1 This is doc/cpp.info, produced by makeinfo version 4.8 from 2 ../../../toolchain/android-toolchain/gcc-4.3.1/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 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 1.1 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 * Computed Includes:: 78 * Wrapper Headers:: 79 * System Headers:: 80 81 Macros 82 83 * Object-like Macros:: 84 * Function-like Macros:: 85 * Macro Arguments:: 86 * Stringification:: 87 * Concatenation:: 88 * Variadic Macros:: 89 * Predefined Macros:: 90 * Undefining and Redefining Macros:: 91 * Directives Within Macro Arguments:: 92 * Macro Pitfalls:: 93 94 Predefined Macros 95 96 * Standard Predefined Macros:: 97 * Common Predefined Macros:: 98 * System-specific Predefined Macros:: 99 * C++ Named Operators:: 100 101 Macro Pitfalls 102 103 * Misnesting:: 104 * Operator Precedence Problems:: 105 * Swallowing the Semicolon:: 106 * Duplication of Side Effects:: 107 * Self-Referential Macros:: 108 * Argument Prescan:: 109 * Newlines in Arguments:: 110 111 Conditionals 112 113 * Conditional Uses:: 114 * Conditional Syntax:: 115 * Deleted Code:: 116 117 Conditional Syntax 118 119 * Ifdef:: 120 * If:: 121 * Defined:: 122 * Else:: 123 * Elif:: 124 125 Implementation Details 126 127 * Implementation-defined behavior:: 128 * Implementation limits:: 129 * Obsolete Features:: 130 * Differences from previous versions:: 131 132 Obsolete Features 133 134 * Assertions:: 135 * Obsolete once-only headers:: 136 137 Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 138 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007 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 1.1 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 * Computed Includes:: 661 * Wrapper Headers:: 662 * System Headers:: 663 664 665 File: cpp.info, Node: Include Syntax, Next: Include Operation, Up: Header Files 666 667 2.1 Include Syntax 668 ================== 669 670 Both user and system header files are included using the preprocessing 671 directive `#include'. It has two variants: 672 673 `#include <FILE>' 674 This variant is used for system header files. It searches for a 675 file named FILE in a standard list of system directories. You can 676 prepend directories to this list with the `-I' option (*note 677 Invocation::). 678 679 `#include "FILE"' 680 This variant is used for header files of your own program. It 681 searches for a file named FILE first in the directory containing 682 the current file, then in the quote directories and then the same 683 directories used for `<FILE>'. You can prepend directories to the 684 list of quote directories with the `-iquote' option. 685 686 The argument of `#include', whether delimited with quote marks or 687 angle brackets, behaves like a string constant in that comments are not 688 recognized, and macro names are not expanded. Thus, `#include <x/*y>' 689 specifies inclusion of a system header file named `x/*y'. 690 691 However, if backslashes occur within FILE, they are considered 692 ordinary text characters, not escape characters. None of the character 693 escape sequences appropriate to string constants in C are processed. 694 Thus, `#include "x\n\\y"' specifies a filename containing three 695 backslashes. (Some systems interpret `\' as a pathname separator. All 696 of these also interpret `/' the same way. It is most portable to use 697 only `/'.) 698 699 It is an error if there is anything (other than comments) on the line 700 after the file name. 701 702 703 File: cpp.info, Node: Include Operation, Next: Search Path, Prev: Include Syntax, Up: Header Files 704 705 2.2 Include Operation 706 ===================== 707 708 The `#include' directive works by directing the C preprocessor to scan 709 the specified file as input before continuing with the rest of the 710 current file. The output from the preprocessor contains the output 711 already generated, followed by the output resulting from the included 712 file, followed by the output that comes from the text after the 713 `#include' directive. For example, if you have a header file 714 `header.h' as follows, 715 716 char *test (void); 717 718 and a main program called `program.c' that uses the header file, like 719 this, 720 721 int x; 722 #include "header.h" 723 724 int 725 main (void) 726 { 727 puts (test ()); 728 } 729 730 the compiler will see the same token stream as it would if `program.c' 731 read 732 733 int x; 734 char *test (void); 735 736 int 737 main (void) 738 { 739 puts (test ()); 740 } 741 742 Included files are not limited to declarations and macro definitions; 743 those are merely the typical uses. Any fragment of a C program can be 744 included from another file. The include file could even contain the 745 beginning of a statement that is concluded in the containing file, or 746 the end of a statement that was started in the including file. However, 747 an included file must consist of complete tokens. Comments and string 748 literals which have not been closed by the end of an included file are 749 invalid. For error recovery, they are considered to end at the end of 750 the file. 751 752 To avoid confusion, it is best if header files contain only complete 753 syntactic units--function declarations or definitions, type 754 declarations, etc. 755 756 The line following the `#include' directive is always treated as a 757 separate line by the C preprocessor, even if the included file lacks a 758 final newline. 759 760 761 File: cpp.info, Node: Search Path, Next: Once-Only Headers, Prev: Include Operation, Up: Header Files 762 763 2.3 Search Path 764 =============== 765 766 GCC looks in several different places for headers. On a normal Unix 767 system, if you do not instruct it otherwise, it will look for headers 768 requested with `#include <FILE>' in: 769 770 /usr/local/include 771 LIBDIR/gcc/TARGET/VERSION/include 772 /usr/TARGET/include 773 /usr/include 774 775 For C++ programs, it will also look in `/usr/include/g++-v3', first. 776 In the above, TARGET is the canonical name of the system GCC was 777 configured to compile code for; often but not always the same as the 778 canonical name of the system it runs on. VERSION is the version of GCC 779 in use. 780 781 You can add to this list with the `-IDIR' command line option. All 782 the directories named by `-I' are searched, in left-to-right order, 783 _before_ the default directories. The only exception is when `dir' is 784 already searched by default. In this case, the option is ignored and 785 the search order for system directories remains unchanged. 786 787 Duplicate directories are removed from the quote and bracket search 788 chains before the two chains are merged to make the final search chain. 789 Thus, it is possible for a directory to occur twice in the final search 790 chain if it was specified in both the quote and bracket chains. 791 792 You can prevent GCC from searching any of the default directories 793 with the `-nostdinc' option. This is useful when you are compiling an 794 operating system kernel or some other program that does not use the 795 standard C library facilities, or the standard C library itself. `-I' 796 options are not ignored as described above when `-nostdinc' is in 797 effect. 798 799 GCC looks for headers requested with `#include "FILE"' first in the 800 directory containing the current file, then in the directories as 801 specified by `-iquote' options, then in the same places it would have 802 looked for a header requested with angle brackets. For example, if 803 `/usr/include/sys/stat.h' contains `#include "types.h"', GCC looks for 804 `types.h' first in `/usr/include/sys', then in its usual search path. 805 806 `#line' (*note Line Control::) does not change GCC's idea of the 807 directory containing the current file. 808 809 You may put `-I-' at any point in your list of `-I' options. This 810 has two effects. First, directories appearing before the `-I-' in the 811 list are searched only for headers requested with quote marks. 812 Directories after `-I-' are searched for all headers. Second, the 813 directory containing the current file is not searched for anything, 814 unless it happens to be one of the directories named by an `-I' switch. 815 `-I-' is deprecated, `-iquote' should be used instead. 816 817 `-I. -I-' is not the same as no `-I' options at all, and does not 818 cause the same behavior for `<>' includes that `""' includes get with 819 no special options. `-I.' searches the compiler's current working 820 directory for header files. That may or may not be the same as the 821 directory containing the current file. 822 823 If you need to look for headers in a directory named `-', write 824 `-I./-'. 825 826 There are several more ways to adjust the header search path. They 827 are generally less useful. *Note Invocation::. 828 829 830 File: cpp.info, Node: Once-Only Headers, Next: Computed Includes, Prev: Search Path, Up: Header Files 831 832 2.4 Once-Only Headers 833 ===================== 834 835 If a header file happens to be included twice, the compiler will process 836 its contents twice. This is very likely to cause an error, e.g. when 837 the compiler sees the same structure definition twice. Even if it does 838 not, it will certainly waste time. 839 840 The standard way to prevent this is to enclose the entire real 841 contents of the file in a conditional, like this: 842 843 /* File foo. */ 844 #ifndef FILE_FOO_SEEN 845 #define FILE_FOO_SEEN 846 847 THE ENTIRE FILE 848 849 #endif /* !FILE_FOO_SEEN */ 850 851 This construct is commonly known as a "wrapper #ifndef". When the 852 header is included again, the conditional will be false, because 853 `FILE_FOO_SEEN' is defined. The preprocessor will skip over the entire 854 contents of the file, and the compiler will not see it twice. 855 856 CPP optimizes even further. It remembers when a header file has a 857 wrapper `#ifndef'. If a subsequent `#include' specifies that header, 858 and the macro in the `#ifndef' is still defined, it does not bother to 859 rescan the file at all. 860 861 You can put comments outside the wrapper. They will not interfere 862 with this optimization. 863 864 The macro `FILE_FOO_SEEN' is called the "controlling macro" or 865 "guard macro". In a user header file, the macro name should not begin 866 with `_'. In a system header file, it should begin with `__' to avoid 867 conflicts with user programs. In any kind of header file, the macro 868 name should contain the name of the file and some additional text, to 869 avoid conflicts with other header files. 870 871 872 File: cpp.info, Node: Computed Includes, Next: Wrapper Headers, Prev: Once-Only Headers, Up: Header Files 873 874 2.5 Computed Includes 875 ===================== 876 877 Sometimes it is necessary to select one of several different header 878 files to be included into your program. They might specify 879 configuration parameters to be used on different sorts of operating 880 systems, for instance. You could do this with a series of conditionals, 881 882 #if SYSTEM_1 883 # include "system_1.h" 884 #elif SYSTEM_2 885 # include "system_2.h" 886 #elif SYSTEM_3 887 ... 888 #endif 889 890 That rapidly becomes tedious. Instead, the preprocessor offers the 891 ability to use a macro for the header name. This is called a "computed 892 include". Instead of writing a header name as the direct argument of 893 `#include', you simply put a macro name there instead: 894 895 #define SYSTEM_H "system_1.h" 896 ... 897 #include SYSTEM_H 898 899 `SYSTEM_H' will be expanded, and the preprocessor will look for 900 `system_1.h' as if the `#include' had been written that way originally. 901 `SYSTEM_H' could be defined by your Makefile with a `-D' option. 902 903 You must be careful when you define the macro. `#define' saves 904 tokens, not text. The preprocessor has no way of knowing that the macro 905 will be used as the argument of `#include', so it generates ordinary 906 tokens, not a header name. This is unlikely to cause problems if you 907 use double-quote includes, which are close enough to string constants. 908 If you use angle brackets, however, you may have trouble. 909 910 The syntax of a computed include is actually a bit more general than 911 the above. If the first non-whitespace character after `#include' is 912 not `"' or `<', then the entire line is macro-expanded like running 913 text would be. 914 915 If the line expands to a single string constant, the contents of that 916 string constant are the file to be included. CPP does not re-examine 917 the string for embedded quotes, but neither does it process backslash 918 escapes in the string. Therefore 919 920 #define HEADER "a\"b" 921 #include HEADER 922 923 looks for a file named `a\"b'. CPP searches for the file according to 924 the rules for double-quoted includes. 925 926 If the line expands to a token stream beginning with a `<' token and 927 including a `>' token, then the tokens between the `<' and the first 928 `>' are combined to form the filename to be included. Any whitespace 929 between tokens is reduced to a single space; then any space after the 930 initial `<' is retained, but a trailing space before the closing `>' is 931 ignored. CPP searches for the file according to the rules for 932 angle-bracket includes. 933 934 In either case, if there are any tokens on the line after the file 935 name, an error occurs and the directive is not processed. It is also 936 an error if the result of expansion does not match either of the two 937 expected forms. 938 939 These rules are implementation-defined behavior according to the C 940 standard. To minimize the risk of different compilers interpreting your 941 computed includes differently, we recommend you use only a single 942 object-like macro which expands to a string constant. This will also 943 minimize confusion for people reading your program. 944 945 946 File: cpp.info, Node: Wrapper Headers, Next: System Headers, Prev: Computed Includes, Up: Header Files 947 948 2.6 Wrapper Headers 949 =================== 950 951 Sometimes it is necessary to adjust the contents of a system-provided 952 header file without editing it directly. GCC's `fixincludes' operation 953 does this, for example. One way to do that would be to create a new 954 header file with the same name and insert it in the search path before 955 the original header. That works fine as long as you're willing to 956 replace the old header entirely. But what if you want to refer to the 957 old header from the new one? 958 959 You cannot simply include the old header with `#include'. That will 960 start from the beginning, and find your new header again. If your 961 header is not protected from multiple inclusion (*note Once-Only 962 Headers::), it will recurse infinitely and cause a fatal error. 963 964 You could include the old header with an absolute pathname: 965 #include "/usr/include/old-header.h" 966 This works, but is not clean; should the system headers ever move, 967 you would have to edit the new headers to match. 968 969 There is no way to solve this problem within the C standard, but you 970 can use the GNU extension `#include_next'. It means, "Include the 971 _next_ file with this name". This directive works like `#include' 972 except in searching for the specified file: it starts searching the 973 list of header file directories _after_ the directory in which the 974 current file was found. 