 2 */ 2898 ... 2899 #endif /* X != 2 */ 2900 #endif /* X != 1 */ 2901 2902 Another conditional directive, `#elif', allows this to be 2903 abbreviated as follows: 2904 2905 #if X == 1 2906 ... 2907 #elif X == 2 2908 ... 2909 #else /* X != 2 and X != 1*/ 2910 ... 2911 #endif /* X != 2 and X != 1*/ 2912 2913 `#elif' stands for "else if". Like `#else', it goes in the middle 2914 of a conditional group and subdivides it; it does not require a 2915 matching `#endif' of its own. Like `#if', the `#elif' directive 2916 includes an expression to be tested. The text following the `#elif' is 2917 processed only if the original `#if'-condition failed and the `#elif' 2918 condition succeeds. 2919 2920 More than one `#elif' can go in the same conditional group. Then 2921 the text after each `#elif' is processed only if the `#elif' condition 2922 succeeds after the original `#if' and all previous `#elif' directives 2923 within it have failed. 2924 2925 `#else' is allowed after any number of `#elif' directives, but 2926 `#elif' may not follow `#else'. 2927 2928 2929 File: cpp.info, Node: Deleted Code, Prev: Conditional Syntax, Up: Conditionals 2930 2931 4.3 Deleted Code 2932 ================ 2933 2934 If you replace or delete a part of the program but want to keep the old 2935 code around for future reference, you often cannot simply comment it 2936 out. Block comments do not nest, so the first comment inside the old 2937 code will end the commenting-out. The probable result is a flood of 2938 syntax errors. 2939 2940 One way to avoid this problem is to use an always-false conditional 2941 instead. For instance, put `#if 0' before the deleted code and 2942 `#endif' after it. This works even if the code being turned off 2943 contains conditionals, but they must be entire conditionals (balanced 2944 `#if' and `#endif'). 2945 2946 Some people use `#ifdef notdef' instead. This is risky, because 2947 `notdef' might be accidentally defined as a macro, and then the 2948 conditional would succeed. `#if 0' can be counted on to fail. 2949 2950 Do not use `#if 0' for comments which are not C code. Use a real 2951 comment, instead. The interior of `#if 0' must consist of complete 2952 tokens; in particular, single-quote characters must balance. Comments 2953 often contain unbalanced single-quote characters (known in English as 2954 apostrophes). These confuse `#if 0'. They don't confuse `/*'. 2955 2956 2957 File: cpp.info, Node: Diagnostics, Next: Line Control, Prev: Conditionals, Up: Top 2958 2959 5 Diagnostics 2960 ************* 2961 2962 The directive `#error' causes the preprocessor to report a fatal error. 2963 The tokens forming the rest of the line following `#error' are used as 2964 the error message. 2965 2966 You would use `#error' inside of a conditional that detects a 2967 combination of parameters which you know the program does not properly 2968 support. For example, if you know that the program will not run 2969 properly on a VAX, you might write 2970 2971 #ifdef __vax__ 2972 #error "Won't work on VAXen. See comments at get_last_object." 2973 #endif 2974 2975 If you have several configuration parameters that must be set up by 2976 the installation in a consistent way, you can use conditionals to detect 2977 an inconsistency and report it with `#error'. For example, 2978 2979 #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO) 2980 #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP." 2981 #endif 2982 2983 The directive `#warning' is like `#error', but causes the 2984 preprocessor to issue a warning and continue preprocessing. The tokens 2985 following `#warning' are used as the warning message. 2986 2987 You might use `#warning' in obsolete header files, with a message 2988 directing the user to the header file which should be used instead. 2989 2990 Neither `#error' nor `#warning' macro-expands its argument. 2991 Internal whitespace sequences are each replaced with a single space. 2992 The line must consist of complete tokens. It is wisest to make the 2993 argument of these directives be a single string constant; this avoids 2994 problems with apostrophes and the like. 2995 2996 2997 File: cpp.info, Node: Line Control, Next: Pragmas, Prev: Diagnostics, Up: Top 2998 2999 6 Line Control 3000 ************** 3001 3002 The C preprocessor informs the C compiler of the location in your source 3003 code where each token came from. Presently, this is just the file name 3004 and line number. All the tokens resulting from macro expansion are 3005 reported as having appeared on the line of the source file where the 3006 outermost macro was used. We intend to be more accurate in the future. 3007 3008 If you write a program which generates source code, such as the 3009 `bison' parser generator, you may want to adjust the preprocessor's 3010 notion of the current file name and line number by hand. Parts of the 3011 output from `bison' are generated from scratch, other parts come from a 3012 standard parser file. The rest are copied verbatim from `bison''s 3013 input. You would like compiler error messages and symbolic debuggers 3014 to be able to refer to `bison''s input file. 3015 3016 `bison' or any such program can arrange this by writing `#line' 3017 directives into the output file. `#line' is a directive that specifies 3018 the original line number and source file name for subsequent input in 3019 the current preprocessor input file. `#line' has three variants: 3020 3021 `#line LINENUM' 3022 LINENUM is a non-negative decimal integer constant. It specifies 3023 the line number which should be reported for the following line of 3024 input. Subsequent lines are counted from LINENUM. 3025 3026 `#line LINENUM FILENAME' 3027 LINENUM is the same as for the first form, and has the same 3028 effect. In addition, FILENAME is a string constant. The 3029 following line and all subsequent lines are reported to come from 3030 the file it specifies, until something else happens to change that. 3031 FILENAME is interpreted according to the normal rules for a string 3032 constant: backslash escapes are interpreted. This is different 3033 from `#include'. 3034 3035 Previous versions of CPP did not interpret escapes in `#line'; we 3036 have changed it because the standard requires they be interpreted, 3037 and most other compilers do. 3038 3039 `#line ANYTHING ELSE' 3040 ANYTHING ELSE is checked for macro calls, which are expanded. The 3041 result should match one of the above two forms. 3042 3043 `#line' directives alter the results of the `__FILE__' and 3044 `__LINE__' predefined macros from that point on. *Note Standard 3045 Predefined Macros::. They do not have any effect on `#include''s idea 3046 of the directory containing the current file. This is a change from 3047 GCC 2.95. Previously, a file reading 3048 3049 #line 1 "../src/gram.y" 3050 #include "gram.h" 3051 3052 would search for `gram.h' in `../src', then the `-I' chain; the 3053 directory containing the physical source file would not be searched. 3054 In GCC 3.0 and later, the `#include' is not affected by the presence of 3055 a `#line' referring to a different directory. 3056 3057 We made this change because the old behavior caused problems when 3058 generated source files were transported between machines. For instance, 3059 it is common practice to ship generated parsers with a source release, 3060 so that people building the distribution do not need to have yacc or 3061 Bison installed. These files frequently have `#line' directives 3062 referring to the directory tree of the system where the distribution was 3063 created. If GCC tries to search for headers in those directories, the 3064 build is likely to fail. 3065 3066 The new behavior can cause failures too, if the generated file is not 3067 in the same directory as its source and it attempts to include a header 3068 which would be visible searching from the directory containing the 3069 source file. However, this problem is easily solved with an additional 3070 `-I' switch on the command line. The failures caused by the old 3071 semantics could sometimes be corrected only by editing the generated 3072 files, which is difficult and error-prone. 3073 3074 3075 File: cpp.info, Node: Pragmas, Next: Other Directives, Prev: Line Control, Up: Top 3076 3077 7 Pragmas 3078 ********* 3079 3080 The `#pragma' directive is the method specified by the C standard for 3081 providing additional information to the compiler, beyond what is 3082 conveyed in the language itself. Three forms of this directive 3083 (commonly known as "pragmas") are specified by the 1999 C standard. A 3084 C compiler is free to attach any meaning it likes to other pragmas. 3085 3086 GCC has historically preferred to use extensions to the syntax of the 3087 language, such as `__attribute__', for this purpose. However, GCC does 3088 define a few pragmas of its own. These mostly have effects on the 3089 entire translation unit or source file. 3090 3091 In GCC version 3, all GNU-defined, supported pragmas have been given 3092 a `GCC' prefix. This is in line with the `STDC' prefix on all pragmas 3093 defined by C99. For backward compatibility, pragmas which were 3094 recognized by previous versions are still recognized without the `GCC' 3095 prefix, but that usage is deprecated. Some older pragmas are 3096 deprecated in their entirety. They are not recognized with the `GCC' 3097 prefix. *Note Obsolete Features::. 3098 3099 C99 introduces the `_Pragma' operator. This feature addresses a 3100 major problem with `#pragma': being a directive, it cannot be produced 3101 as the result of macro expansion. `_Pragma' is an operator, much like 3102 `sizeof' or `defined', and can be embedded in a macro. 3103 3104 Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can 3105 be either a normal or wide-character string literal. It is 3106 destringized, by replacing all `\\' with a single `\' and all `\"' with 3107 a `"'. The result is then processed as if it had appeared as the right 3108 hand side of a `#pragma' directive. For example, 3109 3110 _Pragma ("GCC dependency \"parse.y\"") 3111 3112 has the same effect as `#pragma GCC dependency "parse.y"'. The same 3113 effect could be achieved using macros, for example 3114 3115 #define DO_PRAGMA(x) _Pragma (#x) 3116 DO_PRAGMA (GCC dependency "parse.y") 3117 3118 The standard is unclear on where a `_Pragma' operator can appear. 3119 The preprocessor does not accept it within a preprocessing conditional 3120 directive like `#if'. To be safe, you are probably best keeping it out 3121 of directives other than `#define', and putting it on a line of its own. 3122 3123 This manual documents the pragmas which are meaningful to the 3124 preprocessor itself. Other pragmas are meaningful to the C or C++ 3125 compilers. They are documented in the GCC manual. 3126 3127 `#pragma GCC dependency' 3128 `#pragma GCC dependency' allows you to check the relative dates of 3129 the current file and another file. If the other file is more 3130 recent than the current file, a warning is issued. This is useful 3131 if the current file is derived from the other file, and should be 3132 regenerated. The other file is searched for using the normal 3133 include search path. Optional trailing text can be used to give 3134 more information in the warning message. 3135 3136 #pragma GCC dependency "parse.y" 3137 #pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3138 3139 `#pragma GCC poison' 3140 Sometimes, there is an identifier that you want to remove 3141 completely from your program, and make sure that it never creeps 3142 back in. To enforce this, you can "poison" the identifier with 3143 this pragma. `#pragma GCC poison' is followed by a list of 3144 identifiers to poison. If any of those identifiers appears 3145 anywhere in the source after the directive, it is a hard error. 3146 For example, 3147 3148 #pragma GCC poison printf sprintf fprintf 3149 sprintf(some_string, "hello"); 3150 3151 will produce an error. 3152 3153 If a poisoned identifier appears as part of the expansion of a 3154 macro which was defined before the identifier was poisoned, it 3155 will _not_ cause an error. This lets you poison an identifier 3156 without worrying about system headers defining macros that use it. 3157 3158 For example, 3159 3160 #define strrchr rindex 3161 #pragma GCC poison rindex 3162 strrchr(some_string, 'h'); 3163 3164 will not produce an error. 3165 3166 `#pragma GCC system_header' 3167 This pragma takes no arguments. It causes the rest of the code in 3168 the current file to be treated as if it came from a system header. 3169 *Note System Headers::. 3170 3171 3172 3173 File: cpp.info, Node: Other Directives, Next: Preprocessor Output, Prev: Pragmas, Up: Top 3174 3175 8 Other Directives 3176 ****************** 3177 3178 The `#ident' directive takes one argument, a string constant. On some 3179 systems, that string constant is copied into a special segment of the 3180 object file. On other systems, the directive is ignored. The `#sccs' 3181 directive is a synonym for `#ident'. 3182 3183 These directives are not part of the C standard, but they are not 3184 official GNU extensions either. What historical information we have 3185 been able to find, suggests they originated with System V. 3186 3187 The "null directive" consists of a `#' followed by a newline, with 3188 only whitespace (including comments) in between. A null directive is 3189 understood as a preprocessing directive but has no effect on the 3190 preprocessor output. The primary significance of the existence of the 3191 null directive is that an input line consisting of just a `#' will 3192 produce no output, rather than a line of output containing just a `#'. 3193 Supposedly some old C programs contain such lines. 3194 3195 3196 File: cpp.info, Node: Preprocessor Output, Next: Traditional Mode, Prev: Other Directives, Up: Top 3197 3198 9 Preprocessor Output 3199 ********************* 3200 3201 When the C preprocessor is used with the C, C++, or Objective-C 3202 compilers, it is integrated into the compiler and communicates a stream 3203 of binary tokens directly to the compiler's parser. However, it can 3204 also be used in the more conventional standalone mode, where it produces 3205 textual output. 3206 3207 The output from the C preprocessor looks much like the input, except 3208 that all preprocessing directive lines have been replaced with blank 3209 lines and all comments with spaces. Long runs of blank lines are 3210 discarded. 3211 3212 The ISO standard specifies that it is implementation defined whether 3213 a preprocessor preserves whitespace between tokens, or replaces it with 3214 e.g. a single space. In GNU CPP, whitespace between tokens is collapsed 3215 to become a single space, with the exception that the first token on a 3216 non-directive line is preceded with sufficient spaces that it appears in 3217 the same column in the preprocessed output that it appeared in the 3218 original source file. This is so the output is easy to read. *Note 3219 Differences from previous versions::. CPP does not insert any 3220 whitespace where there was none in the original source, except where 3221 necessary to prevent an accidental token paste. 3222 3223 Source file name and line number information is conveyed by lines of 3224 the form 3225 3226 # LINENUM FILENAME FLAGS 3227 3228 These are called "linemarkers". They are inserted as needed into the 3229 output (but never within a string or character constant). They mean 3230 that the following line originated in file FILENAME at line LINENUM. 3231 FILENAME will never contain any non-printing characters; they are 3232 replaced with octal escape sequences. 3233 3234 After the file name comes zero or more flags, which are `1', `2', 3235 `3', or `4'. If there are multiple flags, spaces separate them. Here 3236 is what the flags mean: 3237 3238 `1' 3239 This indicates the start of a new file. 3240 3241 `2' 3242 This indicates returning to a file (after having included another 3243 file). 3244 3245 `3' 3246 This indicates that the following text comes from a system header 3247 file, so certain warnings should be suppressed. 3248 3249 `4' 3250 This indicates that the following text should be treated as being 3251 wrapped in an implicit `extern "C"' block. 3252 3253 As an extension, the preprocessor accepts linemarkers in 3254 non-assembler input files. They are treated like the corresponding 3255 `#line' directive, (*note Line Control::), except that trailing flags 3256 are permitted, and are interpreted with the meanings described above. 3257 If multiple flags are given, they must be in ascending order. 3258 3259 Some directives may be duplicated in the output of the preprocessor. 3260 These are `#ident' (always), `#pragma' (only if the preprocessor does 3261 not handle the pragma itself), and `#define' and `#undef' (with certain 3262 debugging options). If this happens, the `#' of the directive will 3263 always be in the first column, and there will be no space between the 3264 `#' and the directive name. If macro expansion happens to generate 3265 tokens which might be mistaken for a duplicated directive, a space will 3266 be inserted between the `#' and the directive name. 3267 3268 3269 File: cpp.info, Node: Traditional Mode, Next: Implementation Details, Prev: Preprocessor Output, Up: Top 3270 3271 10 Traditional Mode 3272 ******************* 3273 3274 Traditional (pre-standard) C preprocessing is rather different from the 3275 preprocessing specified by the standard. When GCC is given the 3276 `-traditional-cpp' option, it attempts to emulate a traditional 3277 preprocessor. 3278 3279 GCC versions 3.2 and later only support traditional mode semantics in 3280 the preprocessor, and not in the compiler front ends. This chapter 3281 outlines the traditional preprocessor semantics we implemented. 3282 3283 The implementation does not correspond precisely to the behavior of 3284 earlier versions of GCC, nor to any true traditional preprocessor. 3285 After all, inconsistencies among traditional implementations were a 3286 major motivation for C standardization. However, we intend that it 3287 should be compatible with true traditional preprocessors in all ways 3288 that actually matter. 3289 3290 * Menu: 3291 3292 * Traditional lexical analysis:: 3293 * Traditional macros:: 3294 * Traditional miscellany:: 3295 * Traditional warnings:: 3296 3297 3298 File: cpp.info, Node: Traditional lexical analysis, Next: Traditional macros, Up: Traditional Mode 3299 3300 10.1 Traditional lexical analysis 3301 ================================= 3302 3303 The traditional preprocessor does not decompose its input into tokens 3304 the same way a standards-conforming preprocessor does. The input is 3305 simply treated as a stream of text with minimal internal form. 3306 3307 This implementation does not treat trigraphs (*note trigraphs::) 3308 specially since they were an invention of the standards committee. It 3309 handles arbitrarily-positioned escaped newlines properly and splices 3310 the lines as you would expect; many traditional preprocessors did not 3311 do this. 3312 3313 The form of horizontal whitespace in the input file is preserved in 3314 the output. In particular, hard tabs remain hard tabs. This can be 3315 useful if, for example, you are preprocessing a Makefile. 3316 3317 Traditional CPP only recognizes C-style block comments, and treats 3318 the `/*' sequence as introducing a comment only if it lies outside 3319 quoted text. Quoted text is introduced by the usual single and double 3320 quotes, and also by an initial `<' in a `#include' directive. 3321 3322 Traditionally, comments are completely removed and are not replaced 3323 with a space. Since a traditional compiler does its own tokenization 3324 of the output of the preprocessor, this means that comments can 3325 effectively be used as token paste operators. However, comments behave 3326 like separators for text handled by the preprocessor itself, since it 3327 doesn't re-lex its input. For example, in 3328 3329 #if foo/**/bar 3330 3331 `foo' and `bar' are distinct identifiers and expanded separately if 3332 they happen to be macros. In other words, this directive is equivalent 3333 to 3334 3335 #if foo bar 3336 3337 rather than 3338 3339 #if foobar 3340 3341 Generally speaking, in traditional mode an opening quote need not 3342 have a matching closing quote. In particular, a macro may be defined 3343 with replacement text that contains an unmatched quote. Of course, if 3344 you attempt to compile preprocessed output containing an unmatched quote 3345 you will get a syntax error. 3346 3347 However, all preprocessing directives other than `#define' require 3348 matching quotes. For example: 3349 3350 #define m This macro's fine and has an unmatched quote 3351 "/* This is not a comment. */ 3352 /* This is a comment. The following #include directive 3353 is ill-formed. */ 3354 #include <stdio.h 3355 3356 Just as for the ISO preprocessor, what would be a closing quote can 3357 be escaped with a backslash to prevent the quoted text from closing. 3358 3359 3360 File: cpp.info, Node: Traditional macros, Next: Traditional miscellany, Prev: Traditional lexical analysis, Up: Traditional Mode 3361 3362 10.2 Traditional macros 3363 ======================= 3364 3365 The major difference between traditional and ISO macros is that the 3366 former expand to text rather than to a token sequence. CPP removes all 3367 leading and trailing horizontal whitespace from a macro's replacement 3368 text before storing it, but preserves the form of internal whitespace. 3369 3370 One consequence is that it is legitimate for the replacement text to 3371 contain an unmatched quote (*note Traditional lexical analysis::). An 3372 unclosed string or character constant continues into the text following 3373 the macro call. Similarly, the text at the end of a macro's expansion 3374 can run together with the text after the macro invocation to produce a 3375 single token. 3376 3377 Normally comments are removed from the replacement text after the 3378 macro is expanded, but if the `-CC' option is passed on the command 3379 line comments are preserved. (In fact, the current implementation 3380 removes comments even before saving the macro replacement text, but it 3381 careful to do it in such a way that the observed effect is identical 3382 even in the function-like macro case.) 3383 3384 The ISO stringification operator `#' and token paste operator `##' 3385 have no special meaning. As explained later, an effect similar to 3386 these operators can be obtained in a different way. Macro names that 3387 are embedded in quotes, either from the main file or after macro 3388 replacement, do not expand. 3389 3390 CPP replaces an unquoted object-like macro name with its replacement 3391 text, and then rescans it for further macros to replace. Unlike 3392 standard macro expansion, traditional macro expansion has no provision 3393 to prevent recursion. If an object-like macro appears unquoted in its 3394 replacement text, it will be replaced again during the rescan pass, and 3395 so on _ad infinitum_. GCC detects when it is expanding recursive 3396 macros, emits an error message, and continues after the offending macro 3397 invocation. 3398 3399 #define PLUS + 3400 #define INC(x) PLUS+x 3401 INC(foo); 3402 ==> ++foo; 3403 3404 Function-like macros are similar in form but quite different in 3405 behavior to their ISO counterparts. Their arguments are contained 3406 within parentheses, are comma-separated, and can cross physical lines. 3407 Commas within nested parentheses are not treated as argument 3408 separators. Similarly, a quote in an argument cannot be left unclosed; 3409 a following comma or parenthesis that comes before the closing quote is 3410 treated like any other character. There is no facility for handling 3411 variadic macros. 3412 3413 This implementation removes all comments from macro arguments, unless 3414 the `-C' option is given. The form of all other horizontal whitespace 3415 in arguments is preserved, including leading and trailing whitespace. 3416 In particular 3417 3418 f( ) 3419 3420 is treated as an invocation of the macro `f' with a single argument 3421 consisting of a single space. If you want to invoke a function-like 3422 macro that takes no arguments, you must not leave any whitespace 3423 between the parentheses. 3424 3425 If a macro argument crosses a new line, the new line is replaced with 3426 a space when forming the argument. If the previous line contained an 3427 unterminated quote, the following line inherits the quoted state. 3428 3429 Traditional preprocessors replace parameters in the replacement text 3430 with their arguments regardless of whether the parameters are within 3431 quotes or not. This provides a way to stringize arguments. For example 3432 3433 #define str(x) "x" 3434 str(/* A comment */some text ) 3435 ==> "some text " 3436 3437 Note that the comment is removed, but that the trailing space is 3438 preserved. Here is an example of using a comment to effect token 3439 pasting. 3440 3441 #define suffix(x) foo_/**/x 3442 suffix(bar) 3443 ==> foo_bar 3444 3445 3446 File: cpp.info, Node: Traditional miscellany, Next: Traditional warnings, Prev: Traditional macros, Up: Traditional Mode 3447 3448 10.3 Traditional miscellany 3449 =========================== 3450 3451 Here are some things to be aware of when using the traditional 3452 preprocessor. 3453 3454 * Preprocessing directives are recognized only when their leading 3455 `#' appears in the first column. There can be no whitespace 3456 between the beginning of the line and the `#', but whitespace can 3457 follow the `#'. 3458 3459 * A true traditional C preprocessor does not recognize `#error' or 3460 `#pragma', and may not recognize `#elif'. CPP supports all the 3461 directives in traditional mode that it supports in ISO mode, 3462 including extensions, with the exception that the effects of 3463 `#pragma GCC poison' are undefined. 3464 3465 * __STDC__ is not defined. 3466 3467 * If you use digraphs the behavior is undefined. 3468 3469 * If a line that looks like a directive appears within macro 3470 arguments, the behavior is undefined. 3471 3472 3473 3474 File: cpp.info, Node: Traditional warnings, Prev: Traditional miscellany, Up: Traditional Mode 3475 3476 10.4 Traditional warnings 3477 ========================= 3478 3479 You can request warnings about features that did not exist, or worked 3480 differently, in traditional C with the `-Wtraditional' option. GCC 3481 does not warn about features of ISO C which you must use when you are 3482 using a conforming compiler, such as the `#' and `##' operators. 3483 3484 Presently `-Wtraditional' warns about: 3485 3486 * Macro parameters that appear within string literals in the macro 3487 body. In traditional C macro replacement takes place within 3488 string literals, but does not in ISO C. 3489 3490 * In traditional C, some preprocessor directives did not exist. 3491 Traditional preprocessors would only consider a line to be a 3492 directive if the `#' appeared in column 1 on the line. Therefore 3493 `-Wtraditional' warns about directives that traditional C 3494 understands but would ignore because the `#' does not appear as the 3495 first character on the line. It also suggests you hide directives 3496 like `#pragma' not understood by traditional C by indenting them. 3497 Some traditional implementations would not recognize `#elif', so it 3498 suggests avoiding it altogether. 