975 976 Suppose you specify `-I /usr/local/include', and the list of 977 directories to search also includes `/usr/include'; and suppose both 978 directories contain `signal.h'. Ordinary `#include <signal.h>' finds 979 the file under `/usr/local/include'. If that file contains 980 `#include_next <signal.h>', it starts searching after that directory, 981 and finds the file in `/usr/include'. 982 983 `#include_next' does not distinguish between `<FILE>' and `"FILE"' 984 inclusion, nor does it check that the file you specify has the same 985 name as the current file. It simply looks for the file named, starting 986 with the directory in the search path after the one where the current 987 file was found. 988 989 The use of `#include_next' can lead to great confusion. We 990 recommend it be used only when there is no other alternative. In 991 particular, it should not be used in the headers belonging to a specific 992 program; it should be used only to make global corrections along the 993 lines of `fixincludes'. 994 995 996 File: cpp.info, Node: System Headers, Prev: Wrapper Headers, Up: Header Files 997 998 2.7 System Headers 999 ================== 1000 1001 The header files declaring interfaces to the operating system and 1002 runtime libraries often cannot be written in strictly conforming C. 1003 Therefore, GCC gives code found in "system headers" special treatment. 1004 All warnings, other than those generated by `#warning' (*note 1005 Diagnostics::), are suppressed while GCC is processing a system header. 1006 Macros defined in a system header are immune to a few warnings 1007 wherever they are expanded. This immunity is granted on an ad-hoc 1008 basis, when we find that a warning generates lots of false positives 1009 because of code in macros defined in system headers. 1010 1011 Normally, only the headers found in specific directories are 1012 considered system headers. These directories are determined when GCC 1013 is compiled. There are, however, two ways to make normal headers into 1014 system headers. 1015 1016 The `-isystem' command line option adds its argument to the list of 1017 directories to search for headers, just like `-I'. Any headers found 1018 in that directory will be considered system headers. 1019 1020 All directories named by `-isystem' are searched _after_ all 1021 directories named by `-I', no matter what their order was on the 1022 command line. If the same directory is named by both `-I' and 1023 `-isystem', the `-I' option is ignored. GCC provides an informative 1024 message when this occurs if `-v' is used. 1025 1026 There is also a directive, `#pragma GCC system_header', which tells 1027 GCC to consider the rest of the current include file a system header, 1028 no matter where it was found. Code that comes before the `#pragma' in 1029 the file will not be affected. `#pragma GCC system_header' has no 1030 effect in the primary source file. 1031 1032 On very old systems, some of the pre-defined system header 1033 directories get even more special treatment. GNU C++ considers code in 1034 headers found in those directories to be surrounded by an `extern "C"' 1035 block. There is no way to request this behavior with a `#pragma', or 1036 from the command line. 1037 1038 1039 File: cpp.info, Node: Macros, Next: Conditionals, Prev: Header Files, Up: Top 1040 1041 3 Macros 1042 ******** 1043 1044 A "macro" is a fragment of code which has been given a name. Whenever 1045 the name is used, it is replaced by the contents of the macro. There 1046 are two kinds of macros. They differ mostly in what they look like 1047 when they are used. "Object-like" macros resemble data objects when 1048 used, "function-like" macros resemble function calls. 1049 1050 You may define any valid identifier as a macro, even if it is a C 1051 keyword. The preprocessor does not know anything about keywords. This 1052 can be useful if you wish to hide a keyword such as `const' from an 1053 older compiler that does not understand it. However, the preprocessor 1054 operator `defined' (*note Defined::) can never be defined as a macro, 1055 and C++'s named operators (*note C++ Named Operators::) cannot be 1056 macros when you are compiling C++. 1057 1058 * Menu: 1059 1060 * Object-like Macros:: 1061 * Function-like Macros:: 1062 * Macro Arguments:: 1063 * Stringification:: 1064 * Concatenation:: 1065 * Variadic Macros:: 1066 * Predefined Macros:: 1067 * Undefining and Redefining Macros:: 1068 * Directives Within Macro Arguments:: 1069 * Macro Pitfalls:: 1070 1071 1072 File: cpp.info, Node: Object-like Macros, Next: Function-like Macros, Up: Macros 1073 1074 3.1 Object-like Macros 1075 ====================== 1076 1077 An "object-like macro" is a simple identifier which will be replaced by 1078 a code fragment. It is called object-like because it looks like a data 1079 object in code that uses it. They are most commonly used to give 1080 symbolic names to numeric constants. 1081 1082 You create macros with the `#define' directive. `#define' is 1083 followed by the name of the macro and then the token sequence it should 1084 be an abbreviation for, which is variously referred to as the macro's 1085 "body", "expansion" or "replacement list". For example, 1086 1087 #define BUFFER_SIZE 1024 1088 1089 defines a macro named `BUFFER_SIZE' as an abbreviation for the token 1090 `1024'. If somewhere after this `#define' directive there comes a C 1091 statement of the form 1092 1093 foo = (char *) malloc (BUFFER_SIZE); 1094 1095 then the C preprocessor will recognize and "expand" the macro 1096 `BUFFER_SIZE'. The C compiler will see the same tokens as it would if 1097 you had written 1098 1099 foo = (char *) malloc (1024); 1100 1101 By convention, macro names are written in uppercase. Programs are 1102 easier to read when it is possible to tell at a glance which names are 1103 macros. 1104 1105 The macro's body ends at the end of the `#define' line. You may 1106 continue the definition onto multiple lines, if necessary, using 1107 backslash-newline. When the macro is expanded, however, it will all 1108 come out on one line. For example, 1109 1110 #define NUMBERS 1, \ 1111 2, \ 1112 3 1113 int x[] = { NUMBERS }; 1114 ==> int x[] = { 1, 2, 3 }; 1115 1116 The most common visible consequence of this is surprising line numbers 1117 in error messages. 1118 1119 There is no restriction on what can go in a macro body provided it 1120 decomposes into valid preprocessing tokens. Parentheses need not 1121 balance, and the body need not resemble valid C code. (If it does not, 1122 you may get error messages from the C compiler when you use the macro.) 1123 1124 The C preprocessor scans your program sequentially. Macro 1125 definitions take effect at the place you write them. Therefore, the 1126 following input to the C preprocessor 1127 1128 foo = X; 1129 #define X 4 1130 bar = X; 1131 1132 produces 1133 1134 foo = X; 1135 bar = 4; 1136 1137 When the preprocessor expands a macro name, the macro's expansion 1138 replaces the macro invocation, then the expansion is examined for more 1139 macros to expand. For example, 1140 1141 #define TABLESIZE BUFSIZE 1142 #define BUFSIZE 1024 1143 TABLESIZE 1144 ==> BUFSIZE 1145 ==> 1024 1146 1147 `TABLESIZE' is expanded first to produce `BUFSIZE', then that macro is 1148 expanded to produce the final result, `1024'. 1149 1150 Notice that `BUFSIZE' was not defined when `TABLESIZE' was defined. 1151 The `#define' for `TABLESIZE' uses exactly the expansion you 1152 specify--in this case, `BUFSIZE'--and does not check to see whether it 1153 too contains macro names. Only when you _use_ `TABLESIZE' is the 1154 result of its expansion scanned for more macro names. 1155 1156 This makes a difference if you change the definition of `BUFSIZE' at 1157 some point in the source file. `TABLESIZE', defined as shown, will 1158 always expand using the definition of `BUFSIZE' that is currently in 1159 effect: 1160 1161 #define BUFSIZE 1020 1162 #define TABLESIZE BUFSIZE 1163 #undef BUFSIZE 1164 #define BUFSIZE 37 1165 1166 Now `TABLESIZE' expands (in two stages) to `37'. 1167 1168 If the expansion of a macro contains its own name, either directly or 1169 via intermediate macros, it is not expanded again when the expansion is 1170 examined for more macros. This prevents infinite recursion. *Note 1171 Self-Referential Macros::, for the precise details. 1172 1173 1174 File: cpp.info, Node: Function-like Macros, Next: Macro Arguments, Prev: Object-like Macros, Up: Macros 1175 1176 3.2 Function-like Macros 1177 ======================== 1178 1179 You can also define macros whose use looks like a function call. These 1180 are called "function-like macros". To define a function-like macro, 1181 you use the same `#define' directive, but you put a pair of parentheses 1182 immediately after the macro name. For example, 1183 1184 #define lang_init() c_init() 1185 lang_init() 1186 ==> c_init() 1187 1188 A function-like macro is only expanded if its name appears with a 1189 pair of parentheses after it. If you write just the name, it is left 1190 alone. This can be useful when you have a function and a macro of the 1191 same name, and you wish to use the function sometimes. 1192 1193 extern void foo(void); 1194 #define foo() /* optimized inline version */ 1195 ... 1196 foo(); 1197 funcptr = foo; 1198 1199 Here the call to `foo()' will use the macro, but the function 1200 pointer will get the address of the real function. If the macro were to 1201 be expanded, it would cause a syntax error. 1202 1203 If you put spaces between the macro name and the parentheses in the 1204 macro definition, that does not define a function-like macro, it defines 1205 an object-like macro whose expansion happens to begin with a pair of 1206 parentheses. 1207 1208 #define lang_init () c_init() 1209 lang_init() 1210 ==> () c_init()() 1211 1212 The first two pairs of parentheses in this expansion come from the 1213 macro. The third is the pair that was originally after the macro 1214 invocation. Since `lang_init' is an object-like macro, it does not 1215 consume those parentheses. 1216 1217 1218 File: cpp.info, Node: Macro Arguments, Next: Stringification, Prev: Function-like Macros, Up: Macros 1219 1220 3.3 Macro Arguments 1221 =================== 1222 1223 Function-like macros can take "arguments", just like true functions. 1224 To define a macro that uses arguments, you insert "parameters" between 1225 the pair of parentheses in the macro definition that make the macro 1226 function-like. The parameters must be valid C identifiers, separated 1227 by commas and optionally whitespace. 1228 1229 To invoke a macro that takes arguments, you write the name of the 1230 macro followed by a list of "actual arguments" in parentheses, separated 1231 by commas. The invocation of the macro need not be restricted to a 1232 single logical line--it can cross as many lines in the source file as 1233 you wish. The number of arguments you give must match the number of 1234 parameters in the macro definition. When the macro is expanded, each 1235 use of a parameter in its body is replaced by the tokens of the 1236 corresponding argument. (You need not use all of the parameters in the 1237 macro body.) 1238 1239 As an example, here is a macro that computes the minimum of two 1240 numeric values, as it is defined in many C programs, and some uses. 1241 1242 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1243 x = min(a, b); ==> x = ((a) < (b) ? (a) : (b)); 1244 y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2)); 1245 z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1246 1247 (In this small example you can already see several of the dangers of 1248 macro arguments. *Note Macro Pitfalls::, for detailed explanations.) 1249 1250 Leading and trailing whitespace in each argument is dropped, and all 1251 whitespace between the tokens of an argument is reduced to a single 1252 space. Parentheses within each argument must balance; a comma within 1253 such parentheses does not end the argument. However, there is no 1254 requirement for square brackets or braces to balance, and they do not 1255 prevent a comma from separating arguments. Thus, 1256 1257 macro (array[x = y, x + 1]) 1258 1259 passes two arguments to `macro': `array[x = y' and `x + 1]'. If you 1260 want to supply `array[x = y, x + 1]' as an argument, you can write it 1261 as `array[(x = y, x + 1)]', which is equivalent C code. 1262 1263 All arguments to a macro are completely macro-expanded before they 1264 are substituted into the macro body. After substitution, the complete 1265 text is scanned again for macros to expand, including the arguments. 1266 This rule may seem strange, but it is carefully designed so you need 1267 not worry about whether any function call is actually a macro 1268 invocation. You can run into trouble if you try to be too clever, 1269 though. *Note Argument Prescan::, for detailed discussion. 1270 1271 For example, `min (min (a, b), c)' is first expanded to 1272 1273 min (((a) < (b) ? (a) : (b)), (c)) 1274 1275 and then to 1276 1277 ((((a) < (b) ? (a) : (b))) < (c) 1278 ? (((a) < (b) ? (a) : (b))) 1279 : (c)) 1280 1281 (Line breaks shown here for clarity would not actually be generated.) 1282 1283 You can leave macro arguments empty; this is not an error to the 1284 preprocessor (but many macros will then expand to invalid code). You 1285 cannot leave out arguments entirely; if a macro takes two arguments, 1286 there must be exactly one comma at the top level of its argument list. 1287 Here are some silly examples using `min': 1288 1289 min(, b) ==> (( ) < (b) ? ( ) : (b)) 1290 min(a, ) ==> ((a ) < ( ) ? (a ) : ( )) 1291 min(,) ==> (( ) < ( ) ? ( ) : ( )) 1292 min((,),) ==> (((,)) < ( ) ? ((,)) : ( )) 1293 1294 min() error--> macro "min" requires 2 arguments, but only 1 given 1295 min(,,) error--> macro "min" passed 3 arguments, but takes just 2 1296 1297 Whitespace is not a preprocessing token, so if a macro `foo' takes 1298 one argument, `foo ()' and `foo ( )' both supply it an empty argument. 1299 Previous GNU preprocessor implementations and documentation were 1300 incorrect on this point, insisting that a function-like macro that 1301 takes a single argument be passed a space if an empty argument was 1302 required. 1303 1304 Macro parameters appearing inside string literals are not replaced by 1305 their corresponding actual arguments. 1306 1307 #define foo(x) x, "x" 1308 foo(bar) ==> bar, "x" 1309 1310 1311 File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Macro Arguments, Up: Macros 1312 1313 3.4 Stringification 1314 =================== 1315 1316 Sometimes you may want to convert a macro argument into a string 1317 constant. Parameters are not replaced inside string constants, but you 1318 can use the `#' preprocessing operator instead. When a macro parameter 1319 is used with a leading `#', the preprocessor replaces it with the 1320 literal text of the actual argument, converted to a string constant. 1321 Unlike normal parameter replacement, the argument is not macro-expanded 1322 first. This is called "stringification". 1323 1324 There is no way to combine an argument with surrounding text and 1325 stringify it all together. Instead, you can write a series of adjacent 1326 string constants and stringified arguments. The preprocessor will 1327 replace the stringified arguments with string constants. The C 1328 compiler will then combine all the adjacent string constants into one 1329 long string. 1330 1331 Here is an example of a macro definition that uses stringification: 1332 1333 #define WARN_IF(EXP) \ 1334 do { if (EXP) \ 1335 fprintf (stderr, "Warning: " #EXP "\n"); } \ 1336 while (0) 1337 WARN_IF (x == 0); 1338 ==> do { if (x == 0) 1339 fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0); 1340 1341 The argument for `EXP' is substituted once, as-is, into the `if' 1342 statement, and once, stringified, into the argument to `fprintf'. If 1343 `x' were a macro, it would be expanded in the `if' statement, but not 1344 in the string. 1345 1346 The `do' and `while (0)' are a kludge to make it possible to write 1347 `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function 1348 would make C programmers want to do; see *Note Swallowing the 1349 Semicolon::. 1350 1351 Stringification in C involves more than putting double-quote 1352 characters around the fragment. The preprocessor backslash-escapes the 1353 quotes surrounding embedded string constants, and all backslashes 1354 within string and character constants, in order to get a valid C string 1355 constant with the proper contents. Thus, stringifying `p = "foo\n";' 1356 results in "p = \"foo\\n\";". However, backslashes that are not inside 1357 string or character constants are not duplicated: `\n' by itself 1358 stringifies to "\n". 1359 1360 All leading and trailing whitespace in text being stringified is 1361 ignored. Any sequence of whitespace in the middle of the text is 1362 converted to a single space in the stringified result. Comments are 1363 replaced by whitespace long before stringification happens, so they 1364 never appear in stringified text. 1365 1366 There is no way to convert a macro argument into a character 1367 constant. 1368 1369 If you want to stringify the result of expansion of a macro argument, 1370 you have to use two levels of macros. 1371 1372 #define xstr(s) str(s) 1373 #define str(s) #s 1374 #define foo 4 1375 str (foo) 1376 ==> "foo" 1377 xstr (foo) 1378 ==> xstr (4) 1379 ==> str (4) 1380 ==> "4" 1381 1382 `s' is stringified when it is used in `str', so it is not 1383 macro-expanded first. But `s' is an ordinary argument to `xstr', so it 1384 is completely macro-expanded before `xstr' itself is expanded (*note 1385 Argument Prescan::). Therefore, by the time `str' gets to its 1386 argument, it has already been macro-expanded. 1387 1388 1389 File: cpp.info, Node: Concatenation, Next: Variadic Macros, Prev: Stringification, Up: Macros 1390 1391 3.5 Concatenation 1392 ================= 1393 1394 It is often useful to merge two tokens into one while expanding macros. 1395 This is called "token pasting" or "token concatenation". The `##' 1396 preprocessing operator performs token pasting. When a macro is 1397 expanded, the two tokens on either side of each `##' operator are 1398 combined into a single token, which then replaces the `##' and the two 1399 original tokens in the macro expansion. Usually both will be 1400 identifiers, or one will be an identifier and the other a preprocessing 1401 number. When pasted, they make a longer identifier. This isn't the 1402 only valid case. It is also possible to concatenate two numbers (or a 1403 number and a name, such as `1.5' and `e3') into a number. Also, 1404 multi-character operators such as `+=' can be formed by token pasting. 1405 1406 However, two tokens that don't together form a valid token cannot be 1407 pasted together. For example, you cannot concatenate `x' with `+' in 1408 either order. If you try, the preprocessor issues a warning and emits 1409 the two tokens. Whether it puts white space between the tokens is 1410 undefined. It is common to find unnecessary uses of `##' in complex 1411 macros. If you get this warning, it is likely that you can simply 1412 remove the `##'. 1413 1414 Both the tokens combined by `##' could come from the macro body, but 1415 you could just as well write them as one token in the first place. 1416 Token pasting is most useful when one or both of the tokens comes from a 1417 macro argument. If either of the tokens next to an `##' is a parameter 1418 name, it is replaced by its actual argument before `##' executes. As 1419 with stringification, the actual argument is not macro-expanded first. 1420 If the argument is empty, that `##' has no effect. 1421 1422 Keep in mind that the C preprocessor converts comments to whitespace 1423 before macros are even considered. Therefore, you cannot create a 1424 comment by concatenating `/' and `*'. You can put as much whitespace 1425 between `##' and its operands as you like, including comments, and you 1426 can put comments in arguments that will be concatenated. However, it 1427 is an error if `##' appears at either end of a macro body. 1428 1429 Consider a C program that interprets named commands. There probably 1430 needs to be a table of commands, perhaps an array of structures declared 1431 as follows: 1432 1433 struct command 1434 { 1435 char *name; 1436 void (*function) (void); 1437 }; 1438 1439 struct command commands[] = 1440 { 1441 { "quit", quit_command }, 1442 { "help", help_command }, 1443 ... 1444 }; 1445 1446 It would be cleaner not to have to give each command name twice, 1447 once in the string constant and once in the function name. A macro 1448 which takes the name of a command as an argument can make this 1449 unnecessary. The string constant can be created with stringification, 1450 and the function name by concatenating the argument with `_command'. 1451 Here is how it is done: 1452 1453 #define COMMAND(NAME) { #NAME, NAME ## _command } 1454 1455 struct command commands[] = 1456 { 1457 COMMAND (quit), 1458 COMMAND (help), 1459 ... 1460 }; 1461 1462 1463 File: cpp.info, Node: Variadic Macros, Next: Predefined Macros, Prev: Concatenation, Up: Macros 1464 1465 3.6 Variadic Macros 1466 =================== 1467 1468 A macro can be declared to accept a variable number of arguments much as 1469 a function can. The syntax for defining the macro is similar to that of 1470 a function. Here is an example: 1471 1472 #define eprintf(...) fprintf (stderr, __VA_ARGS__) 1473 1474 This kind of macro is called "variadic". When the macro is invoked, 1475 all the tokens in its argument list after the last named argument (this 1476 macro has none), including any commas, become the "variable argument". 1477 This sequence of tokens replaces the identifier `__VA_ARGS__' in the 1478 macro body wherever it appears. Thus, we have this expansion: 1479 1480 eprintf ("%s:%d: ", input_file, lineno) 1481 ==> fprintf (stderr, "%s:%d: ", input_file, lineno) 1482 1483 The variable argument is completely macro-expanded before it is 1484 inserted into the macro expansion, just like an ordinary argument. You 1485 may use the `#' and `##' operators to stringify the variable argument 1486 or to paste its leading or trailing token with another token. (But see 1487 below for an important special case for `##'.) 1488 1489 If your macro is complicated, you may want a more descriptive name 1490 for the variable argument than `__VA_ARGS__'. CPP permits this, as an 1491 extension. You may write an argument name immediately before the 1492 `...'; that name is used for the variable argument. The `eprintf' 1493 macro above could be written 1494 1495 #define eprintf(args...) fprintf (stderr, args) 1496 1497 using this extension. You cannot use `__VA_ARGS__' and this extension 1498 in the same macro. 1499 1500 You can have named arguments as well as variable arguments in a 1501 variadic macro. We could define `eprintf' like this, instead: 1502 1503 #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__) 1504 1505 This formulation looks more descriptive, but unfortunately it is less 1506 flexible: you must now supply at least one argument after the format 1507 string. In standard C, you cannot omit the comma separating the named 1508 argument from the variable arguments. Furthermore, if you leave the 1509 variable argument empty, you will get a syntax error, because there 1510 will be an extra comma after the format string. 1511 1512 eprintf("success!\n", ); 1513 ==> fprintf(stderr, "success!\n", ); 1514 1515 GNU CPP has a pair of extensions which deal with this problem. 1516 First, you are allowed to leave the variable argument out entirely: 1517 1518 eprintf ("success!\n") 1519 ==> fprintf(stderr, "success!\n", ); 1520 1521 Second, the `##' token paste operator has a special meaning when placed 1522 between a comma and a variable argument. If you write 1523 1524 #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__) 1525 1526 and the variable argument is left out when the `eprintf' macro is used, 1527 then the comma before the `##' will be deleted. This does _not_ happen 1528 if you pass an empty argument, nor does it happen if the token 1529 preceding `##' is anything other than a comma. 1530 1531 eprintf ("success!\n") 1532 ==> fprintf(stderr, "success!\n"); 1533 1534 The above explanation is ambiguous about the case where the only macro 1535 parameter is a variable arguments parameter, as it is meaningless to 1536 try to distinguish whether no argument at all is an empty argument or a 1537 missing argument. In this case the C99 standard is clear that the 1538 comma must remain, however the existing GCC extension used to swallow 1539 the comma. So CPP retains the comma when conforming to a specific C 1540 standard, and drops it otherwise. 1541 1542 C99 mandates that the only place the identifier `__VA_ARGS__' can 1543 appear is in the replacement list of a variadic macro. It may not be 1544 used as a macro name, macro argument name, or within a different type 1545 of macro. It may also be forbidden in open text; the standard is 1546 ambiguous. We recommend you avoid using it except for its defined 1547 purpose. 1548 1549 Variadic macros are a new feature in C99. GNU CPP has supported them 1550 for a long time, but only with a named variable argument (`args...', 1551 not `...' and `__VA_ARGS__'). If you are concerned with portability to 1552 previous versions of GCC, you should use only named variable arguments. 1553 On the other hand, if you are concerned with portability to other 1554 conforming implementations of C99, you should use only `__VA_ARGS__'. 1555 1556 Previous versions of CPP implemented the comma-deletion extension 1557 much more generally. We have restricted it in this release to minimize 1558 the differences from C99. To get the same effect with both this and 1559 previous versions of GCC, the token preceding the special `##' must be 1560 a comma, and there must be white space between that comma and whatever 1561 comes immediately before it: 1562 1563 #define eprintf(format, args...) fprintf (stderr, format , ##args) 1564 1565 *Note Differences from previous versions::, for the gory details. 1566 1567 1568 File: cpp.info, Node: Predefined Macros, Next: Undefining and Redefining Macros, Prev: Variadic Macros, Up: Macros 1569 1570 3.7 Predefined Macros 1571 ===================== 1572 1573 Several object-like macros are predefined; you use them without 1574 supplying their definitions. They fall into three classes: standard, 1575 common, and system-specific. 1576 1577 In C++, there is a fourth category, the named operators. They act 1578 like predefined macros, but you cannot undefine them. 1579 1580 * Menu: 1581 1582 * Standard Predefined Macros:: 1583 * Common Predefined Macros:: 1584 * System-specific Predefined Macros:: 1585 * C++ Named Operators:: 1586 1587 1588 File: cpp.info, Node: Standard Predefined Macros, Next: Common Predefined Macros, Up: Predefined Macros 1589 1590 3.7.1 Standard Predefined Macros 1591 -------------------------------- 1592 1593 The standard predefined macros are specified by the relevant language 1594 standards, so they are available with all compilers that implement 1595 those standards. Older compilers may not provide all of them. Their 1596 names all start with double underscores. 1597 1598 `__FILE__' 1599 This macro expands to the name of the current input file, in the 1600 form of a C string constant. This is the path by which the 1601 preprocessor opened the file, not the short name specified in 1602 `#include' or as the input file name argument. For example, 1603 `"/usr/local/include/myheader.h"' is a possible expansion of this 1604 macro. 1605 1606 `__LINE__' 1607 This macro expands to the current input line number, in the form 1608 of a decimal integer constant. While we call it a predefined 1609 macro, it's a pretty strange macro, since its "definition" changes 1610 with each new line of source code. 1611 1612 `__FILE__' and `__LINE__' are useful in generating an error message 1613 to report an inconsistency detected by the program; the message can 1614 state the source line at which the inconsistency was detected. For 1615 example, 1616 1617 fprintf (stderr, "Internal error: " 1618 "negative string length " 1619 "%d at %s, line %d.", 1620 length, __FILE__, __LINE__); 1621 1622 An `#include' directive changes the expansions of `__FILE__' and 1623 `__LINE__' to correspond to the included file. At the end of that 1624 file, when processing resumes on the input file that contained the 1625 `#include' directive, the expansions of `__FILE__' and `__LINE__' 1626 revert to the values they had before the `#include' (but `__LINE__' is 1627 then incremented by one as processing moves to the line after the 1628 `#include'). 1629 1630 A `#line' directive changes `__LINE__', and may change `__FILE__' as 1631 well. *Note Line Control::. 1632 1633 C99 introduces `__func__', and GCC has provided `__FUNCTION__' for a 1634 long time. Both of these are strings containing the name of the 1635 current function (there are slight semantic differences; see the GCC 1636 manual). Neither of them is a macro; the preprocessor does not know the 1637 name of the current function. They tend to be useful in conjunction 1638 with `__FILE__' and `__LINE__', though. 1639 1640 `__DATE__' 1641 This macro expands to a string constant that describes the date on 1642 which the preprocessor is being run. The string constant contains 1643 eleven characters and looks like `"Feb 12 1996"'. If the day of 1644 the month is less than 10, it is padded with a space on the left. 1645 1646 If GCC cannot determine the current date, it will emit a warning 1647 message (once per compilation) and `__DATE__' will expand to 1648 `"??? ?? ????"'. 1649 1650 `__TIME__' 1651 This macro expands to a string constant that describes the time at 1652 which the preprocessor is being run. The string constant contains 1653 eight characters and looks like `"23:59:01"'. 1654 1655 If GCC cannot determine the current time, it will emit a warning 1656 message (once per compilation) and `__TIME__' will expand to 1657 `"??:??:??"'. 