3499 3500 * A function-like macro that appears without an argument list. In 3501 some traditional preprocessors this was an error. In ISO C it 3502 merely means that the macro is not expanded. 3503 3504 * The unary plus operator. This did not exist in traditional C. 3505 3506 * The `U' and `LL' integer constant suffixes, which were not 3507 available in traditional C. (Traditional C does support the `L' 3508 suffix for simple long integer constants.) You are not warned 3509 about uses of these suffixes in macros defined in system headers. 3510 For instance, `UINT_MAX' may well be defined as `4294967295U', but 3511 you will not be warned if you use `UINT_MAX'. 3512 3513 You can usually avoid the warning, and the related warning about 3514 constants which are so large that they are unsigned, by writing the 3515 integer constant in question in hexadecimal, with no U suffix. 3516 Take care, though, because this gives the wrong result in exotic 3517 cases. 3518 3519 3520 File: cpp.info, Node: Implementation Details, Next: Invocation, Prev: Traditional Mode, Up: Top 3521 3522 11 Implementation Details 3523 ************************* 3524 3525 Here we document details of how the preprocessor's implementation 3526 affects its user-visible behavior. You should try to avoid undue 3527 reliance on behavior described here, as it is possible that it will 3528 change subtly in future implementations. 3529 3530 Also documented here are obsolete features and changes from previous 3531 versions of CPP. 3532 3533 * Menu: 3534 3535 * Implementation-defined behavior:: 3536 * Implementation limits:: 3537 * Obsolete Features:: 3538 * Differences from previous versions:: 3539 3540 3541 File: cpp.info, Node: Implementation-defined behavior, Next: Implementation limits, Up: Implementation Details 3542 3543 11.1 Implementation-defined behavior 3544 ==================================== 3545 3546 This is how CPP behaves in all the cases which the C standard describes 3547 as "implementation-defined". This term means that the implementation 3548 is free to do what it likes, but must document its choice and stick to 3549 it. 3550 3551 * The mapping of physical source file multi-byte characters to the 3552 execution character set. 3553 3554 The input character set can be specified using the 3555 `-finput-charset' option, while the execution character set may be 3556 controlled using the `-fexec-charset' and `-fwide-exec-charset' 3557 options. 3558 3559 * Identifier characters. The C and C++ standards allow identifiers 3560 to be composed of `_' and the alphanumeric characters. C++ and 3561 C99 also allow universal character names, and C99 further permits 3562 implementation-defined characters. GCC currently only permits 3563 universal character names if `-fextended-identifiers' is used, 3564 because the implementation of universal character names in 3565 identifiers is experimental. 3566 3567 GCC allows the `$' character in identifiers as an extension for 3568 most targets. This is true regardless of the `std=' switch, since 3569 this extension cannot conflict with standards-conforming programs. 3570 When preprocessing assembler, however, dollars are not identifier 3571 characters by default. 3572 3573 Currently the targets that by default do not permit `$' are AVR, 3574 IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 3575 operating system. 3576 3577 You can override the default with `-fdollars-in-identifiers' or 3578 `fno-dollars-in-identifiers'. *Note fdollars-in-identifiers::. 3579 3580 * Non-empty sequences of whitespace characters. 3581 3582 In textual output, each whitespace sequence is collapsed to a 3583 single space. For aesthetic reasons, the first token on each 3584 non-directive line of output is preceded with sufficient spaces 3585 that it appears in the same column as it did in the original 3586 source file. 3587 3588 * The numeric value of character constants in preprocessor 3589 expressions. 3590 3591 The preprocessor and compiler interpret character constants in the 3592 same way; i.e. escape sequences such as `\a' are given the values 3593 they would have on the target machine. 3594 3595 The compiler values a multi-character character constant a 3596 character at a time, shifting the previous value left by the 3597 number of bits per target character, and then or-ing in the 3598 bit-pattern of the new character truncated to the width of a 3599 target character. The final bit-pattern is given type `int', and 3600 is therefore signed, regardless of whether single characters are 3601 signed or not (a slight change from versions 3.1 and earlier of 3602 GCC). If there are more characters in the constant than would fit 3603 in the target `int' the compiler issues a warning, and the excess 3604 leading characters are ignored. 3605 3606 For example, `'ab'' for a target with an 8-bit `char' would be 3607 interpreted as 3608 `(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and 3609 `'\234a'' as 3610 `(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'. 3611 3612 * Source file inclusion. 3613 3614 For a discussion on how the preprocessor locates header files, 3615 *note Include Operation::. 3616 3617 * Interpretation of the filename resulting from a macro-expanded 3618 `#include' directive. 3619 3620 *Note Computed Includes::. 3621 3622 * Treatment of a `#pragma' directive that after macro-expansion 3623 results in a standard pragma. 3624 3625 No macro expansion occurs on any `#pragma' directive line, so the 3626 question does not arise. 3627 3628 Note that GCC does not yet implement any of the standard pragmas. 3629 3630 3631 3632 File: cpp.info, Node: Implementation limits, Next: Obsolete Features, Prev: Implementation-defined behavior, Up: Implementation Details 3633 3634 11.2 Implementation limits 3635 ========================== 3636 3637 CPP has a small number of internal limits. This section lists the 3638 limits which the C standard requires to be no lower than some minimum, 3639 and all the others known. It is intended that there should be as few 3640 limits as possible. If you encounter an undocumented or inconvenient 3641 limit, please report that as a bug. *Note Reporting Bugs: (gcc)Bugs. 3642 3643 Where we say something is limited "only by available memory", that 3644 means that internal data structures impose no intrinsic limit, and space 3645 is allocated with `malloc' or equivalent. The actual limit will 3646 therefore depend on many things, such as the size of other things 3647 allocated by the compiler at the same time, the amount of memory 3648 consumed by other processes on the same computer, etc. 3649 3650 * Nesting levels of `#include' files. 3651 3652 We impose an arbitrary limit of 200 levels, to avoid runaway 3653 recursion. The standard requires at least 15 levels. 3654 3655 * Nesting levels of conditional inclusion. 3656 3657 The C standard mandates this be at least 63. CPP is limited only 3658 by available memory. 3659 3660 * Levels of parenthesized expressions within a full expression. 3661 3662 The C standard requires this to be at least 63. In preprocessor 3663 conditional expressions, it is limited only by available memory. 3664 3665 * Significant initial characters in an identifier or macro name. 3666 3667 The preprocessor treats all characters as significant. The C 3668 standard requires only that the first 63 be significant. 3669 3670 * Number of macros simultaneously defined in a single translation 3671 unit. 3672 3673 The standard requires at least 4095 be possible. CPP is limited 3674 only by available memory. 3675 3676 * Number of parameters in a macro definition and arguments in a 3677 macro call. 3678 3679 We allow `USHRT_MAX', which is no smaller than 65,535. The minimum 3680 required by the standard is 127. 3681 3682 * Number of characters on a logical source line. 3683 3684 The C standard requires a minimum of 4096 be permitted. CPP places 3685 no limits on this, but you may get incorrect column numbers 3686 reported in diagnostics for lines longer than 65,535 characters. 3687 3688 * Maximum size of a source file. 3689 3690 The standard does not specify any lower limit on the maximum size 3691 of a source file. GNU cpp maps files into memory, so it is 3692 limited by the available address space. This is generally at 3693 least two gigabytes. Depending on the operating system, the size 3694 of physical memory may or may not be a limitation. 3695 3696 3697 3698 File: cpp.info, Node: Obsolete Features, Next: Differences from previous versions, Prev: Implementation limits, Up: Implementation Details 3699 3700 11.3 Obsolete Features 3701 ====================== 3702 3703 CPP has some features which are present mainly for compatibility with 3704 older programs. We discourage their use in new code. In some cases, 3705 we plan to remove the feature in a future version of GCC. 3706 3707 11.3.1 Assertions 3708 ----------------- 3709 3710 "Assertions" are a deprecated alternative to macros in writing 3711 conditionals to test what sort of computer or system the compiled 3712 program will run on. Assertions are usually predefined, but you can 3713 define them with preprocessing directives or command-line options. 3714 3715 Assertions were intended to provide a more systematic way to describe 3716 the compiler's target system. However, in practice they are just as 3717 unpredictable as the system-specific predefined macros. In addition, 3718 they are not part of any standard, and only a few compilers support 3719 them. Therefore, the use of assertions is *less* portable than the use 3720 of system-specific predefined macros. We recommend you do not use them 3721 at all. 3722 3723 An assertion looks like this: 3724 3725 #PREDICATE (ANSWER) 3726 3727 PREDICATE must be a single identifier. ANSWER can be any sequence of 3728 tokens; all characters are significant except for leading and trailing 3729 whitespace, and differences in internal whitespace sequences are 3730 ignored. (This is similar to the rules governing macro redefinition.) 3731 Thus, `(x + y)' is different from `(x+y)' but equivalent to 3732 `( x + y )'. Parentheses do not nest inside an answer. 3733 3734 To test an assertion, you write it in an `#if'. For example, this 3735 conditional succeeds if either `vax' or `ns16000' has been asserted as 3736 an answer for `machine'. 3737 3738 #if #machine (vax) || #machine (ns16000) 3739 3740 You can test whether _any_ answer is asserted for a predicate by 3741 omitting the answer in the conditional: 3742 3743 #if #machine 3744 3745 Assertions are made with the `#assert' directive. Its sole argument 3746 is the assertion to make, without the leading `#' that identifies 3747 assertions in conditionals. 3748 3749 #assert PREDICATE (ANSWER) 3750 3751 You may make several assertions with the same predicate and different 3752 answers. Subsequent assertions do not override previous ones for the 3753 same predicate. All the answers for any given predicate are 3754 simultaneously true. 3755 3756 Assertions can be canceled with the `#unassert' directive. It has 3757 the same syntax as `#assert'. In that form it cancels only the answer 3758 which was specified on the `#unassert' line; other answers for that 3759 predicate remain true. You can cancel an entire predicate by leaving 3760 out the answer: 3761 3762 #unassert PREDICATE 3763 3764 In either form, if no such assertion has been made, `#unassert' has no 3765 effect. 3766 3767 You can also make or cancel assertions using command line options. 3768 *Note Invocation::. 3769 3770 3771 File: cpp.info, Node: Differences from previous versions, Prev: Obsolete Features, Up: Implementation Details 3772 3773 11.4 Differences from previous versions 3774 ======================================= 3775 3776 This section details behavior which has changed from previous versions 3777 of CPP. We do not plan to change it again in the near future, but we 3778 do not promise not to, either. 3779 3780 The "previous versions" discussed here are 2.95 and before. The 3781 behavior of GCC 3.0 is mostly the same as the behavior of the widely 3782 used 2.96 and 2.97 development snapshots. Where there are differences, 3783 they generally represent bugs in the snapshots. 3784 3785 * -I- deprecated 3786 3787 This option has been deprecated in 4.0. `-iquote' is meant to 3788 replace the need for this option. 3789 3790 * Order of evaluation of `#' and `##' operators 3791 3792 The standard does not specify the order of evaluation of a chain of 3793 `##' operators, nor whether `#' is evaluated before, after, or at 3794 the same time as `##'. You should therefore not write any code 3795 which depends on any specific ordering. It is possible to 3796 guarantee an ordering, if you need one, by suitable use of nested 3797 macros. 3798 3799 An example of where this might matter is pasting the arguments `1', 3800 `e' and `-2'. This would be fine for left-to-right pasting, but 3801 right-to-left pasting would produce an invalid token `e-2'. 3802 3803 GCC 3.0 evaluates `#' and `##' at the same time and strictly left 3804 to right. Older versions evaluated all `#' operators first, then 3805 all `##' operators, in an unreliable order. 3806 3807 * The form of whitespace between tokens in preprocessor output 3808 3809 *Note Preprocessor Output::, for the current textual format. This 3810 is also the format used by stringification. Normally, the 3811 preprocessor communicates tokens directly to the compiler's 3812 parser, and whitespace does not come up at all. 3813 3814 Older versions of GCC preserved all whitespace provided by the 3815 user and inserted lots more whitespace of their own, because they 3816 could not accurately predict when extra spaces were needed to 3817 prevent accidental token pasting. 