1658 1659 `__STDC__' 1660 In normal operation, this macro expands to the constant 1, to 1661 signify that this compiler conforms to ISO Standard C. If GNU CPP 1662 is used with a compiler other than GCC, this is not necessarily 1663 true; however, the preprocessor always conforms to the standard 1664 unless the `-traditional-cpp' option is used. 1665 1666 This macro is not defined if the `-traditional-cpp' option is used. 1667 1668 On some hosts, the system compiler uses a different convention, 1669 where `__STDC__' is normally 0, but is 1 if the user specifies 1670 strict conformance to the C Standard. CPP follows the host 1671 convention when processing system header files, but when 1672 processing user files `__STDC__' is always 1. This has been 1673 reported to cause problems; for instance, some versions of Solaris 1674 provide X Windows headers that expect `__STDC__' to be either 1675 undefined or 1. *Note Invocation::. 1676 1677 `__STDC_VERSION__' 1678 This macro expands to the C Standard's version number, a long 1679 integer constant of the form `YYYYMML' where YYYY and MM are the 1680 year and month of the Standard version. This signifies which 1681 version of the C Standard the compiler conforms to. Like 1682 `__STDC__', this is not necessarily accurate for the entire 1683 implementation, unless GNU CPP is being used with GCC. 1684 1685 The value `199409L' signifies the 1989 C standard as amended in 1686 1994, which is the current default; the value `199901L' signifies 1687 the 1999 revision of the C standard. Support for the 1999 1688 revision is not yet complete. 1689 1690 This macro is not defined if the `-traditional-cpp' option is 1691 used, nor when compiling C++ or Objective-C. 1692 1693 `__STDC_HOSTED__' 1694 This macro is defined, with value 1, if the compiler's target is a 1695 "hosted environment". A hosted environment has the complete 1696 facilities of the standard C library available. 1697 1698 `__cplusplus' 1699 This macro is defined when the C++ compiler is in use. You can use 1700 `__cplusplus' to test whether a header is compiled by a C compiler 1701 or a C++ compiler. This macro is similar to `__STDC_VERSION__', in 1702 that it expands to a version number. A fully conforming 1703 implementation of the 1998 C++ standard will define this macro to 1704 `199711L'. The GNU C++ compiler is not yet fully conforming, so 1705 it uses `1' instead. It is hoped to complete the implementation 1706 of standard C++ in the near future. 1707 1708 `__OBJC__' 1709 This macro is defined, with value 1, when the Objective-C compiler 1710 is in use. You can use `__OBJC__' to test whether a header is 1711 compiled by a C compiler or a Objective-C compiler. 1712 1713 `__ASSEMBLER__' 1714 This macro is defined with value 1 when preprocessing assembly 1715 language. 1716 1717 1718 1719 File: cpp.info, Node: Common Predefined Macros, Next: System-specific Predefined Macros, Prev: Standard Predefined Macros, Up: Predefined Macros 1720 1721 3.7.2 Common Predefined Macros 1722 ------------------------------ 1723 1724 The common predefined macros are GNU C extensions. They are available 1725 with the same meanings regardless of the machine or operating system on 1726 which you are using GNU C or GNU Fortran. Their names all start with 1727 double underscores. 1728 1729 `__COUNTER__' 1730 This macro expands to sequential integral values starting from 0. 1731 In conjunction with the `##' operator, this provides a convenient 1732 means to generate unique identifiers. Care must be taken to 1733 ensure that `__COUNTER__' is not expanded prior to inclusion of 1734 precompiled headers which use it. Otherwise, the precompiled 1735 headers will not be used. 1736 1737 `__GFORTRAN__' 1738 The GNU Fortran compiler defines this. 1739 1740 `__GNUC__' 1741 `__GNUC_MINOR__' 1742 `__GNUC_PATCHLEVEL__' 1743 These macros are defined by all GNU compilers that use the C 1744 preprocessor: C, C++, Objective-C and Fortran. Their values are 1745 the major version, minor version, and patch level of the compiler, 1746 as integer constants. For example, GCC 3.2.1 will define 1747 `__GNUC__' to 3, `__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__' 1748 to 1. These macros are also defined if you invoke the 1749 preprocessor directly. 1750 1751 `__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the 1752 widely-used development snapshots leading up to 3.0 (which identify 1753 themselves as GCC 2.96 or 2.97, depending on which snapshot you 1754 have). 1755 1756 If all you need to know is whether or not your program is being 1757 compiled by GCC, or a non-GCC compiler that claims to accept the 1758 GNU C dialects, you can simply test `__GNUC__'. If you need to 1759 write code which depends on a specific version, you must be more 1760 careful. Each time the minor version is increased, the patch 1761 level is reset to zero; each time the major version is increased 1762 (which happens rarely), the minor version and patch level are 1763 reset. If you wish to use the predefined macros directly in the 1764 conditional, you will need to write it like this: 1765 1766 /* Test for GCC > 3.2.0 */ 1767 #if __GNUC__ > 3 || \ 1768 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1769 (__GNUC_MINOR__ == 2 && \ 1770 __GNUC_PATCHLEVEL__ > 0)) 1771 1772 Another approach is to use the predefined macros to calculate a 1773 single number, then compare that against a threshold: 1774 1775 #define GCC_VERSION (__GNUC__ * 10000 \ 1776 + __GNUC_MINOR__ * 100 \ 1777 + __GNUC_PATCHLEVEL__) 1778 ... 1779 /* Test for GCC > 3.2.0 */ 1780 #if GCC_VERSION > 30200 1781 1782 Many people find this form easier to understand. 1783 1784 `__GNUG__' 1785 The GNU C++ compiler defines this. Testing it is equivalent to 1786 testing `(__GNUC__ && __cplusplus)'. 1787 1788 `__STRICT_ANSI__' 1789 GCC defines this macro if and only if the `-ansi' switch, or a 1790 `-std' switch specifying strict conformance to some version of ISO 1791 C, was specified when GCC was invoked. It is defined to `1'. 1792 This macro exists primarily to direct GNU libc's header files to 1793 restrict their definitions to the minimal set found in the 1989 C 1794 standard. 1795 1796 `__BASE_FILE__' 1797 This macro expands to the name of the main input file, in the form 1798 of a C string constant. This is the source file that was specified 1799 on the command line of the preprocessor or C compiler. 1800 1801 `__INCLUDE_LEVEL__' 1802 This macro expands to a decimal integer constant that represents 1803 the depth of nesting in include files. The value of this macro is 1804 incremented on every `#include' directive and decremented at the 1805 end of every included file. It starts out at 0, its value within 1806 the base file specified on the command line. 1807 1808 `__ELF__' 1809 This macro is defined if the target uses the ELF object format. 1810 1811 `__VERSION__' 1812 This macro expands to a string constant which describes the 1813 version of the compiler in use. You should not rely on its 1814 contents having any particular form, but it can be counted on to 1815 contain at least the release number. 1816 1817 `__OPTIMIZE__' 1818 `__OPTIMIZE_SIZE__' 1819 `__NO_INLINE__' 1820 These macros describe the compilation mode. `__OPTIMIZE__' is 1821 defined in all optimizing compilations. `__OPTIMIZE_SIZE__' is 1822 defined if the compiler is optimizing for size, not speed. 1823 `__NO_INLINE__' is defined if no functions will be inlined into 1824 their callers (when not optimizing, or when inlining has been 1825 specifically disabled by `-fno-inline'). 1826 1827 These macros cause certain GNU header files to provide optimized 1828 definitions, using macros or inline functions, of system library 1829 functions. You should not use these macros in any way unless you 1830 make sure that programs will execute with the same effect whether 1831 or not they are defined. If they are defined, their value is 1. 1832 1833 `__GNUC_GNU_INLINE__' 1834 GCC defines this macro if functions declared `inline' will be 1835 handled in GCC's traditional gnu89 mode. Object files will contain 1836 externally visible definitions of all functions declared `inline' 1837 without `extern' or `static'. They will not contain any 1838 definitions of any functions declared `extern inline'. 1839 1840 `__GNUC_STDC_INLINE__' 1841 GCC defines this macro if functions declared `inline' will be 1842 handled according to the ISO C99 standard. Object files will 1843 contain externally visible definitions of all functions declared 1844 `extern inline'. They will not contain definitions of any 1845 functions declared `inline' without `extern'. 1846 1847 If this macro is defined, GCC supports the `gnu_inline' function 1848 attribute as a way to always get the gnu89 behavior. Support for 1849 this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3. If neither 1850 macro is defined, an older version of GCC is being used: `inline' 1851 functions will be compiled in gnu89 mode, and the `gnu_inline' 1852 function attribute will not be recognized. 1853 1854 `__CHAR_UNSIGNED__' 1855 GCC defines this macro if and only if the data type `char' is 1856 unsigned on the target machine. It exists to cause the standard 1857 header file `limits.h' to work correctly. You should not use this 1858 macro yourself; instead, refer to the standard macros defined in 1859 `limits.h'. 1860 1861 `__WCHAR_UNSIGNED__' 1862 Like `__CHAR_UNSIGNED__', this macro is defined if and only if the 1863 data type `wchar_t' is unsigned and the front-end is in C++ mode. 1864 1865 `__REGISTER_PREFIX__' 1866 This macro expands to a single token (not a string constant) which 1867 is the prefix applied to CPU register names in assembly language 1868 for this target. You can use it to write assembly that is usable 1869 in multiple environments. For example, in the `m68k-aout' 1870 environment it expands to nothing, but in the `m68k-coff' 1871 environment it expands to a single `%'. 1872 1873 `__USER_LABEL_PREFIX__' 1874 This macro expands to a single token which is the prefix applied to 1875 user labels (symbols visible to C code) in assembly. For example, 1876 in the `m68k-aout' environment it expands to an `_', but in the 1877 `m68k-coff' environment it expands to nothing. 1878 1879 This macro will have the correct definition even if 1880 `-f(no-)underscores' is in use, but it will not be correct if 1881 target-specific options that adjust this prefix are used (e.g. the 1882 OSF/rose `-mno-underscores' option). 1883 1884 `__SIZE_TYPE__' 1885 `__PTRDIFF_TYPE__' 1886 `__WCHAR_TYPE__' 1887 `__WINT_TYPE__' 1888 `__INTMAX_TYPE__' 1889 `__UINTMAX_TYPE__' 1890 These macros are defined to the correct underlying types for the 1891 `size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', and 1892 `uintmax_t' typedefs, respectively. They exist to make the 1893 standard header files `stddef.h' and `wchar.h' work correctly. 1894 You should not use these macros directly; instead, include the 1895 appropriate headers and use the typedefs. 1896 1897 `__CHAR_BIT__' 1898 Defined to the number of bits used in the representation of the 1899 `char' data type. It exists to make the standard header given 1900 numerical limits work correctly. You should not use this macro 1901 directly; instead, include the appropriate headers. 1902 1903 `__SCHAR_MAX__' 1904 `__WCHAR_MAX__' 1905 `__SHRT_MAX__' 1906 `__INT_MAX__' 1907 `__LONG_MAX__' 1908 `__LONG_LONG_MAX__' 1909 `__INTMAX_MAX__' 1910 Defined to the maximum value of the `signed char', `wchar_t', 1911 `signed short', `signed int', `signed long', `signed long long', 1912 and `intmax_t' types respectively. They exist to make the 1913 standard header given numerical limits work correctly. You should 1914 not use these macros directly; instead, include the appropriate 1915 headers. 1916 1917 `__SIZEOF_INT__' 1918 `__SIZEOF_LONG__' 1919 `__SIZEOF_LONG_LONG__' 1920 `__SIZEOF_SHORT__' 1921 `__SIZEOF_POINTER__' 1922 `__SIZEOF_FLOAT__' 1923 `__SIZEOF_DOUBLE__' 1924 `__SIZEOF_LONG_DOUBLE__' 1925 `__SIZEOF_SIZE_T__' 1926 `__SIZEOF_WCHAR_T__' 1927 `__SIZEOF_WINT_T__' 1928 `__SIZEOF_PTRDIFF_T__' 1929 Defined to the number of bytes of the C standard data types: `int', 1930 `long', `long long', `short', `void *', `float', `double', `long 1931 double', `size_t', `wchar_t', `wint_t' and `ptrdiff_t'. 1932 1933 `__DEPRECATED' 1934 This macro is defined, with value 1, when compiling a C++ source 1935 file with warnings about deprecated constructs enabled. These 1936 warnings are enabled by default, but can be disabled with 1937 `-Wno-deprecated'. 1938 1939 `__EXCEPTIONS' 1940 This macro is defined, with value 1, when compiling a C++ source 1941 file with exceptions enabled. If `-fno-exceptions' is used when 1942 compiling the file, then this macro is not defined. 1943 1944 `__GXX_RTTI' 1945 This macro is defined, with value 1, when compiling a C++ source 1946 file with runtime type identification enabled. If `-fno-rtti' is 1947 used when compiling the file, then this macro is not defined. 1948 1949 `__USING_SJLJ_EXCEPTIONS__' 1950 This macro is defined, with value 1, if the compiler uses the old 1951 mechanism based on `setjmp' and `longjmp' for exception handling. 1952 1953 `__GXX_EXPERIMENTAL_CXX0X__' 1954 This macro is defined when compiling a C++ source file with the 1955 option `-std=c++0x' or `-std=gnu++0x'. It indicates that some 1956 features likely to be included in C++0x are available. Note that 1957 these features are experimental, and may change or be removed in 1958 future versions of GCC. 1959 1960 `__GXX_WEAK__' 1961 This macro is defined when compiling a C++ source file. It has the 1962 value 1 if the compiler will use weak symbols, COMDAT sections, or 1963 other similar techniques to collapse symbols with "vague linkage" 1964 that are defined in multiple translation units. If the compiler 1965 will not collapse such symbols, this macro is defined with value 1966 0. In general, user code should not need to make use of this 1967 macro; the purpose of this macro is to ease implementation of the 1968 C++ runtime library provided with G++. 1969 1970 `__NEXT_RUNTIME__' 1971 This macro is defined, with value 1, if (and only if) the NeXT 1972 runtime (as in `-fnext-runtime') is in use for Objective-C. If 1973 the GNU runtime is used, this macro is not defined, so that you 1974 can use this macro to determine which runtime (NeXT or GNU) is 1975 being used. 1976 1977 `__LP64__' 1978 `_LP64' 1979 These macros are defined, with value 1, if (and only if) the 1980 compilation is for a target where `long int' and pointer both use 1981 64-bits and `int' uses 32-bit. 1982 1983 `__SSP__' 1984 This macro is defined, with value 1, when `-fstack-protector' is in 1985 use. 1986 1987 `__SSP_ALL__' 1988 This macro is defined, with value 2, when `-fstack-protector-all' 1989 is in use. 1990 1991 `__TIMESTAMP__' 1992 This macro expands to a string constant that describes the date 1993 and time of the last modification of the current source file. The 1994 string constant contains abbreviated day of the week, month, day 1995 of the month, time in hh:mm:ss form, year and looks like 1996 `"Sun Sep 16 01:03:52 1973"'. If the day of the month is less 1997 than 10, it is padded with a space on the left. 1998 1999 If GCC cannot determine the current date, it will emit a warning 2000 message (once per compilation) and `__TIMESTAMP__' will expand to 2001 `"??? ??? ?? ??:??:?? ????"'. 2002 2003 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1' 2004 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2' 2005 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4' 2006 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8' 2007 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16' 2008 These macros are defined when the target processor supports atomic 2009 compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in 2010 length, respectively. 2011 2012 2013 2014 File: cpp.info, Node: System-specific Predefined Macros, Next: C++ Named Operators, Prev: Common Predefined Macros, Up: Predefined Macros 2015 2016 3.7.3 System-specific Predefined Macros 2017 --------------------------------------- 2018 2019 The C preprocessor normally predefines several macros that indicate what 2020 type of system and machine is in use. They are obviously different on 2021 each target supported by GCC. This manual, being for all systems and 2022 machines, cannot tell you what their names are, but you can use `cpp 2023 -dM' to see them all. *Note Invocation::. All system-specific 2024 predefined macros expand to the constant 1, so you can test them with 2025 either `#ifdef' or `#if'. 2026 2027 The C standard requires that all system-specific macros be part of 2028 the "reserved namespace". All names which begin with two underscores, 2029 or an underscore and a capital letter, are reserved for the compiler and 2030 library to use as they wish. However, historically system-specific 2031 macros have had names with no special prefix; for instance, it is common 2032 to find `unix' defined on Unix systems. For all such macros, GCC 2033 provides a parallel macro with two underscores added at the beginning 2034 and the end. If `unix' is defined, `__unix__' will be defined too. 2035 There will never be more than two underscores; the parallel of `_mips' 2036 is `__mips__'. 2037 2038 When the `-ansi' option, or any `-std' option that requests strict 2039 conformance, is given to the compiler, all the system-specific 2040 predefined macros outside the reserved namespace are suppressed. The 2041 parallel macros, inside the reserved namespace, remain defined. 2042 2043 We are slowly phasing out all predefined macros which are outside the 2044 reserved namespace. You should never use them in new programs, and we 2045 encourage you to correct older code to use the parallel macros whenever 2046 you find it. We don't recommend you use the system-specific macros that 2047 are in the reserved namespace, either. It is better in the long run to 2048 check specifically for features you need, using a tool such as 2049 `autoconf'. 2050 2051 2052 File: cpp.info, Node: C++ Named Operators, Prev: System-specific Predefined Macros, Up: Predefined Macros 2053 2054 3.7.4 C++ Named Operators 2055 ------------------------- 2056 2057 In C++, there are eleven keywords which are simply alternate spellings 2058 of operators normally written with punctuation. These keywords are 2059 treated as such even in the preprocessor. They function as operators in 2060 `#if', and they cannot be defined as macros or poisoned. In C, you can 2061 request that those keywords take their C++ meaning by including 2062 `iso646.h'. That header defines each one as a normal object-like macro 2063 expanding to the appropriate punctuator. 2064 2065 These are the named operators and their corresponding punctuators: 2066 2067 Named Operator Punctuator 2068 `and' `&&' 2069 `and_eq' `&=' 2070 `bitand' `&' 2071 `bitor' `|' 2072 `compl' `~' 2073 `not' `!' 2074 `not_eq' `!=' 2075 `or' `||' 2076 `or_eq' `|=' 2077 `xor' `^' 2078 `xor_eq' `^=' 2079 2080 2081 File: cpp.info, Node: Undefining and Redefining Macros, Next: Directives Within Macro Arguments, Prev: Predefined Macros, Up: Macros 2082 2083 3.8 Undefining and Redefining Macros 2084 ==================================== 2085 2086 If a macro ceases to be useful, it may be "undefined" with the `#undef' 2087 directive. `#undef' takes a single argument, the name of the macro to 2088 undefine. You use the bare macro name, even if the macro is 2089 function-like. It is an error if anything appears on the line after 2090 the macro name. `#undef' has no effect if the name is not a macro. 2091 2092 #define FOO 4 2093 x = FOO; ==> x = 4; 2094 #undef FOO 2095 x = FOO; ==> x = FOO; 2096 2097 Once a macro has been undefined, that identifier may be "redefined" 2098 as a macro by a subsequent `#define' directive. The new definition 2099 need not have any resemblance to the old definition. 2100 2101 However, if an identifier which is currently a macro is redefined, 2102 then the new definition must be "effectively the same" as the old one. 2103 Two macro definitions are effectively the same if: 2104 * Both are the same type of macro (object- or function-like). 2105 2106 * All the tokens of the replacement list are the same. 2107 2108 * If there are any parameters, they are the same. 2109 2110 * Whitespace appears in the same places in both. It need not be 2111 exactly the same amount of whitespace, though. Remember that 2112 comments count as whitespace. 2113 2114 These definitions are effectively the same: 2115 #define FOUR (2 + 2) 2116 #define FOUR (2 + 2) 2117 #define FOUR (2 /* two */ + 2) 2118 but these are not: 2119 #define FOUR (2 + 2) 2120 #define FOUR ( 2+2 ) 2121 #define FOUR (2 * 2) 2122 #define FOUR(score,and,seven,years,ago) (2 + 2) 2123 2124 If a macro is redefined with a definition that is not effectively the 2125 same as the old one, the preprocessor issues a warning and changes the 2126 macro to use the new definition. If the new definition is effectively 2127 the same, the redefinition is silently ignored. This allows, for 2128 instance, two different headers to define a common macro. The 2129 preprocessor will only complain if the definitions do not match. 2130 2131 2132 File: cpp.info, Node: Directives Within Macro Arguments, Next: Macro Pitfalls, Prev: Undefining and Redefining Macros, Up: Macros 2133 2134 3.9 Directives Within Macro Arguments 2135 ===================================== 2136 2137 Occasionally it is convenient to use preprocessor directives within the 2138 arguments of a macro. The C and C++ standards declare that behavior in 2139 these cases is undefined. 2140 2141 Versions of CPP prior to 3.2 would reject such constructs with an 2142 error message. This was the only syntactic difference between normal 2143 functions and function-like macros, so it seemed attractive to remove 2144 this limitation, and people would often be surprised that they could 2145 not use macros in this way. Moreover, sometimes people would use 2146 conditional compilation in the argument list to a normal library 2147 function like `printf', only to find that after a library upgrade 2148 `printf' had changed to be a function-like macro, and their code would 2149 no longer compile. So from version 3.2 we changed CPP to successfully 2150 process arbitrary directives within macro arguments in exactly the same 2151 way as it would have processed the directive were the function-like 2152 macro invocation not present. 2153 2154 If, within a macro invocation, that macro is redefined, then the new 2155 definition takes effect in time for argument pre-expansion, but the 2156 original definition is still used for argument replacement. Here is a 2157 pathological example: 2158 2159 #define f(x) x x 2160 f (1 2161 #undef f 2162 #define f 2 2163 f) 2164 2165 which expands to 2166 2167 1 2 1 2 2168 2169 with the semantics described above. 2170 2171 2172 File: cpp.info, Node: Macro Pitfalls, Prev: Directives Within Macro Arguments, Up: Macros 2173 2174 3.10 Macro Pitfalls 2175 =================== 2176 2177 In this section we describe some special rules that apply to macros and 2178 macro expansion, and point out certain cases in which the rules have 2179 counter-intuitive consequences that you must watch out for. 2180 2181 * Menu: 2182 2183 * Misnesting:: 2184 * Operator Precedence Problems:: 2185 * Swallowing the Semicolon:: 2186 * Duplication of Side Effects:: 2187 * Self-Referential Macros:: 2188 * Argument Prescan:: 2189 * Newlines in Arguments:: 2190 2191 2192 File: cpp.info, Node: Misnesting, Next: Operator Precedence Problems, Up: Macro Pitfalls 2193 2194 3.10.1 Misnesting 2195 ----------------- 2196 2197 When a macro is called with arguments, the arguments are substituted 2198 into the macro body and the result is checked, together with the rest of 2199 the input file, for more macro calls. It is possible to piece together 2200 a macro call coming partially from the macro body and partially from the 2201 arguments. For example, 2202 2203 #define twice(x) (2*(x)) 2204 #define call_with_1(x) x(1) 2205 call_with_1 (twice) 2206 ==> twice(1) 2207 ==> (2*(1)) 2208 2209 Macro definitions do not have to have balanced parentheses. By 2210 writing an unbalanced open parenthesis in a macro body, it is possible 2211 to create a macro call that begins inside the macro body but ends 2212 outside of it. For example, 2213 2214 #define strange(file) fprintf (file, "%s %d", 2215 ... 2216 strange(stderr) p, 35) 2217 ==> fprintf (stderr, "%s %d", p, 35) 2218 2219 The ability to piece together a macro call can be useful, but the 2220 use of unbalanced open parentheses in a macro body is just confusing, 2221 and should be avoided. 2222 2223 2224 File: cpp.info, Node: Operator Precedence Problems, Next: Swallowing the Semicolon, Prev: Misnesting, Up: Macro Pitfalls 2225 2226 3.10.2 Operator Precedence Problems 2227 ----------------------------------- 2228 2229 You may have noticed that in most of the macro definition examples shown 2230 above, each occurrence of a macro argument name had parentheses around 2231 it. In addition, another pair of parentheses usually surround the 2232 entire macro definition. Here is why it is best to write macros that 2233 way. 2234 2235 Suppose you define a macro as follows, 2236 2237 #define ceil_div(x, y) (x + y - 1) / y 2238 2239 whose purpose is to divide, rounding up. (One use for this operation is 2240 to compute how many `int' objects are needed to hold a certain number 2241 of `char' objects.) Then suppose it is used as follows: 2242 2243 a = ceil_div (b & c, sizeof (int)); 2244 ==> a = (b & c + sizeof (int) - 1) / sizeof (int); 2245 2246 This does not do what is intended. The operator-precedence rules of C 2247 make it equivalent to this: 2248 2249 a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2250 2251 What we want is this: 2252 2253 a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2254 2255 Defining the macro as 2256 2257 #define ceil_div(x, y) ((x) + (y) - 1) / (y) 2258 2259 provides the desired result. 2260 2261 Unintended grouping can result in another way. Consider `sizeof 2262 ceil_div(1, 2)'. That has the appearance of a C expression that would 2263 compute the size of the type of `ceil_div (1, 2)', but in fact it means 2264 something very different. Here is what it expands to: 2265 2266 sizeof ((1) + (2) - 1) / (2) 2267 2268 This would take the size of an integer and divide it by two. The 2269 precedence rules have put the division outside the `sizeof' when it was 2270 intended to be inside. 2271 2272 Parentheses around the entire macro definition prevent such problems. 2273 Here, then, is the recommended way to define `ceil_div': 2274 2275 #define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2276 2277 2278 File: cpp.info, Node: Swallowing the Semicolon, Next: Duplication of Side Effects, Prev: Operator Precedence Problems, Up: Macro Pitfalls 2279 2280 3.10.3 Swallowing the Semicolon 2281 ------------------------------- 2282 2283 Often it is desirable to define a macro that expands into a compound 2284 statement. Consider, for example, the following macro, that advances a 2285 pointer (the argument `p' says where to find it) across whitespace 2286 characters: 2287 2288 #define SKIP_SPACES(p, limit) \ 2289 { char *lim = (limit); \ 2290 while (p < lim) { \ 2291 if (*p++ != ' ') { \ 2292 p--; break; }}} 2293 2294 Here backslash-newline is used to split the macro definition, which must 2295 be a single logical line, so that it resembles the way such code would 2296 be laid out if not part of a macro definition. 2297 2298 A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly 2299 speaking, the call expands to a compound statement, which is a complete 2300 statement with no need for a semicolon to end it. However, since it 2301 looks like a function call, it minimizes confusion if you can use it 2302 like a function call, writing a semicolon afterward, as in `SKIP_SPACES 2303 (p, lim);' 2304 2305 This can cause trouble before `else' statements, because the 2306 semicolon is actually a null statement. Suppose you write 2307 2308 if (*p != 0) 2309 SKIP_SPACES (p, lim); 2310 else ... 2311 2312 The presence of two statements--the compound statement and a null 2313 statement--in between the `if' condition and the `else' makes invalid C 2314 code. 2315 2316 The definition of the macro `SKIP_SPACES' can be altered to solve 2317 this problem, using a `do ... while' statement. Here is how: 2318 2319 #define SKIP_SPACES(p, limit) \ 2320 do { char *lim = (limit); \ 2321 while (p < lim) { \ 2322 if (*p++ != ' ') { \ 2323 p--; break; }}} \ 2324 while (0) 2325 2326 Now `SKIP_SPACES (p, lim);' expands into 2327 2328 do {...} while (0); 2329 2330 which is one statement. The loop executes exactly once; most compilers 2331 generate no extra code for it. 2332 2333 2334 File: cpp.info, Node: Duplication of Side Effects, Next: Self-Referential Macros, Prev: Swallowing the Semicolon, Up: Macro Pitfalls 2335 2336 3.10.4 Duplication of Side Effects 2337 ---------------------------------- 2338 2339 Many C programs define a macro `min', for "minimum", like this: 2340 2341 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2342 2343 When you use this macro with an argument containing a side effect, 2344 as shown here, 2345 2346 next = min (x + y, foo (z)); 2347 2348 it expands as follows: 2349 2350 next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2351 2352 where `x + y' has been substituted for `X' and `foo (z)' for `Y'. 2353 2354 The function `foo' is used only once in the statement as it appears 2355 in the program, but the expression `foo (z)' has been substituted twice 2356 into the macro expansion. As a result, `foo' might be called two times 2357 when the statement is executed. If it has side effects or if it takes 2358 a long time to compute, the results might not be what you intended. We 2359 say that `min' is an "unsafe" macro. 2360 2361 The best solution to this problem is to define `min' in a way that 2362 computes the value of `foo (z)' only once. The C language offers no 2363 standard way to do this, but it can be done with GNU extensions as 2364 follows: 2365 2366 #define min(X, Y) \ 2367 ({ typeof (X) x_ = (X); \ 2368 typeof (Y) y_ = (Y); \ 2369 (x_ < y_) ? x_ : y_; }) 2370 2371 The `({ ... })' notation produces a compound statement that acts as 2372 an expression. Its value is the value of its last statement. This 2373 permits us to define local variables and assign each argument to one. 2374 The local variables have underscores after their names to reduce the 2375 risk of conflict with an identifier of wider scope (it is impossible to 2376 avoid this entirely). Now each argument is evaluated exactly once. 2377 2378 If you do not wish to use GNU C extensions, the only solution is to 2379 be careful when _using_ the macro `min'. For example, you can 2380 calculate the value of `foo (z)', save it in a variable, and use that 2381 variable in `min': 2382 2383 #define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2384 ... 