3818 3819 * Optional argument when invoking rest argument macros 3820 3821 As an extension, GCC permits you to omit the variable arguments 3822 entirely when you use a variable argument macro. This is 3823 forbidden by the 1999 C standard, and will provoke a pedantic 3824 warning with GCC 3.0. Previous versions accepted it silently. 3825 3826 * `##' swallowing preceding text in rest argument macros 3827 3828 Formerly, in a macro expansion, if `##' appeared before a variable 3829 arguments parameter, and the set of tokens specified for that 3830 argument in the macro invocation was empty, previous versions of 3831 CPP would back up and remove the preceding sequence of 3832 non-whitespace characters (*not* the preceding token). This 3833 extension is in direct conflict with the 1999 C standard and has 3834 been drastically pared back. 3835 3836 In the current version of the preprocessor, if `##' appears between 3837 a comma and a variable arguments parameter, and the variable 3838 argument is omitted entirely, the comma will be removed from the 3839 expansion. If the variable argument is empty, or the token before 3840 `##' is not a comma, then `##' behaves as a normal token paste. 3841 3842 * `#line' and `#include' 3843 3844 The `#line' directive used to change GCC's notion of the 3845 "directory containing the current file", used by `#include' with a 3846 double-quoted header file name. In 3.0 and later, it does not. 3847 *Note Line Control::, for further explanation. 3848 3849 * Syntax of `#line' 3850 3851 In GCC 2.95 and previous, the string constant argument to `#line' 3852 was treated the same way as the argument to `#include': backslash 3853 escapes were not honored, and the string ended at the second `"'. 3854 This is not compliant with the C standard. In GCC 3.0, an attempt 3855 was made to correct the behavior, so that the string was treated 3856 as a real string constant, but it turned out to be buggy. In 3.1, 3857 the bugs have been fixed. (We are not fixing the bugs in 3.0 3858 because they affect relatively few people and the fix is quite 3859 invasive.) 3860 3861 3862 3863 File: cpp.info, Node: Invocation, Next: Environment Variables, Prev: Implementation Details, Up: Top 3864 3865 12 Invocation 3866 ************* 3867 3868 Most often when you use the C preprocessor you will not have to invoke 3869 it explicitly: the C compiler will do so automatically. However, the 3870 preprocessor is sometimes useful on its own. All the options listed 3871 here are also acceptable to the C compiler and have the same meaning, 3872 except that the C compiler has different rules for specifying the output 3873 file. 3874 3875 _Note:_ Whether you use the preprocessor by way of `gcc' or `cpp', 3876 the "compiler driver" is run first. This program's purpose is to 3877 translate your command into invocations of the programs that do the 3878 actual work. Their command line interfaces are similar but not 3879 identical to the documented interface, and may change without notice. 3880 3881 The C preprocessor expects two file names as arguments, INFILE and 3882 OUTFILE. The preprocessor reads INFILE together with any other files 3883 it specifies with `#include'. All the output generated by the combined 3884 input files is written in OUTFILE. 3885 3886 Either INFILE or OUTFILE may be `-', which as INFILE means to read 3887 from standard input and as OUTFILE means to write to standard output. 3888 Also, if either file is omitted, it means the same as if `-' had been 3889 specified for that file. 3890 3891 Unless otherwise noted, or the option ends in `=', all options which 3892 take an argument may have that argument appear either immediately after 3893 the option, or with a space between option and argument: `-Ifoo' and 3894 `-I foo' have the same effect. 3895 3896 Many options have multi-letter names; therefore multiple 3897 single-letter options may _not_ be grouped: `-dM' is very different from 3898 `-d -M'. 3899 3900 `-D NAME' 3901 Predefine NAME as a macro, with definition `1'. 3902 3903 `-D NAME=DEFINITION' 3904 The contents of DEFINITION are tokenized and processed as if they 3905 appeared during translation phase three in a `#define' directive. 3906 In particular, the definition will be truncated by embedded 3907 newline characters. 3908 3909 If you are invoking the preprocessor from a shell or shell-like 3910 program you may need to use the shell's quoting syntax to protect 3911 characters such as spaces that have a meaning in the shell syntax. 3912 3913 If you wish to define a function-like macro on the command line, 3914 write its argument list with surrounding parentheses before the 3915 equals sign (if any). Parentheses are meaningful to most shells, 3916 so you will need to quote the option. With `sh' and `csh', 3917 `-D'NAME(ARGS...)=DEFINITION'' works. 3918 3919 `-D' and `-U' options are processed in the order they are given on 3920 the command line. All `-imacros FILE' and `-include FILE' options 3921 are processed after all `-D' and `-U' options. 3922 3923 `-U NAME' 3924 Cancel any previous definition of NAME, either built in or 3925 provided with a `-D' option. 3926 3927 `-undef' 3928 Do not predefine any system-specific or GCC-specific macros. The 3929 standard predefined macros remain defined. *Note Standard 3930 Predefined Macros::. 3931 3932 `-I DIR' 3933 Add the directory DIR to the list of directories to be searched 3934 for header files. *Note Search Path::. Directories named by `-I' 3935 are searched before the standard system include directories. If 3936 the directory DIR is a standard system include directory, the 3937 option is ignored to ensure that the default search order for 3938 system directories and the special treatment of system headers are 3939 not defeated (*note System Headers::) . If DIR begins with `=', 3940 then the `=' will be replaced by the sysroot prefix; see 3941 `--sysroot' and `-isysroot'. 3942 3943 `-o FILE' 3944 Write output to FILE. This is the same as specifying FILE as the 3945 second non-option argument to `cpp'. `gcc' has a different 3946 interpretation of a second non-option argument, so you must use 3947 `-o' to specify the output file. 3948 3949 `-Wall' 3950 Turns on all optional warnings which are desirable for normal code. 3951 At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a 3952 warning about integer promotion causing a change of sign in `#if' 3953 expressions. Note that many of the preprocessor's warnings are on 3954 by default and have no options to control them. 3955 3956 `-Wcomment' 3957 `-Wcomments' 3958 Warn whenever a comment-start sequence `/*' appears in a `/*' 3959 comment, or whenever a backslash-newline appears in a `//' comment. 3960 (Both forms have the same effect.) 3961 3962 `-Wtrigraphs' 3963 Most trigraphs in comments cannot affect the meaning of the 3964 program. However, a trigraph that would form an escaped newline 3965 (`??/' at the end of a line) can, by changing where the comment 3966 begins or ends. Therefore, only trigraphs that would form escaped 3967 newlines produce warnings inside a comment. 3968 3969 This option is implied by `-Wall'. If `-Wall' is not given, this 3970 option is still enabled unless trigraphs are enabled. To get 3971 trigraph conversion without warnings, but get the other `-Wall' 3972 warnings, use `-trigraphs -Wall -Wno-trigraphs'. 3973 3974 `-Wtraditional' 3975 Warn about certain constructs that behave differently in 3976 traditional and ISO C. Also warn about ISO C constructs that have 3977 no traditional C equivalent, and problematic constructs which 3978 should be avoided. *Note Traditional Mode::. 3979 3980 `-Wundef' 3981 Warn whenever an identifier which is not a macro is encountered in 3982 an `#if' directive, outside of `defined'. Such identifiers are 3983 replaced with zero. 3984 3985 `-Wunused-macros' 3986 Warn about macros defined in the main file that are unused. A 3987 macro is "used" if it is expanded or tested for existence at least 3988 once. The preprocessor will also warn if the macro has not been 3989 used at the time it is redefined or undefined. 3990 3991 Built-in macros, macros defined on the command line, and macros 3992 defined in include files are not warned about. 3993 3994 _Note:_ If a macro is actually used, but only used in skipped 3995 conditional blocks, then CPP will report it as unused. To avoid 3996 the warning in such a case, you might improve the scope of the 3997 macro's definition by, for example, moving it into the first 3998 skipped block. Alternatively, you could provide a dummy use with 3999 something like: 4000 4001 #if defined the_macro_causing_the_warning 4002 #endif 4003 4004 `-Wendif-labels' 4005 Warn whenever an `#else' or an `#endif' are followed by text. 4006 This usually happens in code of the form 4007 4008 #if FOO 4009 ... 4010 #else FOO 4011 ... 4012 #endif FOO 4013 4014 The second and third `FOO' should be in comments, but often are not 4015 in older programs. This warning is on by default. 4016 4017 `-Werror' 4018 Make all warnings into hard errors. Source code which triggers 4019 warnings will be rejected. 4020 4021 `-Wsystem-headers' 4022 Issue warnings for code in system headers. These are normally 4023 unhelpful in finding bugs in your own code, therefore suppressed. 4024 If you are responsible for the system library, you may want to see 4025 them. 4026 4027 `-w' 4028 Suppress all warnings, including those which GNU CPP issues by 4029 default. 4030 4031 `-pedantic' 4032 Issue all the mandatory diagnostics listed in the C standard. 4033 Some of them are left out by default, since they trigger 4034 frequently on harmless code. 4035 4036 `-pedantic-errors' 4037 Issue all the mandatory diagnostics, and make all mandatory 4038 diagnostics into errors. This includes mandatory diagnostics that 4039 GCC issues without `-pedantic' but treats as warnings. 4040 4041 `-M' 4042 Instead of outputting the result of preprocessing, output a rule 4043 suitable for `make' describing the dependencies of the main source 4044 file. The preprocessor outputs one `make' rule containing the 4045 object file name for that source file, a colon, and the names of 4046 all the included files, including those coming from `-include' or 4047 `-imacros' command line options. 4048 4049 Unless specified explicitly (with `-MT' or `-MQ'), the object file 4050 name consists of the name of the source file with any suffix 4051 replaced with object file suffix and with any leading directory 4052 parts removed. If there are many included files then the rule is 4053 split into several lines using `\'-newline. The rule has no 4054 commands. 4055 4056 This option does not suppress the preprocessor's debug output, 4057 such as `-dM'. To avoid mixing such debug output with the 4058 dependency rules you should explicitly specify the dependency 4059 output file with `-MF', or use an environment variable like 4060 `DEPENDENCIES_OUTPUT' (*note Environment Variables::). Debug 4061 output will still be sent to the regular output stream as normal. 4062 4063 Passing `-M' to the driver implies `-E', and suppresses warnings 4064 with an implicit `-w'. 4065 4066 `-MM' 4067 Like `-M' but do not mention header files that are found in system 4068 header directories, nor header files that are included, directly 4069 or indirectly, from such a header. 4070 4071 This implies that the choice of angle brackets or double quotes in 4072 an `#include' directive does not in itself determine whether that 4073 header will appear in `-MM' dependency output. This is a slight 4074 change in semantics from GCC versions 3.0 and earlier. 4075 4076 `-MF FILE' 4077 When used with `-M' or `-MM', specifies a file to write the 4078 dependencies to. If no `-MF' switch is given the preprocessor 4079 sends the rules to the same place it would have sent preprocessed 4080 output. 4081 4082 When used with the driver options `-MD' or `-MMD', `-MF' overrides 4083 the default dependency output file. 4084 4085 `-MG' 4086 In conjunction with an option such as `-M' requesting dependency 4087 generation, `-MG' assumes missing header files are generated files 4088 and adds them to the dependency list without raising an error. 4089 The dependency filename is taken directly from the `#include' 4090 directive without prepending any path. `-MG' also suppresses 4091 preprocessed output, as a missing header file renders this useless. 4092 4093 This feature is used in automatic updating of makefiles. 4094 4095 `-MP' 4096 This option instructs CPP to add a phony target for each dependency 4097 other than the main file, causing each to depend on nothing. These 4098 dummy rules work around errors `make' gives if you remove header 4099 files without updating the `Makefile' to match. 4100 4101 This is typical output: 4102 4103 test.o: test.c test.h 4104 4105 test.h: 4106 4107 `-MT TARGET' 4108 Change the target of the rule emitted by dependency generation. By 4109 default CPP takes the name of the main input file, deletes any 4110 directory components and any file suffix such as `.c', and appends 4111 the platform's usual object suffix. The result is the target. 4112 4113 An `-MT' option will set the target to be exactly the string you 4114 specify. If you want multiple targets, you can specify them as a 4115 single argument to `-MT', or use multiple `-MT' options. 4116 4117 For example, `-MT '$(objpfx)foo.o'' might give 4118 4119 $(objpfx)foo.o: foo.c 4120 4121 `-MQ TARGET' 4122 Same as `-MT', but it quotes any characters which are special to 4123 Make. `-MQ '$(objpfx)foo.o'' gives 4124 4125 $$(objpfx)foo.o: foo.c 4126 4127 The default target is automatically quoted, as if it were given 4128 with `-MQ'. 