2385 { 2386 int tem = foo (z); 2387 next = min (x + y, tem); 2388 } 2389 2390 (where we assume that `foo' returns type `int'). 2391 2392 2393 File: cpp.info, Node: Self-Referential Macros, Next: Argument Prescan, Prev: Duplication of Side Effects, Up: Macro Pitfalls 2394 2395 3.10.5 Self-Referential Macros 2396 ------------------------------ 2397 2398 A "self-referential" macro is one whose name appears in its definition. 2399 Recall that all macro definitions are rescanned for more macros to 2400 replace. If the self-reference were considered a use of the macro, it 2401 would produce an infinitely large expansion. To prevent this, the 2402 self-reference is not considered a macro call. It is passed into the 2403 preprocessor output unchanged. Consider an example: 2404 2405 #define foo (4 + foo) 2406 2407 where `foo' is also a variable in your program. 2408 2409 Following the ordinary rules, each reference to `foo' will expand 2410 into `(4 + foo)'; then this will be rescanned and will expand into `(4 2411 + (4 + foo))'; and so on until the computer runs out of memory. 2412 2413 The self-reference rule cuts this process short after one step, at 2414 `(4 + foo)'. Therefore, this macro definition has the possibly useful 2415 effect of causing the program to add 4 to the value of `foo' wherever 2416 `foo' is referred to. 2417 2418 In most cases, it is a bad idea to take advantage of this feature. A 2419 person reading the program who sees that `foo' is a variable will not 2420 expect that it is a macro as well. The reader will come across the 2421 identifier `foo' in the program and think its value should be that of 2422 the variable `foo', whereas in fact the value is four greater. 2423 2424 One common, useful use of self-reference is to create a macro which 2425 expands to itself. If you write 2426 2427 #define EPERM EPERM 2428 2429 then the macro `EPERM' expands to `EPERM'. Effectively, it is left 2430 alone by the preprocessor whenever it's used in running text. You can 2431 tell that it's a macro with `#ifdef'. You might do this if you want to 2432 define numeric constants with an `enum', but have `#ifdef' be true for 2433 each constant. 2434 2435 If a macro `x' expands to use a macro `y', and the expansion of `y' 2436 refers to the macro `x', that is an "indirect self-reference" of `x'. 2437 `x' is not expanded in this case either. Thus, if we have 2438 2439 #define x (4 + y) 2440 #define y (2 * x) 2441 2442 then `x' and `y' expand as follows: 2443 2444 x ==> (4 + y) 2445 ==> (4 + (2 * x)) 2446 2447 y ==> (2 * x) 2448 ==> (2 * (4 + y)) 2449 2450 Each macro is expanded when it appears in the definition of the other 2451 macro, but not when it indirectly appears in its own definition. 2452 2453 2454 File: cpp.info, Node: Argument Prescan, Next: Newlines in Arguments, Prev: Self-Referential Macros, Up: Macro Pitfalls 2455 2456 3.10.6 Argument Prescan 2457 ----------------------- 2458 2459 Macro arguments are completely macro-expanded before they are 2460 substituted into a macro body, unless they are stringified or pasted 2461 with other tokens. After substitution, the entire macro body, including 2462 the substituted arguments, is scanned again for macros to be expanded. 2463 The result is that the arguments are scanned _twice_ to expand macro 2464 calls in them. 2465 2466 Most of the time, this has no effect. If the argument contained any 2467 macro calls, they are expanded during the first scan. The result 2468 therefore contains no macro calls, so the second scan does not change 2469 it. If the argument were substituted as given, with no prescan, the 2470 single remaining scan would find the same macro calls and produce the 2471 same results. 2472 2473 You might expect the double scan to change the results when a 2474 self-referential macro is used in an argument of another macro (*note 2475 Self-Referential Macros::): the self-referential macro would be 2476 expanded once in the first scan, and a second time in the second scan. 2477 However, this is not what happens. The self-references that do not 2478 expand in the first scan are marked so that they will not expand in the 2479 second scan either. 2480 2481 You might wonder, "Why mention the prescan, if it makes no 2482 difference? And why not skip it and make the preprocessor faster?" 2483 The answer is that the prescan does make a difference in three special 2484 cases: 2485 2486 * Nested calls to a macro. 2487 2488 We say that "nested" calls to a macro occur when a macro's argument 2489 contains a call to that very macro. For example, if `f' is a macro 2490 that expects one argument, `f (f (1))' is a nested pair of calls to 2491 `f'. The desired expansion is made by expanding `f (1)' and 2492 substituting that into the definition of `f'. The prescan causes 2493 the expected result to happen. Without the prescan, `f (1)' itself 2494 would be substituted as an argument, and the inner use of `f' would 2495 appear during the main scan as an indirect self-reference and 2496 would not be expanded. 2497 2498 * Macros that call other macros that stringify or concatenate. 2499 2500 If an argument is stringified or concatenated, the prescan does not 2501 occur. If you _want_ to expand a macro, then stringify or 2502 concatenate its expansion, you can do that by causing one macro to 2503 call another macro that does the stringification or concatenation. 2504 For instance, if you have 2505 2506 #define AFTERX(x) X_ ## x 2507 #define XAFTERX(x) AFTERX(x) 2508 #define TABLESIZE 1024 2509 #define BUFSIZE TABLESIZE 2510 2511 then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and 2512 `XAFTERX(BUFSIZE)' expands to `X_1024'. (Not to `X_TABLESIZE'. 2513 Prescan always does a complete expansion.) 2514 2515 * Macros used in arguments, whose expansions contain unshielded 2516 commas. 2517 2518 This can cause a macro expanded on the second scan to be called 2519 with the wrong number of arguments. Here is an example: 2520 2521 #define foo a,b 2522 #define bar(x) lose(x) 2523 #define lose(x) (1 + (x)) 2524 2525 We would like `bar(foo)' to turn into `(1 + (foo))', which would 2526 then turn into `(1 + (a,b))'. Instead, `bar(foo)' expands into 2527 `lose(a,b)', and you get an error because `lose' requires a single 2528 argument. In this case, the problem is easily solved by the same 2529 parentheses that ought to be used to prevent misnesting of 2530 arithmetic operations: 2531 2532 #define foo (a,b) 2533 or 2534 #define bar(x) lose((x)) 2535 2536 The extra pair of parentheses prevents the comma in `foo''s 2537 definition from being interpreted as an argument separator. 2538 2539 2540 2541 File: cpp.info, Node: Newlines in Arguments, Prev: Argument Prescan, Up: Macro Pitfalls 2542 2543 3.10.7 Newlines in Arguments 2544 ---------------------------- 2545 2546 The invocation of a function-like macro can extend over many logical 2547 lines. However, in the present implementation, the entire expansion 2548 comes out on one line. Thus line numbers emitted by the compiler or 2549 debugger refer to the line the invocation started on, which might be 2550 different to the line containing the argument causing the problem. 2551 2552 Here is an example illustrating this: 2553 2554 #define ignore_second_arg(a,b,c) a; c 2555 2556 ignore_second_arg (foo (), 2557 ignored (), 2558 syntax error); 2559 2560 The syntax error triggered by the tokens `syntax error' results in an 2561 error message citing line three--the line of ignore_second_arg-- even 2562 though the problematic code comes from line five. 2563 2564 We consider this a bug, and intend to fix it in the near future. 2565 2566 2567 File: cpp.info, Node: Conditionals, Next: Diagnostics, Prev: Macros, Up: Top 2568 2569 4 Conditionals 2570 ************** 2571 2572 A "conditional" is a directive that instructs the preprocessor to 2573 select whether or not to include a chunk of code in the final token 2574 stream passed to the compiler. Preprocessor conditionals can test 2575 arithmetic expressions, or whether a name is defined as a macro, or both 2576 simultaneously using the special `defined' operator. 2577 2578 A conditional in the C preprocessor resembles in some ways an `if' 2579 statement in C, but it is important to understand the difference between 2580 them. The condition in an `if' statement is tested during the 2581 execution of your program. Its purpose is to allow your program to 2582 behave differently from run to run, depending on the data it is 2583 operating on. The condition in a preprocessing conditional directive is 2584 tested when your program is compiled. Its purpose is to allow different 2585 code to be included in the program depending on the situation at the 2586 time of compilation. 2587 2588 However, the distinction is becoming less clear. Modern compilers 2589 often do test `if' statements when a program is compiled, if their 2590 conditions are known not to vary at run time, and eliminate code which 2591 can never be executed. If you can count on your compiler to do this, 2592 you may find that your program is more readable if you use `if' 2593 statements with constant conditions (perhaps determined by macros). Of 2594 course, you can only use this to exclude code, not type definitions or 2595 other preprocessing directives, and you can only do it if the code 2596 remains syntactically valid when it is not to be used. 2597 2598 GCC version 3 eliminates this kind of never-executed code even when 2599 not optimizing. Older versions did it only when optimizing. 2600 2601 * Menu: 2602 2603 * Conditional Uses:: 2604 * Conditional Syntax:: 2605 * Deleted Code:: 2606 2607 2608 File: cpp.info, Node: Conditional Uses, Next: Conditional Syntax, Up: Conditionals 2609 2610 4.1 Conditional Uses 2611 ==================== 2612 2613 There are three general reasons to use a conditional. 2614 2615 * A program may need to use different code depending on the machine 2616 or operating system it is to run on. In some cases the code for 2617 one operating system may be erroneous on another operating system; 2618 for example, it might refer to data types or constants that do not 2619 exist on the other system. When this happens, it is not enough to 2620 avoid executing the invalid code. Its mere presence will cause 2621 the compiler to reject the program. With a preprocessing 2622 conditional, the offending code can be effectively excised from 2623 the program when it is not valid. 2624 2625 * You may want to be able to compile the same source file into two 2626 different programs. One version might make frequent time-consuming 2627 consistency checks on its intermediate data, or print the values of 2628 those data for debugging, and the other not. 2629 2630 * A conditional whose condition is always false is one way to 2631 exclude code from the program but keep it as a sort of comment for 2632 future reference. 2633 2634 Simple programs that do not need system-specific logic or complex 2635 debugging hooks generally will not need to use preprocessing 2636 conditionals. 2637 2638 2639 File: cpp.info, Node: Conditional Syntax, Next: Deleted Code, Prev: Conditional Uses, Up: Conditionals 2640 2641 4.2 Conditional Syntax 2642 ====================== 2643 2644 A conditional in the C preprocessor begins with a "conditional 2645 directive": `#if', `#ifdef' or `#ifndef'. 2646 2647 * Menu: 2648 2649 * Ifdef:: 2650 * If:: 2651 * Defined:: 2652 * Else:: 2653 * Elif:: 2654 2655 2656 File: cpp.info, Node: Ifdef, Next: If, Up: Conditional Syntax 2657 2658 4.2.1 Ifdef 2659 ----------- 2660 2661 The simplest sort of conditional is 2662 2663 #ifdef MACRO 2664 2665 CONTROLLED TEXT 2666 2667 #endif /* MACRO */ 2668 2669 This block is called a "conditional group". CONTROLLED TEXT will be 2670 included in the output of the preprocessor if and only if MACRO is 2671 defined. We say that the conditional "succeeds" if MACRO is defined, 2672 "fails" if it is not. 2673 2674 The CONTROLLED TEXT inside of a conditional can include 2675 preprocessing directives. They are executed only if the conditional 2676 succeeds. You can nest conditional groups inside other conditional 2677 groups, but they must be completely nested. In other words, `#endif' 2678 always matches the nearest `#ifdef' (or `#ifndef', or `#if'). Also, 2679 you cannot start a conditional group in one file and end it in another. 2680 2681 Even if a conditional fails, the CONTROLLED TEXT inside it is still 2682 run through initial transformations and tokenization. Therefore, it 2683 must all be lexically valid C. Normally the only way this matters is 2684 that all comments and string literals inside a failing conditional group 2685 must still be properly ended. 2686 2687 The comment following the `#endif' is not required, but it is a good 2688 practice if there is a lot of CONTROLLED TEXT, because it helps people 2689 match the `#endif' to the corresponding `#ifdef'. Older programs 2690 sometimes put MACRO directly after the `#endif' without enclosing it in 2691 a comment. This is invalid code according to the C standard. CPP 2692 accepts it with a warning. It never affects which `#ifndef' the 2693 `#endif' matches. 2694 2695 Sometimes you wish to use some code if a macro is _not_ defined. 2696 You can do this by writing `#ifndef' instead of `#ifdef'. One common 2697 use of `#ifndef' is to include code only the first time a header file 2698 is included. *Note Once-Only Headers::. 2699 2700 Macro definitions can vary between compilations for several reasons. 2701 Here are some samples. 2702 2703 * Some macros are predefined on each kind of machine (*note 2704 System-specific Predefined Macros::). This allows you to provide 2705 code specially tuned for a particular machine. 2706 2707 * System header files define more macros, associated with the 2708 features they implement. You can test these macros with 2709 conditionals to avoid using a system feature on a machine where it 2710 is not implemented. 2711 2712 * Macros can be defined or undefined with the `-D' and `-U' command 2713 line options when you compile the program. You can arrange to 2714 compile the same source file into two different programs by 2715 choosing a macro name to specify which program you want, writing 2716 conditionals to test whether or how this macro is defined, and 2717 then controlling the state of the macro with command line options, 2718 perhaps set in the Makefile. *Note Invocation::. 