4129 4130 `-MD' 4131 `-MD' is equivalent to `-M -MF FILE', except that `-E' is not 4132 implied. The driver determines FILE based on whether an `-o' 4133 option is given. If it is, the driver uses its argument but with 4134 a suffix of `.d', otherwise it takes the name of the input file, 4135 removes any directory components and suffix, and applies a `.d' 4136 suffix. 4137 4138 If `-MD' is used in conjunction with `-E', any `-o' switch is 4139 understood to specify the dependency output file (*note -MF: 4140 dashMF.), but if used without `-E', each `-o' is understood to 4141 specify a target object file. 4142 4143 Since `-E' is not implied, `-MD' can be used to generate a 4144 dependency output file as a side-effect of the compilation process. 4145 4146 `-MMD' 4147 Like `-MD' except mention only user header files, not system 4148 header files. 4149 4150 `-x c' 4151 `-x c++' 4152 `-x objective-c' 4153 `-x assembler-with-cpp' 4154 Specify the source language: C, C++, Objective-C, or assembly. 4155 This has nothing to do with standards conformance or extensions; 4156 it merely selects which base syntax to expect. If you give none 4157 of these options, cpp will deduce the language from the extension 4158 of the source file: `.c', `.cc', `.m', or `.S'. Some other common 4159 extensions for C++ and assembly are also recognized. If cpp does 4160 not recognize the extension, it will treat the file as C; this is 4161 the most generic mode. 4162 4163 _Note:_ Previous versions of cpp accepted a `-lang' option which 4164 selected both the language and the standards conformance level. 4165 This option has been removed, because it conflicts with the `-l' 4166 option. 4167 4168 `-std=STANDARD' 4169 `-ansi' 4170 Specify the standard to which the code should conform. Currently 4171 CPP knows about C and C++ standards; others may be added in the 4172 future. 4173 4174 STANDARD may be one of: 4175 `iso9899:1990' 4176 `c89' 4177 The ISO C standard from 1990. `c89' is the customary 4178 shorthand for this version of the standard. 4179 4180 The `-ansi' option is equivalent to `-std=c89'. 4181 4182 `iso9899:199409' 4183 The 1990 C standard, as amended in 1994. 4184 4185 `iso9899:1999' 4186 `c99' 4187 `iso9899:199x' 4188 `c9x' 4189 The revised ISO C standard, published in December 1999. 4190 Before publication, this was known as C9X. 4191 4192 `gnu89' 4193 The 1990 C standard plus GNU extensions. This is the default. 4194 4195 `gnu99' 4196 `gnu9x' 4197 The 1999 C standard plus GNU extensions. 4198 4199 `c++98' 4200 The 1998 ISO C++ standard plus amendments. 4201 4202 `gnu++98' 4203 The same as `-std=c++98' plus GNU extensions. This is the 4204 default for C++ code. 4205 4206 `-I-' 4207 Split the include path. Any directories specified with `-I' 4208 options before `-I-' are searched only for headers requested with 4209 `#include "FILE"'; they are not searched for `#include <FILE>'. 4210 If additional directories are specified with `-I' options after 4211 the `-I-', those directories are searched for all `#include' 4212 directives. 4213 4214 In addition, `-I-' inhibits the use of the directory of the current 4215 file directory as the first search directory for `#include "FILE"'. 4216 *Note Search Path::. This option has been deprecated. 4217 4218 `-nostdinc' 4219 Do not search the standard system directories for header files. 4220 Only the directories you have specified with `-I' options (and the 4221 directory of the current file, if appropriate) are searched. 4222 4223 `-nostdinc++' 4224 Do not search for header files in the C++-specific standard 4225 directories, but do still search the other standard directories. 4226 (This option is used when building the C++ library.) 4227 4228 `-include FILE' 4229 Process FILE as if `#include "file"' appeared as the first line of 4230 the primary source file. However, the first directory searched 4231 for FILE is the preprocessor's working directory _instead of_ the 4232 directory containing the main source file. If not found there, it 4233 is searched for in the remainder of the `#include "..."' search 4234 chain as normal. 4235 4236 If multiple `-include' options are given, the files are included 4237 in the order they appear on the command line. 4238 4239 `-imacros FILE' 4240 Exactly like `-include', except that any output produced by 4241 scanning FILE is thrown away. Macros it defines remain defined. 4242 This allows you to acquire all the macros from a header without 4243 also processing its declarations. 4244 4245 All files specified by `-imacros' are processed before all files 4246 specified by `-include'. 4247 4248 `-idirafter DIR' 4249 Search DIR for header files, but do it _after_ all directories 4250 specified with `-I' and the standard system directories have been 4251 exhausted. DIR is treated as a system include directory. If DIR 4252 begins with `=', then the `=' will be replaced by the sysroot 4253 prefix; see `--sysroot' and `-isysroot'. 4254 4255 `-iprefix PREFIX' 4256 Specify PREFIX as the prefix for subsequent `-iwithprefix' 4257 options. If the prefix represents a directory, you should include 4258 the final `/'. 4259 4260 `-iwithprefix DIR' 4261 `-iwithprefixbefore DIR' 4262 Append DIR to the prefix specified previously with `-iprefix', and 4263 add the resulting directory to the include search path. 4264 `-iwithprefixbefore' puts it in the same place `-I' would; 4265 `-iwithprefix' puts it where `-idirafter' would. 4266 4267 `-isysroot DIR' 4268 This option is like the `--sysroot' option, but applies only to 4269 header files. See the `--sysroot' option for more information. 4270 4271 `-imultilib DIR' 4272 Use DIR as a subdirectory of the directory containing 4273 target-specific C++ headers. 4274 4275 `-isystem DIR' 4276 Search DIR for header files, after all directories specified by 4277 `-I' but before the standard system directories. Mark it as a 4278 system directory, so that it gets the same special treatment as is 4279 applied to the standard system directories. *Note System 4280 Headers::. If DIR begins with `=', then the `=' will be replaced 4281 by the sysroot prefix; see `--sysroot' and `-isysroot'. 4282 4283 `-iquote DIR' 4284 Search DIR only for header files requested with `#include "FILE"'; 4285 they are not searched for `#include <FILE>', before all 4286 directories specified by `-I' and before the standard system 4287 directories. *Note Search Path::. If DIR begins with `=', then 4288 the `=' will be replaced by the sysroot prefix; see `--sysroot' 4289 and `-isysroot'. 4290 4291 `-fdirectives-only' 4292 When preprocessing, handle directives, but do not expand macros. 4293 4294 The option's behavior depends on the `-E' and `-fpreprocessed' 4295 options. 4296 4297 With `-E', preprocessing is limited to the handling of directives 4298 such as `#define', `#ifdef', and `#error'. Other preprocessor 4299 operations, such as macro expansion and trigraph conversion are 4300 not performed. In addition, the `-dD' option is implicitly 4301 enabled. 4302 4303 With `-fpreprocessed', predefinition of command line and most 4304 builtin macros is disabled. Macros such as `__LINE__', which are 4305 contextually dependent, are handled normally. This enables 4306 compilation of files previously preprocessed with `-E 4307 -fdirectives-only'. 4308 4309 With both `-E' and `-fpreprocessed', the rules for 4310 `-fpreprocessed' take precedence. This enables full preprocessing 4311 of files previously preprocessed with `-E -fdirectives-only'. 4312 4313 `-fdollars-in-identifiers' 4314 Accept `$' in identifiers. *Note Identifier characters::. 4315 4316 `-fextended-identifiers' 4317 Accept universal character names in identifiers. This option is 4318 experimental; in a future version of GCC, it will be enabled by 4319 default for C99 and C++. 4320 4321 `-fpreprocessed' 4322 Indicate to the preprocessor that the input file has already been 4323 preprocessed. This suppresses things like macro expansion, 4324 trigraph conversion, escaped newline splicing, and processing of 4325 most directives. The preprocessor still recognizes and removes 4326 comments, so that you can pass a file preprocessed with `-C' to 4327 the compiler without problems. In this mode the integrated 4328 preprocessor is little more than a tokenizer for the front ends. 4329 4330 `-fpreprocessed' is implicit if the input file has one of the 4331 extensions `.i', `.ii' or `.mi'. These are the extensions that 4332 GCC uses for preprocessed files created by `-save-temps'. 4333 4334 `-ftabstop=WIDTH' 4335 Set the distance between tab stops. This helps the preprocessor 4336 report correct column numbers in warnings or errors, even if tabs 4337 appear on the line. If the value is less than 1 or greater than 4338 100, the option is ignored. The default is 8. 4339 4340 `-fexec-charset=CHARSET' 4341 Set the execution character set, used for string and character 4342 constants. The default is UTF-8. CHARSET can be any encoding 4343 supported by the system's `iconv' library routine. 4344 4345 `-fwide-exec-charset=CHARSET' 4346 Set the wide execution character set, used for wide string and 4347 character constants. The default is UTF-32 or UTF-16, whichever 4348 corresponds to the width of `wchar_t'. As with `-fexec-charset', 4349 CHARSET can be any encoding supported by the system's `iconv' 4350 library routine; however, you will have problems with encodings 4351 that do not fit exactly in `wchar_t'. 4352 4353 `-finput-charset=CHARSET' 4354 Set the input character set, used for translation from the 4355 character set of the input file to the source character set used 4356 by GCC. If the locale does not specify, or GCC cannot get this 4357 information from the locale, the default is UTF-8. This can be 4358 overridden by either the locale or this command line option. 4359 Currently the command line option takes precedence if there's a 4360 conflict. CHARSET can be any encoding supported by the system's 4361 `iconv' library routine. 4362 4363 `-fworking-directory' 4364 Enable generation of linemarkers in the preprocessor output that 4365 will let the compiler know the current working directory at the 4366 time of preprocessing. When this option is enabled, the 4367 preprocessor will emit, after the initial linemarker, a second 4368 linemarker with the current working directory followed by two 4369 slashes. GCC will use this directory, when it's present in the 4370 preprocessed input, as the directory emitted as the current 4371 working directory in some debugging information formats. This 4372 option is implicitly enabled if debugging information is enabled, 4373 but this can be inhibited with the negated form 4374 `-fno-working-directory'. If the `-P' flag is present in the 4375 command line, this option has no effect, since no `#line' 4376 directives are emitted whatsoever. 4377 4378 `-fno-show-column' 4379 Do not print column numbers in diagnostics. This may be necessary 4380 if diagnostics are being scanned by a program that does not 4381 understand the column numbers, such as `dejagnu'. 4382 4383 `-A PREDICATE=ANSWER' 4384 Make an assertion with the predicate PREDICATE and answer ANSWER. 4385 This form is preferred to the older form `-A PREDICATE(ANSWER)', 4386 which is still supported, because it does not use shell special 4387 characters. *Note Obsolete Features::. 4388 4389 `-A -PREDICATE=ANSWER' 4390 Cancel an assertion with the predicate PREDICATE and answer ANSWER. 4391 4392 `-dCHARS' 4393 CHARS is a sequence of one or more of the following characters, 4394 and must not be preceded by a space. Other characters are 4395 interpreted by the compiler proper, or reserved for future 4396 versions of GCC, and so are silently ignored. If you specify 4397 characters whose behavior conflicts, the result is undefined. 4398 4399 `M' 4400 Instead of the normal output, generate a list of `#define' 4401 directives for all the macros defined during the execution of 4402 the preprocessor, including predefined macros. This gives 4403 you a way of finding out what is predefined in your version 4404 of the preprocessor. Assuming you have no file `foo.h', the 4405 command 4406 4407 touch foo.h; cpp -dM foo.h 4408 4409 will show all the predefined macros. 4410 4411 If you use `-dM' without the `-E' option, `-dM' is 4412 interpreted as a synonym for `-fdump-rtl-mach'. *Note 4413 Debugging Options: (gcc)Debugging Options. 4414 4415 `D' 4416 Like `M' except in two respects: it does _not_ include the 4417 predefined macros, and it outputs _both_ the `#define' 4418 directives and the result of preprocessing. Both kinds of 4419 output go to the standard output file. 4420 4421 `N' 4422 Like `D', but emit only the macro names, not their expansions. 4423 4424 `I' 4425 Output `#include' directives in addition to the result of 4426 preprocessing. 4427 4428 `U' 4429 Like `D' except that only macros that are expanded, or whose 4430 definedness is tested in preprocessor directives, are output; 4431 the output is delayed until the use or test of the macro; and 4432 `#undef' directives are also output for macros tested but 4433 undefined at the time. 4434 4435 `-P' 4436 Inhibit generation of linemarkers in the output from the 4437 preprocessor. This might be useful when running the preprocessor 4438 on something that is not C code, and will be sent to a program 4439 which might be confused by the linemarkers. *Note Preprocessor 4440 Output::. 4441 4442 `-C' 4443 Do not discard comments. All comments are passed through to the 4444 output file, except for comments in processed directives, which 4445 are deleted along with the directive. 4446 4447 You should be prepared for side effects when using `-C'; it causes 4448 the preprocessor to treat comments as tokens in their own right. 4449 For example, comments appearing at the start of what would be a 4450 directive line have the effect of turning that line into an 4451 ordinary source line, since the first token on the line is no 4452 longer a `#'. 4453 4454 `-CC' 4455 Do not discard comments, including during macro expansion. This is 4456 like `-C', except that comments contained within macros are also 4457 passed through to the output file where the macro is expanded. 4458 4459 In addition to the side-effects of the `-C' option, the `-CC' 4460 option causes all C++-style comments inside a macro to be 4461 converted to C-style comments. This is to prevent later use of 4462 that macro from inadvertently commenting out the remainder of the 4463 source line. 4464 4465 The `-CC' option is generally used to support lint comments. 4466 4467 `-traditional-cpp' 4468 Try to imitate the behavior of old-fashioned C preprocessors, as 4469 opposed to ISO C preprocessors. *Note Traditional Mode::. 4470 4471 `-trigraphs' 4472 Process trigraph sequences. *Note Initial processing::. 4473 4474 `-remap' 4475 Enable special code to work around file systems which only permit 4476 very short file names, such as MS-DOS. 4477 4478 `--help' 4479 `--target-help' 4480 Print text describing all the command line options instead of 4481 preprocessing anything. 4482 4483 `-v' 4484 Verbose mode. Print out GNU CPP's version number at the beginning 4485 of execution, and report the final form of the include path. 4486 4487 `-H' 4488 Print the name of each header file used, in addition to other 4489 normal activities. Each name is indented to show how deep in the 4490 `#include' stack it is. Precompiled header files are also 4491 printed, even if they are found to be invalid; an invalid 4492 precompiled header file is printed with `...x' and a valid one 4493 with `...!' . 4494 4495 `-version' 4496 `--version' 4497 Print out GNU CPP's version number. With one dash, proceed to 4498 preprocess as normal. With two dashes, exit immediately. 4499 4500 4501 File: cpp.info, Node: Environment Variables, Next: GNU Free Documentation License, Prev: Invocation, Up: Top 4502 4503 13 Environment Variables 4504 ************************ 4505 4506 This section describes the environment variables that affect how CPP 4507 operates. You can use them to specify directories or prefixes to use 4508 when searching for include files, or to control dependency output. 4509 4510 Note that you can also specify places to search using options such as 4511 `-I', and control dependency output with options like `-M' (*note 4512 Invocation::). These take precedence over environment variables, which 4513 in turn take precedence over the configuration of GCC. 4514 4515 `CPATH' 4516 `C_INCLUDE_PATH' 4517 `CPLUS_INCLUDE_PATH' 4518 `OBJC_INCLUDE_PATH' 4519 Each variable's value is a list of directories separated by a 4520 special character, much like `PATH', in which to look for header 4521 files. The special character, `PATH_SEPARATOR', is 4522 target-dependent and determined at GCC build time. For Microsoft 4523 Windows-based targets it is a semicolon, and for almost all other 4524 targets it is a colon. 4525 4526 `CPATH' specifies a list of directories to be searched as if 4527 specified with `-I', but after any paths given with `-I' options 4528 on the command line. This environment variable is used regardless 4529 of which language is being preprocessed. 4530 4531 The remaining environment variables apply only when preprocessing 4532 the particular language indicated. Each specifies a list of 4533 directories to be searched as if specified with `-isystem', but 4534 after any paths given with `-isystem' options on the command line. 4535 4536 In all these variables, an empty element instructs the compiler to 4537 search its current working directory. Empty elements can appear 4538 at the beginning or end of a path. For instance, if the value of 4539 `CPATH' is `:/special/include', that has the same effect as 4540 `-I. -I/special/include'. 4541 4542 See also *note Search Path::. 4543 4544 `DEPENDENCIES_OUTPUT' 4545 If this variable is set, its value specifies how to output 4546 dependencies for Make based on the non-system header files 4547 processed by the compiler. System header files are ignored in the 4548 dependency output. 4549 4550 The value of `DEPENDENCIES_OUTPUT' can be just a file name, in 4551 which case the Make rules are written to that file, guessing the 4552 target name from the source file name. Or the value can have the 4553 form `FILE TARGET', in which case the rules are written to file 4554 FILE using TARGET as the target name. 4555 4556 In other words, this environment variable is equivalent to 4557 combining the options `-MM' and `-MF' (*note Invocation::), with 4558 an optional `-MT' switch too. 4559 4560 `SUNPRO_DEPENDENCIES' 4561 This variable is the same as `DEPENDENCIES_OUTPUT' (see above), 4562 except that system header files are not ignored, so it implies 4563 `-M' rather than `-MM'. However, the dependence on the main input 4564 file is omitted. *Note Invocation::. 4565 4566 4567 File: cpp.info, Node: GNU Free Documentation License, Next: Index of Directives, Prev: Environment Variables, Up: Top 4568 4569 GNU Free Documentation License 4570 ****************************** 4571 4572 Version 1.2, November 2002 4573 4574 Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 4575 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA 4576 4577 Everyone is permitted to copy and distribute verbatim copies 4578 of this license document, but changing it is not allowed. 4579 4580 0. PREAMBLE 4581 4582 The purpose of this License is to make a manual, textbook, or other 4583 functional and useful document "free" in the sense of freedom: to 4584 assure everyone the effective freedom to copy and redistribute it, 4585 with or without modifying it, either commercially or 4586 noncommercially. Secondarily, this License preserves for the 4587 author and publisher a way to get credit for their work, while not 4588 being considered responsible for modifications made by others. 4589 4590 This License is a kind of "copyleft", which means that derivative 4591 works of the document must themselves be free in the same sense. 4592 It complements the GNU General Public License, which is a copyleft 4593 license designed for free software. 4594 4595 We have designed this License in order to use it for manuals for 4596 free software, because free software needs free documentation: a 4597 free program should come with manuals providing the same freedoms 4598 that the software does. But this License is not limited to 4599 software manuals; it can be used for any textual work, regardless 4600 of subject matter or whether it is published as a printed book. 4601 We recommend this License principally for works whose purpose is 4602 instruction or reference. 4603 4604 1. APPLICABILITY AND DEFINITIONS 4605 4606 This License applies to any manual or other work, in any medium, 4607 that contains a notice placed by the copyright holder saying it 4608 can be distributed under the terms of this License. Such a notice 4609 grants a world-wide, royalty-free license, unlimited in duration, 4610 to use that work under the conditions stated herein. The 4611 "Document", below, refers to any such manual or work. Any member 4612 of the public is a licensee, and is addressed as "you". You 4613 accept the license if you copy, modify or distribute the work in a 4614 way requiring permission under copyright law. 4615 4616 A "Modified Version" of the Document means any work containing the 4617 Document or a portion of it, either copied verbatim, or with 4618 modifications and/or translated into another language. 4619 4620 A "Secondary Section" is a named appendix or a front-matter section 4621 of the Document that deals exclusively with the relationship of the 4622 publishers or authors of the Document to the Document's overall 4623 subject (or to related matters) and contains nothing that could 4624 fall directly within that overall subject. (Thus, if the Document 4625 is in part a textbook of mathematics, a Secondary Section may not 4626 explain any mathematics.) The relationship could be a matter of 4627 historical connection with the subject or with related matters, or 4628 of legal, commercial, philosophical, ethical or political position 4629 regarding them. 4630 4631 The "Invariant Sections" are certain Secondary Sections whose 4632 titles are designated, as being those of Invariant Sections, in 4633 the notice that says that the Document is released under this 4634 License. If a section does not fit the above definition of 4635 Secondary then it is not allowed to be designated as Invariant. 4636 The Document may contain zero Invariant Sections. If the Document 4637 does not identify any Invariant Sections then there are none. 4638 4639 The "Cover Texts" are certain short passages of text that are 4640 listed, as Front-Cover Texts or Back-Cover Texts, in the notice 4641 that says that the Document is released under this License. A 4642 Front-Cover Text may be at most 5 words, and a Back-Cover Text may 4643 be at most 25 words. 4644 4645 A "Transparent" copy of the Document means a machine-readable copy, 4646 represented in a format whose specification is available to the 4647 general public, that is suitable for revising the document 4648 straightforwardly with generic text editors or (for images 4649 composed of pixels) generic paint programs or (for drawings) some 4650 widely available drawing editor, and that is suitable for input to 4651 text formatters or for automatic translation to a variety of 4652 formats suitable for input to text formatters. A copy made in an 4653 otherwise Transparent file format whose markup, or absence of 4654 markup, has been arranged to thwart or discourage subsequent 4655 modification by readers is not Transparent. An image format is 4656 not Transparent if used for any substantial amount of text. A 4657 copy that is not "Transparent" is called "Opaque". 4658 4659 Examples of suitable formats for Transparent copies include plain 4660 ASCII without markup, Texinfo input format, LaTeX input format, 4661 SGML or XML using a publicly available DTD, and 4662 standard-conforming simple HTML, PostScript or PDF designed for 4663 human modification. Examples of transparent image formats include 4664 PNG, XCF and JPG. Opaque formats include proprietary formats that 4665 can be read and edited only by proprietary word processors, SGML or 4666 XML for which the DTD and/or processing tools are not generally 4667 available, and the machine-generated HTML, PostScript or PDF 4668 produced by some word processors for output purposes only. 4669 4670 The "Title Page" means, for a printed book, the title page itself, 4671 plus such following pages as are needed to hold, legibly, the 4672 material this License requires to appear in the title page. For 4673 works in formats which do not have any title page as such, "Title 4674 Page" means the text near the most prominent appearance of the 4675 work's title, preceding the beginning of the body of the text. 4676 4677 A section "Entitled XYZ" means a named subunit of the Document 4678 whose title either is precisely XYZ or contains XYZ in parentheses 4679 following text that translates XYZ in another language. (Here XYZ 4680 stands for a specific section name mentioned below, such as 4681 "Acknowledgements", "Dedications", "Endorsements", or "History".) 4682 To "Preserve the Title" of such a section when you modify the 4683 Document means that it remains a section "Entitled XYZ" according 4684 to this definition. 4685 4686 The Document may include Warranty Disclaimers next to the notice 4687 which states that this License applies to the Document. These 4688 Warranty Disclaimers are considered to be included by reference in 4689 this License, but only as regards disclaiming warranties: any other 4690 implication that these Warranty Disclaimers may have is void and 4691 has no effect on the meaning of this License. 4692 4693 2. VERBATIM COPYING 4694 4695 You may copy and distribute the Document in any medium, either 4696 commercially or noncommercially, provided that this License, the 4697 copyright notices, and the license notice saying this License 4698 applies to the Document are reproduced in all copies, and that you 4699 add no other conditions whatsoever to those of this License. You 4700 may not use technical measures to obstruct or control the reading 4701 or further copying of the copies you make or distribute. However, 4702 you may accept compensation in exchange for copies. If you 4703 distribute a large enough number of copies you must also follow 4704 the conditions in section 3. 4705 4706 You may also lend copies, under the same conditions stated above, 4707 and you may publicly display copies. 4708 4709 3. COPYING IN QUANTITY 4710 4711 If you publish printed copies (or copies in media that commonly 4712 have printed covers) of the Document, numbering more than 100, and 4713 the Document's license notice requires Cover Texts, you must 4714 enclose the copies in covers that carry, clearly and legibly, all 4715 these Cover Texts: Front-Cover Texts on the front cover, and 4716 Back-Cover Texts on the back cover. Both covers must also clearly 4717 and legibly identify you as the publisher of these copies. The 4718 front cover must present the full title with all words of the 4719 title equally prominent and visible. You may add other material 4720 on the covers in addition. Copying with changes limited to the 4721 covers, as long as they preserve the title of the Document and 4722 satisfy these conditions, can be treated as verbatim copying in 4723 other respects. 