2719 2720 * Your program might have a special header file (often called 2721 `config.h') that is adjusted when the program is compiled. It can 2722 define or not define macros depending on the features of the 2723 system and the desired capabilities of the program. The 2724 adjustment can be automated by a tool such as `autoconf', or done 2725 by hand. 2726 2727 2728 File: cpp.info, Node: If, Next: Defined, Prev: Ifdef, Up: Conditional Syntax 2729 2730 4.2.2 If 2731 -------- 2732 2733 The `#if' directive allows you to test the value of an arithmetic 2734 expression, rather than the mere existence of one macro. Its syntax is 2735 2736 #if EXPRESSION 2737 2738 CONTROLLED TEXT 2739 2740 #endif /* EXPRESSION */ 2741 2742 EXPRESSION is a C expression of integer type, subject to stringent 2743 restrictions. It may contain 2744 2745 * Integer constants. 2746 2747 * Character constants, which are interpreted as they would be in 2748 normal code. 2749 2750 * Arithmetic operators for addition, subtraction, multiplication, 2751 division, bitwise operations, shifts, comparisons, and logical 2752 operations (`&&' and `||'). The latter two obey the usual 2753 short-circuiting rules of standard C. 2754 2755 * Macros. All macros in the expression are expanded before actual 2756 computation of the expression's value begins. 2757 2758 * Uses of the `defined' operator, which lets you check whether macros 2759 are defined in the middle of an `#if'. 2760 2761 * Identifiers that are not macros, which are all considered to be the 2762 number zero. This allows you to write `#if MACRO' instead of 2763 `#ifdef MACRO', if you know that MACRO, when defined, will always 2764 have a nonzero value. Function-like macros used without their 2765 function call parentheses are also treated as zero. 2766 2767 In some contexts this shortcut is undesirable. The `-Wundef' 2768 option causes GCC to warn whenever it encounters an identifier 2769 which is not a macro in an `#if'. 2770 2771 The preprocessor does not know anything about types in the language. 2772 Therefore, `sizeof' operators are not recognized in `#if', and neither 2773 are `enum' constants. They will be taken as identifiers which are not 2774 macros, and replaced by zero. In the case of `sizeof', this is likely 2775 to cause the expression to be invalid. 2776 2777 The preprocessor calculates the value of EXPRESSION. It carries out 2778 all calculations in the widest integer type known to the compiler; on 2779 most machines supported by GCC this is 64 bits. This is not the same 2780 rule as the compiler uses to calculate the value of a constant 2781 expression, and may give different results in some cases. If the value 2782 comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is 2783 included; otherwise it is skipped. 2784 2785 2786 File: cpp.info, Node: Defined, Next: Else, Prev: If, Up: Conditional Syntax 2787 2788 4.2.3 Defined 2789 ------------- 2790 2791 The special operator `defined' is used in `#if' and `#elif' expressions 2792 to test whether a certain name is defined as a macro. `defined NAME' 2793 and `defined (NAME)' are both expressions whose value is 1 if NAME is 2794 defined as a macro at the current point in the program, and 0 2795 otherwise. Thus, `#if defined MACRO' is precisely equivalent to 2796 `#ifdef MACRO'. 2797 2798 `defined' is useful when you wish to test more than one macro for 2799 existence at once. For example, 2800 2801 #if defined (__vax__) || defined (__ns16000__) 2802 2803 would succeed if either of the names `__vax__' or `__ns16000__' is 2804 defined as a macro. 2805 2806 Conditionals written like this: 2807 2808 #if defined BUFSIZE && BUFSIZE >= 1024 2809 2810 can generally be simplified to just `#if BUFSIZE >= 1024', since if 2811 `BUFSIZE' is not defined, it will be interpreted as having the value 2812 zero. 2813 2814 If the `defined' operator appears as a result of a macro expansion, 2815 the C standard says the behavior is undefined. GNU cpp treats it as a 2816 genuine `defined' operator and evaluates it normally. It will warn 2817 wherever your code uses this feature if you use the command-line option 2818 `-pedantic', since other compilers may handle it differently. 2819 2820 2821 File: cpp.info, Node: Else, Next: Elif, Prev: Defined, Up: Conditional Syntax 2822 2823 4.2.4 Else 2824 ---------- 2825 2826 The `#else' directive can be added to a conditional to provide 2827 alternative text to be used if the condition fails. This is what it 2828 looks like: 2829 2830 #if EXPRESSION 2831 TEXT-IF-TRUE 2832 #else /* Not EXPRESSION */ 2833 TEXT-IF-FALSE 2834 #endif /* Not EXPRESSION */ 2835 2836 If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the 2837 TEXT-IF-FALSE is skipped. If EXPRESSION is zero, the opposite happens. 2838 2839 You can use `#else' with `#ifdef' and `#ifndef', too. 2840 2841 2842 File: cpp.info, Node: Elif, Prev: Else, Up: Conditional Syntax 2843 2844 4.2.5 Elif 2845 ---------- 2846 2847 One common case of nested conditionals is used to check for more than 2848 two possible alternatives. For example, you might have 2849 2850 #if X == 1 2851 ... 2852 #else /* X != 1 */ 2853 #if X == 2 2854 ... 2855 #else /* X != 2 */ 2856 ... 2857 #endif /* X != 2 */ 2858 #endif /* X != 1 */ 2859 2860 Another conditional directive, `#elif', allows this to be 2861 abbreviated as follows: 2862 2863 #if X == 1 2864 ... 2865 #elif X == 2 2866 ... 2867 #else /* X != 2 and X != 1*/ 2868 ... 2869 #endif /* X != 2 and X != 1*/ 2870 2871 `#elif' stands for "else if". Like `#else', it goes in the middle 2872 of a conditional group and subdivides it; it does not require a 2873 matching `#endif' of its own. Like `#if', the `#elif' directive 2874 includes an expression to be tested. The text following the `#elif' is 2875 processed only if the original `#if'-condition failed and the `#elif' 2876 condition succeeds. 2877 2878 More than one `#elif' can go in the same conditional group. Then 2879 the text after each `#elif' is processed only if the `#elif' condition 2880 succeeds after the original `#if' and all previous `#elif' directives 2881 within it have failed. 2882 2883 `#else' is allowed after any number of `#elif' directives, but 2884 `#elif' may not follow `#else'. 2885 2886 2887 File: cpp.info, Node: Deleted Code, Prev: Conditional Syntax, Up: Conditionals 2888 2889 4.3 Deleted Code 2890 ================ 2891 2892 If you replace or delete a part of the program but want to keep the old 2893 code around for future reference, you often cannot simply comment it 2894 out. Block comments do not nest, so the first comment inside the old 2895 code will end the commenting-out. The probable result is a flood of 2896 syntax errors. 2897 2898 One way to avoid this problem is to use an always-false conditional 2899 instead. For instance, put `#if 0' before the deleted code and 2900 `#endif' after it. This works even if the code being turned off 2901 contains conditionals, but they must be entire conditionals (balanced 2902 `#if' and `#endif'). 2903 2904 Some people use `#ifdef notdef' instead. This is risky, because 2905 `notdef' might be accidentally defined as a macro, and then the 2906 conditional would succeed. `#if 0' can be counted on to fail. 2907 2908 Do not use `#if 0' for comments which are not C code. Use a real 2909 comment, instead. The interior of `#if 0' must consist of complete 2910 tokens; in particular, single-quote characters must balance. Comments 2911 often contain unbalanced single-quote characters (known in English as 2912 apostrophes). These confuse `#if 0'. They don't confuse `/*'. 2913 2914 2915 File: cpp.info, Node: Diagnostics, Next: Line Control, Prev: Conditionals, Up: Top 2916 2917 5 Diagnostics 2918 ************* 2919 2920 The directive `#error' causes the preprocessor to report a fatal error. 2921 The tokens forming the rest of the line following `#error' are used as 2922 the error message. 2923 2924 You would use `#error' inside of a conditional that detects a 2925 combination of parameters which you know the program does not properly 2926 support. For example, if you know that the program will not run 2927 properly on a VAX, you might write 2928 2929 #ifdef __vax__ 2930 #error "Won't work on VAXen. See comments at get_last_object." 2931 #endif 2932 2933 If you have several configuration parameters that must be set up by 2934 the installation in a consistent way, you can use conditionals to detect 2935 an inconsistency and report it with `#error'. For example, 2936 2937 #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO) 2938 #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP." 2939 #endif 2940 2941 The directive `#warning' is like `#error', but causes the 2942 preprocessor to issue a warning and continue preprocessing. The tokens 2943 following `#warning' are used as the warning message. 2944 2945 You might use `#warning' in obsolete header files, with a message 2946 directing the user to the header file which should be used instead. 2947 2948 Neither `#error' nor `#warning' macro-expands its argument. 2949 Internal whitespace sequences are each replaced with a single space. 2950 The line must consist of complete tokens. It is wisest to make the 2951 argument of these directives be a single string constant; this avoids 2952 problems with apostrophes and the like. 2953 2954 2955 File: cpp.info, Node: Line Control, Next: Pragmas, Prev: Diagnostics, Up: Top 2956 2957 6 Line Control 2958 ************** 2959 2960 The C preprocessor informs the C compiler of the location in your source 2961 code where each token came from. Presently, this is just the file name 2962 and line number. All the tokens resulting from macro expansion are 2963 reported as having appeared on the line of the source file where the 2964 outermost macro was used. We intend to be more accurate in the future. 2965 2966 If you write a program which generates source code, such as the 2967 `bison' parser generator, you may want to adjust the preprocessor's 2968 notion of the current file name and line number by hand. Parts of the 2969 output from `bison' are generated from scratch, other parts come from a 2970 standard parser file. The rest are copied verbatim from `bison''s 2971 input. You would like compiler error messages and symbolic debuggers 2972 to be able to refer to `bison''s input file. 2973 2974 `bison' or any such program can arrange this by writing `#line' 2975 directives into the output file. `#line' is a directive that specifies 2976 the original line number and source file name for subsequent input in 2977 the current preprocessor input file. `#line' has three variants: 2978 2979 `#line LINENUM' 2980 LINENUM is a non-negative decimal integer constant. It specifies 2981 the line number which should be reported for the following line of 2982 input. Subsequent lines are counted from LINENUM. 2983 2984 `#line LINENUM FILENAME' 2985 LINENUM is the same as for the first form, and has the same 2986 effect. In addition, FILENAME is a string constant. The 2987 following line and all subsequent lines are reported to come from 2988 the file it specifies, until something else happens to change that. 2989 FILENAME is interpreted according to the normal rules for a string 2990 constant: backslash escapes are interpreted. This is different 2991 from `#include'. 2992 2993 Previous versions of CPP did not interpret escapes in `#line'; we 2994 have changed it because the standard requires they be interpreted, 2995 and most other compilers do. 2996 2997 `#line ANYTHING ELSE' 2998 ANYTHING ELSE is checked for macro calls, which are expanded. The 2999 result should match one of the above two forms. 3000 3001 `#line' directives alter the results of the `__FILE__' and 3002 `__LINE__' predefined macros from that point on. *Note Standard 3003 Predefined Macros::. They do not have any effect on `#include''s idea 3004 of the directory containing the current file. This is a change from 3005 GCC 2.95. Previously, a file reading 3006 3007 #line 1 "../src/gram.y" 3008 #include "gram.h" 3009 3010 would search for `gram.h' in `../src', then the `-I' chain; the 3011 directory containing the physical source file would not be searched. 3012 In GCC 3.0 and later, the `#include' is not affected by the presence of 3013 a `#line' referring to a different directory. 3014 3015 We made this change because the old behavior caused problems when 3016 generated source files were transported between machines. For instance, 3017 it is common practice to ship generated parsers with a source release, 3018 so that people building the distribution do not need to have yacc or 3019 Bison installed. These files frequently have `#line' directives 3020 referring to the directory tree of the system where the distribution was 3021 created. If GCC tries to search for headers in those directories, the 3022 build is likely to fail. 3023 3024 The new behavior can cause failures too, if the generated file is not 3025 in the same directory as its source and it attempts to include a header 3026 which would be visible searching from the directory containing the 3027 source file. However, this problem is easily solved with an additional 3028 `-I' switch on the command line. The failures caused by the old 3029 semantics could sometimes be corrected only by editing the generated 3030 files, which is difficult and error-prone. 3031 3032 3033 File: cpp.info, Node: Pragmas, Next: Other Directives, Prev: Line Control, Up: Top 3034 3035 7 Pragmas 3036 ********* 3037 3038 The `#pragma' directive is the method specified by the C standard for 3039 providing additional information to the compiler, beyond what is 3040 conveyed in the language itself. Three forms of this directive 3041 (commonly known as "pragmas") are specified by the 1999 C standard. A 3042 C compiler is free to attach any meaning it likes to other pragmas. 3043 3044 GCC has historically preferred to use extensions to the syntax of the 3045 language, such as `__attribute__', for this purpose. However, GCC does 3046 define a few pragmas of its own. These mostly have effects on the 3047 entire translation unit or source file. 3048 3049 In GCC version 3, all GNU-defined, supported pragmas have been given 3050 a `GCC' prefix. This is in line with the `STDC' prefix on all pragmas 3051 defined by C99. For backward compatibility, pragmas which were 3052 recognized by previous versions are still recognized without the `GCC' 3053 prefix, but that usage is deprecated. Some older pragmas are 3054 deprecated in their entirety. They are not recognized with the `GCC' 3055 prefix. *Note Obsolete Features::. 3056 3057 C99 introduces the `_Pragma' operator. This feature addresses a 3058 major problem with `#pragma': being a directive, it cannot be produced 3059 as the result of macro expansion. `_Pragma' is an operator, much like 3060 `sizeof' or `defined', and can be embedded in a macro. 3061 3062 Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can 3063 be either a normal or wide-character string literal. It is 3064 destringized, by replacing all `\\' with a single `\' and all `\"' with 3065 a `"'. The result is then processed as if it had appeared as the right 3066 hand side of a `#pragma' directive. For example, 3067 3068 _Pragma ("GCC dependency \"parse.y\"") 3069 3070 has the same effect as `#pragma GCC dependency "parse.y"'. The same 3071 effect could be achieved using macros, for example 3072 3073 #define DO_PRAGMA(x) _Pragma (#x) 3074 DO_PRAGMA (GCC dependency "parse.y") 3075 3076 The standard is unclear on where a `_Pragma' operator can appear. 