4724 4725 If the required texts for either cover are too voluminous to fit 4726 legibly, you should put the first ones listed (as many as fit 4727 reasonably) on the actual cover, and continue the rest onto 4728 adjacent pages. 4729 4730 If you publish or distribute Opaque copies of the Document 4731 numbering more than 100, you must either include a 4732 machine-readable Transparent copy along with each Opaque copy, or 4733 state in or with each Opaque copy a computer-network location from 4734 which the general network-using public has access to download 4735 using public-standard network protocols a complete Transparent 4736 copy of the Document, free of added material. If you use the 4737 latter option, you must take reasonably prudent steps, when you 4738 begin distribution of Opaque copies in quantity, to ensure that 4739 this Transparent copy will remain thus accessible at the stated 4740 location until at least one year after the last time you 4741 distribute an Opaque copy (directly or through your agents or 4742 retailers) of that edition to the public. 4743 4744 It is requested, but not required, that you contact the authors of 4745 the Document well before redistributing any large number of 4746 copies, to give them a chance to provide you with an updated 4747 version of the Document. 4748 4749 4. MODIFICATIONS 4750 4751 You may copy and distribute a Modified Version of the Document 4752 under the conditions of sections 2 and 3 above, provided that you 4753 release the Modified Version under precisely this License, with 4754 the Modified Version filling the role of the Document, thus 4755 licensing distribution and modification of the Modified Version to 4756 whoever possesses a copy of it. In addition, you must do these 4757 things in the Modified Version: 4758 4759 A. Use in the Title Page (and on the covers, if any) a title 4760 distinct from that of the Document, and from those of 4761 previous versions (which should, if there were any, be listed 4762 in the History section of the Document). You may use the 4763 same title as a previous version if the original publisher of 4764 that version gives permission. 4765 4766 B. List on the Title Page, as authors, one or more persons or 4767 entities responsible for authorship of the modifications in 4768 the Modified Version, together with at least five of the 4769 principal authors of the Document (all of its principal 4770 authors, if it has fewer than five), unless they release you 4771 from this requirement. 4772 4773 C. State on the Title page the name of the publisher of the 4774 Modified Version, as the publisher. 4775 4776 D. Preserve all the copyright notices of the Document. 4777 4778 E. Add an appropriate copyright notice for your modifications 4779 adjacent to the other copyright notices. 4780 4781 F. Include, immediately after the copyright notices, a license 4782 notice giving the public permission to use the Modified 4783 Version under the terms of this License, in the form shown in 4784 the Addendum below. 4785 4786 G. Preserve in that license notice the full lists of Invariant 4787 Sections and required Cover Texts given in the Document's 4788 license notice. 4789 4790 H. Include an unaltered copy of this License. 4791 4792 I. Preserve the section Entitled "History", Preserve its Title, 4793 and add to it an item stating at least the title, year, new 4794 authors, and publisher of the Modified Version as given on 4795 the Title Page. If there is no section Entitled "History" in 4796 the Document, create one stating the title, year, authors, 4797 and publisher of the Document as given on its Title Page, 4798 then add an item describing the Modified Version as stated in 4799 the previous sentence. 4800 4801 J. Preserve the network location, if any, given in the Document 4802 for public access to a Transparent copy of the Document, and 4803 likewise the network locations given in the Document for 4804 previous versions it was based on. These may be placed in 4805 the "History" section. You may omit a network location for a 4806 work that was published at least four years before the 4807 Document itself, or if the original publisher of the version 4808 it refers to gives permission. 4809 4810 K. For any section Entitled "Acknowledgements" or "Dedications", 4811 Preserve the Title of the section, and preserve in the 4812 section all the substance and tone of each of the contributor 4813 acknowledgements and/or dedications given therein. 4814 4815 L. Preserve all the Invariant Sections of the Document, 4816 unaltered in their text and in their titles. Section numbers 4817 or the equivalent are not considered part of the section 4818 titles. 4819 4820 M. Delete any section Entitled "Endorsements". Such a section 4821 may not be included in the Modified Version. 4822 4823 N. Do not retitle any existing section to be Entitled 4824 "Endorsements" or to conflict in title with any Invariant 4825 Section. 4826 4827 O. Preserve any Warranty Disclaimers. 4828 4829 If the Modified Version includes new front-matter sections or 4830 appendices that qualify as Secondary Sections and contain no 4831 material copied from the Document, you may at your option 4832 designate some or all of these sections as invariant. To do this, 4833 add their titles to the list of Invariant Sections in the Modified 4834 Version's license notice. These titles must be distinct from any 4835 other section titles. 4836 4837 You may add a section Entitled "Endorsements", provided it contains 4838 nothing but endorsements of your Modified Version by various 4839 parties--for example, statements of peer review or that the text 4840 has been approved by an organization as the authoritative 4841 definition of a standard. 4842 4843 You may add a passage of up to five words as a Front-Cover Text, 4844 and a passage of up to 25 words as a Back-Cover Text, to the end 4845 of the list of Cover Texts in the Modified Version. Only one 4846 passage of Front-Cover Text and one of Back-Cover Text may be 4847 added by (or through arrangements made by) any one entity. If the 4848 Document already includes a cover text for the same cover, 4849 previously added by you or by arrangement made by the same entity 4850 you are acting on behalf of, you may not add another; but you may 4851 replace the old one, on explicit permission from the previous 4852 publisher that added the old one. 4853 4854 The author(s) and publisher(s) of the Document do not by this 4855 License give permission to use their names for publicity for or to 4856 assert or imply endorsement of any Modified Version. 4857 4858 5. COMBINING DOCUMENTS 4859 4860 You may combine the Document with other documents released under 4861 this License, under the terms defined in section 4 above for 4862 modified versions, provided that you include in the combination 4863 all of the Invariant Sections of all of the original documents, 4864 unmodified, and list them all as Invariant Sections of your 4865 combined work in its license notice, and that you preserve all 4866 their Warranty Disclaimers. 4867 4868 The combined work need only contain one copy of this License, and 4869 multiple identical Invariant Sections may be replaced with a single 4870 copy. If there are multiple Invariant Sections with the same name 4871 but different contents, make the title of each such section unique 4872 by adding at the end of it, in parentheses, the name of the 4873 original author or publisher of that section if known, or else a 4874 unique number. Make the same adjustment to the section titles in 4875 the list of Invariant Sections in the license notice of the 4876 combined work. 4877 4878 In the combination, you must combine any sections Entitled 4879 "History" in the various original documents, forming one section 4880 Entitled "History"; likewise combine any sections Entitled 4881 "Acknowledgements", and any sections Entitled "Dedications". You 4882 must delete all sections Entitled "Endorsements." 4883 4884 6. COLLECTIONS OF DOCUMENTS 4885 4886 You may make a collection consisting of the Document and other 4887 documents released under this License, and replace the individual 4888 copies of this License in the various documents with a single copy 4889 that is included in the collection, provided that you follow the 4890 rules of this License for verbatim copying of each of the 4891 documents in all other respects. 4892 4893 You may extract a single document from such a collection, and 4894 distribute it individually under this License, provided you insert 4895 a copy of this License into the extracted document, and follow 4896 this License in all other respects regarding verbatim copying of 4897 that document. 4898 4899 7. AGGREGATION WITH INDEPENDENT WORKS 4900 4901 A compilation of the Document or its derivatives with other 4902 separate and independent documents or works, in or on a volume of 4903 a storage or distribution medium, is called an "aggregate" if the 4904 copyright resulting from the compilation is not used to limit the 4905 legal rights of the compilation's users beyond what the individual 4906 works permit. When the Document is included in an aggregate, this 4907 License does not apply to the other works in the aggregate which 4908 are not themselves derivative works of the Document. 4909 4910 If the Cover Text requirement of section 3 is applicable to these 4911 copies of the Document, then if the Document is less than one half 4912 of the entire aggregate, the Document's Cover Texts may be placed 4913 on covers that bracket the Document within the aggregate, or the 4914 electronic equivalent of covers if the Document is in electronic 4915 form. Otherwise they must appear on printed covers that bracket 4916 the whole aggregate. 4917 4918 8. TRANSLATION 4919 4920 Translation is considered a kind of modification, so you may 4921 distribute translations of the Document under the terms of section 4922 4. Replacing Invariant Sections with translations requires special 4923 permission from their copyright holders, but you may include 4924 translations of some or all Invariant Sections in addition to the 4925 original versions of these Invariant Sections. You may include a 4926 translation of this License, and all the license notices in the 4927 Document, and any Warranty Disclaimers, provided that you also 4928 include the original English version of this License and the 4929 original versions of those notices and disclaimers. In case of a 4930 disagreement between the translation and the original version of 4931 this License or a notice or disclaimer, the original version will 4932 prevail. 4933 4934 If a section in the Document is Entitled "Acknowledgements", 4935 "Dedications", or "History", the requirement (section 4) to 4936 Preserve its Title (section 1) will typically require changing the 4937 actual title. 4938 4939 9. TERMINATION 4940 4941 You may not copy, modify, sublicense, or distribute the Document 4942 except as expressly provided for under this License. Any other 4943 attempt to copy, modify, sublicense or distribute the Document is 4944 void, and will automatically terminate your rights under this 4945 License. However, parties who have received copies, or rights, 4946 from you under this License will not have their licenses 4947 terminated so long as such parties remain in full compliance. 4948 4949 10. FUTURE REVISIONS OF THIS LICENSE 4950 4951 The Free Software Foundation may publish new, revised versions of 4952 the GNU Free Documentation License from time to time. Such new 4953 versions will be similar in spirit to the present version, but may 4954 differ in detail to address new problems or concerns. See 4955 `http://www.gnu.org/copyleft/'. 4956 4957 Each version of the License is given a distinguishing version 4958 number. If the Document specifies that a particular numbered 4959 version of this License "or any later version" applies to it, you 4960 have the option of following the terms and conditions either of 4961 that specified version or of any later version that has been 4962 published (not as a draft) by the Free Software Foundation. If 4963 the Document does not specify a version number of this License, 4964 you may choose any version ever published (not as a draft) by the 4965 Free Software Foundation. 4966 4967 ADDENDUM: How to use this License for your documents 4968 ==================================================== 4969 4970 To use this License in a document you have written, include a copy of 4971 the License in the document and put the following copyright and license 4972 notices just after the title page: 4973 4974 Copyright (C) YEAR YOUR NAME. 4975 Permission is granted to copy, distribute and/or modify this document 4976 under the terms of the GNU Free Documentation License, Version 1.2 4977 or any later version published by the Free Software Foundation; 4978 with no Invariant Sections, no Front-Cover Texts, and no Back-Cover 4979 Texts. A copy of the license is included in the section entitled ``GNU 4980 Free Documentation License''. 4981 4982 If you have Invariant Sections, Front-Cover Texts and Back-Cover 4983 Texts, replace the "with...Texts." line with this: 4984 4985 with the Invariant Sections being LIST THEIR TITLES, with 4986 the Front-Cover Texts being LIST, and with the Back-Cover Texts 4987 being LIST. 4988 4989 If you have Invariant Sections without Cover Texts, or some other 4990 combination of the three, merge those two alternatives to suit the 4991 situation. 4992 4993 If your document contains nontrivial examples of program code, we 4994 recommend releasing these examples in parallel under your choice of 4995 free software license, such as the GNU General Public License, to 4996 permit their use in free software. 4997 4998 4999 File: cpp.info, Node: Index of Directives, Next: Option Index, Prev: GNU Free Documentation License, Up: Top 5000 5001 Index of Directives 5002 ******************* 5003 5004 [index] 5005 * Menu: 5006 5007 * #assert: Obsolete Features. (line 48) 5008 * #define: Object-like Macros. (line 11) 5009 * #elif: Elif. (line 6) 5010 * #else: Else. (line 6) 5011 * #endif: Ifdef. (line 6) 5012 * #error: Diagnostics. (line 6) 5013 * #ident: Other Directives. (line 6) 5014 * #if: Conditional Syntax. (line 6) 5015 * #ifdef: Ifdef. (line 6) 5016 * #ifndef: Ifdef. (line 40) 5017 * #import: Alternatives to Wrapper #ifndef. 5018 (line 11) 5019 * #include: Include Syntax. (line 6) 5020 * #include_next: Wrapper Headers. (line 6) 5021 * #line: Line Control. (line 20) 5022 * #pragma GCC dependency: Pragmas. (line 53) 5023 * #pragma GCC poison: Pragmas. (line 65) 5024 * #pragma GCC system_header <1>: Pragmas. (line 92) 5025 * #pragma GCC system_header: System Headers. (line 31) 5026 * #sccs: Other Directives. (line 6) 5027 * #unassert: Obsolete Features. (line 59) 5028 * #undef: Undefining and Redefining Macros. 5029 (line 6) 5030 * #warning: Diagnostics. (line 27) 5031 5032 5033 File: cpp.info, Node: Option Index, Next: Concept Index, Prev: Index of Directives, Up: Top 5034 5035 Option Index 5036 ************ 5037 5038 CPP's command line options and environment variables are indexed here 5039 without any initial `-' or `--'. 5040 5041 [index] 5042 * Menu: 5043 5044 * A: Invocation. (line 522) 5045 * ansi: Invocation. (line 308) 5046 * C: Invocation. (line 581) 5047 * C_INCLUDE_PATH: Environment Variables. 5048 (line 16) 5049 * CPATH: Environment Variables. 5050 (line 15) 5051 * CPLUS_INCLUDE_PATH: Environment Variables. 5052 (line 17) 5053 * D: Invocation. (line 39) 5054 * dD: Invocation. (line 554) 5055 * DEPENDENCIES_OUTPUT: Environment Variables. 5056 (line 44) 5057 * dI: Invocation. (line 563) 5058 * dM: Invocation. (line 538) 5059 * dN: Invocation. (line 560) 5060 * dU: Invocation. (line 567) 5061 * fdirectives-only: Invocation. (line 430) 5062 * fdollars-in-identifiers: Invocation. (line 452) 5063 * fexec-charset: Invocation. (line 479) 5064 * fextended-identifiers: Invocation. (line 455) 5065 * finput-charset: Invocation. (line 492) 5066 * fno-show-column: Invocation. (line 517) 5067 * fno-working-directory: Invocation. (line 502) 5068 * fpreprocessed: Invocation. (line 460) 5069 * ftabstop: Invocation. (line 473) 5070 * fwide-exec-charset: Invocation. (line 484) 5071 * fworking-directory: Invocation. (line 502) 5072 * H: Invocation. (line 626) 5073 * help: Invocation. (line 618) 5074 * I: Invocation. (line 71) 5075 * I-: Invocation. (line 345) 5076 * idirafter: Invocation. (line 387) 5077 * imacros: Invocation. (line 378) 5078 * imultilib: Invocation. (line 410) 5079 * include: Invocation. (line 367) 5080 * iprefix: Invocation. (line 394) 5081 * iquote: Invocation. (line 422) 5082 * isysroot: Invocation. (line 406) 5083 * isystem: Invocation. (line 414) 5084 * iwithprefix: Invocation. (line 400) 5085 * iwithprefixbefore: Invocation. (line 400) 5086 * M: Invocation. (line 180) 5087 * MD: Invocation. (line 269) 5088 * MF: Invocation. (line 215) 5089 * MG: Invocation. (line 224) 5090 * MM: Invocation. (line 205) 5091 * MMD: Invocation. (line 285) 5092 * MP: Invocation. (line 234) 5093 * MQ: Invocation. (line 260) 5094 * MT: Invocation. (line 246) 5095 * nostdinc: Invocation. (line 357) 5096 * nostdinc++: Invocation. (line 362) 5097 * o: Invocation. (line 82) 5098 * OBJC_INCLUDE_PATH: Environment Variables. 5099 (line 18) 5100 * P: Invocation. (line 574) 5101 * pedantic: Invocation. (line 170) 5102 * pedantic-errors: Invocation. (line 175) 5103 * remap: Invocation. (line 613) 5104 * std=: Invocation. (line 308) 5105 * SUNPRO_DEPENDENCIES: Environment Variables. 5106 (line 60) 5107 * target-help: Invocation. (line 618) 5108 * traditional-cpp: Invocation. (line 606) 5109 * trigraphs: Invocation. (line 610) 5110 * U: Invocation. (line 62) 5111 * undef: Invocation. (line 66) 5112 * v: Invocation. (line 622) 5113 * version: Invocation. (line 635) 5114 * w: Invocation. (line 166) 5115 * Wall: Invocation. (line 88) 5116 * Wcomment: Invocation. (line 96) 5117 * Wcomments: Invocation. (line 96) 5118 * Wendif-labels: Invocation. (line 143) 5119 * Werror: Invocation. (line 156) 5120 * Wsystem-headers: Invocation. (line 160) 5121 * Wtraditional: Invocation. (line 113) 5122 * Wtrigraphs: Invocation. (line 101) 5123 * Wundef: Invocation. (line 119) 5124 * Wunused-macros: Invocation. (line 124) 5125 * x: Invocation. (line 292) 5126 5127 5128 File: cpp.info, Node: Concept Index, Prev: Option Index, Up: Top 5129 5130 Concept Index 5131 ************* 5132 5133 [index] 5134 * Menu: 5135 5136 * # operator: Stringification. (line 6) 5137 * ## operator: Concatenation. (line 6) 5138 * _Pragma: Pragmas. (line 25) 5139 * alternative tokens: Tokenization. (line 106) 5140 * arguments: Macro Arguments. (line 6) 5141 * arguments in macro definitions: Macro Arguments. (line 6) 5142 * assertions: Obsolete Features. (line 13) 5143 * assertions, canceling: Obsolete Features. (line 59) 5144 * backslash-newline: Initial processing. (line 61) 5145 * block comments: Initial processing. (line 77) 5146 * C++ named operators: C++ Named Operators. (line 6) 5147 * character constants: Tokenization. (line 85) 5148 * character set, execution: Invocation. (line 479) 5149 * character set, input: Invocation. (line 492) 5150 * character set, wide execution: Invocation. (line 484) 5151 * command line: Invocation. (line 6) 5152 * commenting out code: Deleted Code. (line 6) 5153 * comments: Initial processing. (line 77) 5154 * common predefined macros: Common Predefined Macros. 5155 (line 6) 5156 * computed includes: Computed Includes. (line 6) 5157 * concatenation: Concatenation. (line 6) 5158 * conditional group: Ifdef. (line 14) 5159 * conditionals: Conditionals. (line 6) 5160 * continued lines: Initial processing. (line 61) 5161 * controlling macro: Once-Only Headers. (line 35) 5162 * defined: Defined. (line 6) 5163 * dependencies for make as output: Environment Variables. 5164 (line 45) 5165 * dependencies, make: Invocation. (line 180) 5166 * diagnostic: Diagnostics. (line 6) 5167 * differences from previous versions: Differences from previous versions. 5168 (line 6) 5169 * digraphs: Tokenization. (line 106) 5170 * directive line: The preprocessing language. 5171 (line 6) 5172 * directive name: The preprocessing language. 5173 (line 6) 5174 * directives: The preprocessing language. 5175 (line 6) 5176 * empty macro arguments: Macro Arguments. (line 66) 5177 * environment variables: Environment Variables. 5178 (line 6) 5179 * expansion of arguments: Argument Prescan. (line 6) 5180 * FDL, GNU Free Documentation License: GNU Free Documentation License. 5181 (line 6) 5182 * function-like macros: Function-like Macros. 5183 (line 6) 5184 * grouping options: Invocation. (line 34) 5185 * guard macro: Once-Only Headers. (line 35) 5186 * header file: Header Files. (line 6) 5187 * header file names: Tokenization. (line 85) 5188 * identifiers: Tokenization. (line 34) 5189 * implementation limits: Implementation limits. 5190 (line 6) 5191 * implementation-defined behavior: Implementation-defined behavior. 5192 (line 6) 5193 * including just once: Once-Only Headers. (line 6) 5194 * invocation: Invocation. (line 6) 5195 * iso646.h: C++ Named Operators. (line 6) 5196 * line comments: Initial processing. (line 77) 5197 * line control: Line Control. (line 6) 5198 * line endings: Initial processing. (line 14) 5199 * linemarkers: Preprocessor Output. (line 28) 5200 * macro argument expansion: Argument Prescan. (line 6) 5201 * macro arguments and directives: Directives Within Macro Arguments. 5202 (line 6) 5203 * macros in include: Computed Includes. (line 6) 5204 * macros with arguments: Macro Arguments. (line 6) 5205 * macros with variable arguments: Variadic Macros. (line 6) 5206 * make: Invocation. (line 180) 5207 * manifest constants: Object-like Macros. (line 6) 5208 * named operators: C++ Named Operators. (line 6) 5209 * newlines in macro arguments: Newlines in Arguments. 5210 (line 6) 5211 * null directive: Other Directives. (line 15) 5212 * numbers: Tokenization. (line 61) 5213 * object-like macro: Object-like Macros. (line 6) 5214 * options: Invocation. (line 38) 5215 * options, grouping: Invocation. (line 34) 5216 * other tokens: Tokenization. (line 120) 5217 * output format: Preprocessor Output. (line 12) 5218 * overriding a header file: Wrapper Headers. (line 6) 5219 * parentheses in macro bodies: Operator Precedence Problems. 5220 (line 6) 5221 * pitfalls of macros: Macro Pitfalls. (line 6) 5222 * predefined macros: Predefined Macros. (line 6) 5223 * predefined macros, system-specific: System-specific Predefined Macros. 5224 (line 6) 5225 * predicates: Obsolete Features. (line 26) 5226 * preprocessing directives: The preprocessing language. 5227 (line 6) 5228 * preprocessing numbers: Tokenization. (line 61) 5229 * preprocessing tokens: Tokenization. (line 6) 5230 * prescan of macro arguments: Argument Prescan. (line 6) 5231 * problems with macros: Macro Pitfalls. (line 6) 5232 * punctuators: Tokenization. (line 106) 5233 * redefining macros: Undefining and Redefining Macros. 5234 (line 6) 5235 * repeated inclusion: Once-Only Headers. (line 6) 5236 * reporting errors: Diagnostics. (line 6) 5237 * reporting warnings: Diagnostics. (line 6) 5238 * reserved namespace: System-specific Predefined Macros. 5239 (line 6) 5240 * self-reference: Self-Referential Macros. 5241 (line 6) 5242 * semicolons (after macro calls): Swallowing the Semicolon. 5243 (line 6) 5244 * side effects (in macro arguments): Duplication of Side Effects. 5245 (line 6) 5246 * standard predefined macros.: Standard Predefined Macros. 5247 (line 6) 5248 * string constants: Tokenization. (line 85) 5249 * string literals: Tokenization. (line 85) 5250 * stringification: Stringification. (line 6) 5251 * symbolic constants: Object-like Macros. (line 6) 5252 * system header files <1>: System Headers. (line 6) 5253 * system header files: Header Files. (line 13) 5254 * system-specific predefined macros: System-specific Predefined Macros. 5255 (line 6) 5256 * testing predicates: Obsolete Features. (line 37) 5257 * token concatenation: Concatenation. (line 6) 5258 * token pasting: Concatenation. (line 6) 5259 * tokens: Tokenization. (line 6) 5260 * trigraphs: Initial processing. (line 32) 5261 * undefining macros: Undefining and Redefining Macros. 5262 (line 6) 5263 * unsafe macros: Duplication of Side Effects. 5264 (line 6) 5265 * variable number of arguments: Variadic Macros. (line 6) 5266 * variadic macros: Variadic Macros. (line 6) 5267 * wrapper #ifndef: Once-Only Headers. (line 6) 5268 * wrapper headers: Wrapper Headers. (line 6) 5269 5270 5271 5272 Tag Table: 5273 Node: Top1138 5274 Node: Overview3852 5275 Node: Character sets6673 5276 Ref: Character sets-Footnote-18856 5277 Node: Initial processing9037 5278 Ref: trigraphs10596 5279 Node: Tokenization14798 5280 Ref: Tokenization-Footnote-121934 5281 Node: The preprocessing language22045 5282 Node: Header Files24923 5283 Node: Include Syntax26839 5284 Node: Include Operation28476 5285 Node: Search Path30324 5286 Node: Once-Only Headers33514 5287 Node: Alternatives to Wrapper #ifndef35173 5288 Node: Computed Includes36916 5289 Node: Wrapper Headers40074 5290 Node: System Headers42500 5291 Node: Macros44550 5292 Node: Object-like Macros45691 5293 Node: Function-like Macros49281 5294 Node: Macro Arguments50897 5295 Node: Stringification55042 5296 Node: Concatenation58248 5297 Node: Variadic Macros61356 5298 Node: Predefined Macros66143 5299 Node: Standard Predefined Macros66731 5300 Node: Common Predefined Macros72667 5301 Node: System-specific Predefined Macros85577 5302 Node: C++ Named Operators87598 5303 Node: Undefining and Redefining Macros88562 5304 Node: Directives Within Macro Arguments90666 5305 Node: Macro Pitfalls92214 5306 Node: Misnesting92747 5307 Node: Operator Precedence Problems93859 5308 Node: Swallowing the Semicolon95725 5309 Node: Duplication of Side Effects97748 5310 Node: Self-Referential Macros99931 5311 Node: Argument Prescan102340 5312 Node: Newlines in Arguments106094 5313 Node: Conditionals107045 5314 Node: Conditional Uses108875 5315 Node: Conditional Syntax110233 5316 Node: Ifdef110553 5317 Node: If113714 5318 Node: Defined116018 5319 Node: Else117301 5320 Node: Elif117871 5321 Node: Deleted Code119160 5322 Node: Diagnostics120407 5323 Node: Line Control122024 5324 Node: Pragmas125828 5325 Node: Other Directives130098 5326 Node: Preprocessor Output131148 5327 Node: Traditional Mode134349 5328 Node: Traditional lexical analysis135407 5329 Node: Traditional macros137910 5330 Node: Traditional miscellany141712 5331 Node: Traditional warnings142709 5332 Node: Implementation Details144906 5333 Node: Implementation-defined behavior145527 5334 Ref: Identifier characters146279 5335 Node: Implementation limits149354 5336 Node: Obsolete Features152028 5337 Node: Differences from previous versions154865 5338 Node: Invocation159073 5339 Ref: Wtrigraphs163525 5340 Ref: dashMF168300 5341 Ref: fdollars-in-identifiers177683 5342 Node: Environment Variables185846 5343 Node: GNU Free Documentation License188812 5344 Node: Index of Directives211245 5345 Node: Option Index213179 5346 Node: Concept Index219363 5347 5348 End Tag Table 5349