3077 The preprocessor does not accept it within a preprocessing conditional 3078 directive like `#if'. To be safe, you are probably best keeping it out 3079 of directives other than `#define', and putting it on a line of its own. 3080 3081 This manual documents the pragmas which are meaningful to the 3082 preprocessor itself. Other pragmas are meaningful to the C or C++ 3083 compilers. They are documented in the GCC manual. 3084 3085 `#pragma GCC dependency' 3086 `#pragma GCC dependency' allows you to check the relative dates of 3087 the current file and another file. If the other file is more 3088 recent than the current file, a warning is issued. This is useful 3089 if the current file is derived from the other file, and should be 3090 regenerated. The other file is searched for using the normal 3091 include search path. Optional trailing text can be used to give 3092 more information in the warning message. 3093 3094 #pragma GCC dependency "parse.y" 3095 #pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3096 3097 `#pragma GCC poison' 3098 Sometimes, there is an identifier that you want to remove 3099 completely from your program, and make sure that it never creeps 3100 back in. To enforce this, you can "poison" the identifier with 3101 this pragma. `#pragma GCC poison' is followed by a list of 3102 identifiers to poison. If any of those identifiers appears 3103 anywhere in the source after the directive, it is a hard error. 3104 For example, 3105 3106 #pragma GCC poison printf sprintf fprintf 3107 sprintf(some_string, "hello"); 3108 3109 will produce an error. 3110 3111 If a poisoned identifier appears as part of the expansion of a 3112 macro which was defined before the identifier was poisoned, it 3113 will _not_ cause an error. This lets you poison an identifier 3114 without worrying about system headers defining macros that use it. 3115 3116 For example, 3117 3118 #define strrchr rindex 3119 #pragma GCC poison rindex 3120 strrchr(some_string, 'h'); 3121 3122 will not produce an error. 3123 3124 `#pragma GCC system_header' 3125 This pragma takes no arguments. It causes the rest of the code in 3126 the current file to be treated as if it came from a system header. 3127 *Note System Headers::. 3128 3129 3130 3131 File: cpp.info, Node: Other Directives, Next: Preprocessor Output, Prev: Pragmas, Up: Top 3132 3133 8 Other Directives 3134 ****************** 3135 3136 The `#ident' directive takes one argument, a string constant. On some 3137 systems, that string constant is copied into a special segment of the 3138 object file. On other systems, the directive is ignored. The `#sccs' 3139 directive is a synonym for `#ident'. 3140 3141 These directives are not part of the C standard, but they are not 3142 official GNU extensions either. What historical information we have 3143 been able to find, suggests they originated with System V. 3144 3145 The "null directive" consists of a `#' followed by a newline, with 3146 only whitespace (including comments) in between. A null directive is 3147 understood as a preprocessing directive but has no effect on the 3148 preprocessor output. The primary significance of the existence of the 3149 null directive is that an input line consisting of just a `#' will 3150 produce no output, rather than a line of output containing just a `#'. 3151 Supposedly some old C programs contain such lines. 3152 3153 3154 File: cpp.info, Node: Preprocessor Output, Next: Traditional Mode, Prev: Other Directives, Up: Top 3155 3156 9 Preprocessor Output 3157 ********************* 3158 3159 When the C preprocessor is used with the C, C++, or Objective-C 3160 compilers, it is integrated into the compiler and communicates a stream 3161 of binary tokens directly to the compiler's parser. However, it can 3162 also be used in the more conventional standalone mode, where it produces 3163 textual output. 3164 3165 The output from the C preprocessor looks much like the input, except 3166 that all preprocessing directive lines have been replaced with blank 3167 lines and all comments with spaces. Long runs of blank lines are 3168 discarded. 3169 3170 The ISO standard specifies that it is implementation defined whether 3171 a preprocessor preserves whitespace between tokens, or replaces it with 3172 e.g. a single space. In GNU CPP, whitespace between tokens is collapsed 3173 to become a single space, with the exception that the first token on a 3174 non-directive line is preceded with sufficient spaces that it appears in 3175 the same column in the preprocessed output that it appeared in the 3176 original source file. This is so the output is easy to read. *Note 3177 Differences from previous versions::. CPP does not insert any 3178 whitespace where there was none in the original source, except where 3179 necessary to prevent an accidental token paste. 3180 3181 Source file name and line number information is conveyed by lines of 3182 the form 3183 3184 # LINENUM FILENAME FLAGS 3185 3186 These are called "linemarkers". They are inserted as needed into the 3187 output (but never within a string or character constant). They mean 3188 that the following line originated in file FILENAME at line LINENUM. 3189 FILENAME will never contain any non-printing characters; they are 3190 replaced with octal escape sequences. 3191 3192 After the file name comes zero or more flags, which are `1', `2', 3193 `3', or `4'. If there are multiple flags, spaces separate them. Here 3194 is what the flags mean: 3195 3196 `1' 3197 This indicates the start of a new file. 3198 3199 `2' 3200 This indicates returning to a file (after having included another 3201 file). 3202 3203 `3' 3204 This indicates that the following text comes from a system header 3205 file, so certain warnings should be suppressed. 3206 3207 `4' 3208 This indicates that the following text should be treated as being 3209 wrapped in an implicit `extern "C"' block. 3210 3211 As an extension, the preprocessor accepts linemarkers in 3212 non-assembler input files. They are treated like the corresponding 3213 `#line' directive, (*note Line Control::), except that trailing flags 3214 are permitted, and are interpreted with the meanings described above. 3215 If multiple flags are given, they must be in ascending order. 3216 3217 Some directives may be duplicated in the output of the preprocessor. 3218 These are `#ident' (always), `#pragma' (only if the preprocessor does 3219 not handle the pragma itself), and `#define' and `#undef' (with certain 3220 debugging options). If this happens, the `#' of the directive will 3221 always be in the first column, and there will be no space between the 3222 `#' and the directive name. If macro expansion happens to generate 3223 tokens which might be mistaken for a duplicated directive, a space will 3224 be inserted between the `#' and the directive name. 3225 3226 3227 File: cpp.info, Node: Traditional Mode, Next: Implementation Details, Prev: Preprocessor Output, Up: Top 3228 3229 10 Traditional Mode 3230 ******************* 3231 3232 Traditional (pre-standard) C preprocessing is rather different from the 3233 preprocessing specified by the standard. When GCC is given the 3234 `-traditional-cpp' option, it attempts to emulate a traditional 3235 preprocessor. 3236 3237 GCC versions 3.2 and later only support traditional mode semantics in 3238 the preprocessor, and not in the compiler front ends. This chapter 3239 outlines the traditional preprocessor semantics we implemented. 3240 3241 The implementation does not correspond precisely to the behavior of 3242 earlier versions of GCC, nor to any true traditional preprocessor. 3243 After all, inconsistencies among traditional implementations were a 3244 major motivation for C standardization. However, we intend that it 3245 should be compatible with true traditional preprocessors in all ways 3246 that actually matter. 3247 3248 * Menu: 3249 3250 * Traditional lexical analysis:: 3251 * Traditional macros:: 3252 * Traditional miscellany:: 3253 * Traditional warnings:: 3254 3255 3256 File: cpp.info, Node: Traditional lexical analysis, Next: Traditional macros, Up: Traditional Mode 3257 3258 10.1 Traditional lexical analysis 3259 ================================= 3260 3261 The traditional preprocessor does not decompose its input into tokens 3262 the same way a standards-conforming preprocessor does. The input is 3263 simply treated as a stream of text with minimal internal form. 3264 3265 This implementation does not treat trigraphs (*note trigraphs::) 3266 specially since they were an invention of the standards committee. It 3267 handles arbitrarily-positioned escaped newlines properly and splices 3268 the lines as you would expect; many traditional preprocessors did not 3269 do this. 3270 3271 The form of horizontal whitespace in the input file is preserved in 3272 the output. In particular, hard tabs remain hard tabs. This can be 3273 useful if, for example, you are preprocessing a Makefile. 3274 3275 Traditional CPP only recognizes C-style block comments, and treats 3276 the `/*' sequence as introducing a comment only if it lies outside 3277 quoted text. Quoted text is introduced by the usual single and double 3278 quotes, and also by an initial `<' in a `#include' directive. 3279 3280 Traditionally, comments are completely removed and are not replaced 3281 with a space. Since a traditional compiler does its own tokenization 3282 of the output of the preprocessor, this means that comments can 3283 effectively be used as token paste operators. However, comments behave 3284 like separators for text handled by the preprocessor itself, since it 3285 doesn't re-lex its input. For example, in 3286 3287 #if foo/**/bar 3288 3289 `foo' and `bar' are distinct identifiers and expanded separately if 3290 they happen to be macros. In other words, this directive is equivalent 3291 to 3292 3293 #if foo bar 3294 3295 rather than 3296 3297 #if foobar 3298 3299 Generally speaking, in traditional mode an opening quote need not 3300 have a matching closing quote. In particular, a macro may be defined 3301 with replacement text that contains an unmatched quote. Of course, if 3302 you attempt to compile preprocessed output containing an unmatched quote 3303 you will get a syntax error. 3304 3305 However, all preprocessing directives other than `#define' require 3306 matching quotes. For example: 3307 3308 #define m This macro's fine and has an unmatched quote 3309 "/* This is not a comment. */ 3310 /* This is a comment. The following #include directive 3311 is ill-formed. */ 3312 #include <stdio.h 3313 3314 Just as for the ISO preprocessor, what would be a closing quote can 3315 be escaped with a backslash to prevent the quoted text from closing. 3316 3317 3318 File: cpp.info, Node: Traditional macros, Next: Traditional miscellany, Prev: Traditional lexical analysis, Up: Traditional Mode 3319 3320 10.2 Traditional macros 3321 ======================= 3322 3323 The major difference between traditional and ISO macros is that the 3324 former expand to text rather than to a token sequence. CPP removes all 3325 leading and trailing horizontal whitespace from a macro's replacement 3326 text before storing it, but preserves the form of internal whitespace. 3327 3328 One consequence is that it is legitimate for the replacement text to 3329 contain an unmatched quote (*note Traditional lexical analysis::). An 3330 unclosed string or character constant continues into the text following 3331 the macro call. Similarly, the text at the end of a macro's expansion 3332 can run together with the text after the macro invocation to produce a 3333 single token. 3334 3335 Normally comments are removed from the replacement text after the 3336 macro is expanded, but if the `-CC' option is passed on the command 3337 line comments are preserved. (In fact, the current implementation 3338 removes comments even before saving the macro replacement text, but it 3339 careful to do it in such a way that the observed effect is identical 3340 even in the function-like macro case.) 3341 3342 The ISO stringification operator `#' and token paste operator `##' 3343 have no special meaning. As explained later, an effect similar to 3344 these operators can be obtained in a different way. Macro names that 3345 are embedded in quotes, either from the main file or after macro 3346 replacement, do not expand. 3347 3348 CPP replaces an unquoted object-like macro name with its replacement 3349 text, and then rescans it for further macros to replace. Unlike 3350 standard macro expansion, traditional macro expansion has no provision 3351 to prevent recursion. If an object-like macro appears unquoted in its 3352 replacement text, it will be replaced again during the rescan pass, and 3353 so on _ad infinitum_. GCC detects when it is expanding recursive 3354 macros, emits an error message, and continues after the offending macro 3355 invocation. 3356 3357 #define PLUS + 3358 #define INC(x) PLUS+x 3359 INC(foo); 3360 ==> ++foo; 3361 3362 Function-like macros are similar in form but quite different in 3363 behavior to their ISO counterparts. Their arguments are contained 3364 within parentheses, are comma-separated, and can cross physical lines. 3365 Commas within nested parentheses are not treated as argument 3366 separators. Similarly, a quote in an argument cannot be left unclosed; 3367 a following comma or parenthesis that comes before the closing quote is 3368 treated like any other character. There is no facility for handling 3369 variadic macros. 3370 3371 This implementation removes all comments from macro arguments, unless 3372 the `-C' option is given. The form of all other horizontal whitespace 3373 in arguments is preserved, including leading and trailing whitespace. 3374 In particular 3375 3376 f( ) 3377 3378 is treated as an invocation of the macro `f' with a single argument 3379 consisting of a single space. If you want to invoke a function-like 3380 macro that takes no arguments, you must not leave any whitespace 3381 between the parentheses. 3382 3383 If a macro argument crosses a new line, the new line is replaced with 3384 a space when forming the argument. If the previous line contained an 3385 unterminated quote, the following line inherits the quoted state. 3386 3387 Traditional preprocessors replace parameters in the replacement text 3388 with their arguments regardless of whether the parameters are within 3389 quotes or not. This provides a way to stringize arguments. For example 3390 3391 #define str(x) "x" 3392 str(/* A comment */some text ) 3393 ==> "some text " 3394 3395 Note that the comment is removed, but that the trailing space is 3396 preserved. Here is an example of using a comment to effect token 3397 pasting. 3398 3399 #define suffix(x) foo_/**/x 3400 suffix(bar) 3401 ==> foo_bar 3402 3403 3404 File: cpp.info, Node: Traditional miscellany, Next: Traditional warnings, Prev: Traditional macros, Up: Traditional Mode 3405 3406 10.3 Traditional miscellany 3407 =========================== 3408 3409 Here are some things to be aware of when using the traditional 3410 preprocessor. 3411 3412 * Preprocessing directives are recognized only when their leading 3413 `#' appears in the first column. There can be no whitespace 3414 between the beginning of the line and the `#', but whitespace can 3415 follow the `#'. 3416 3417 * A true traditional C preprocessor does not recognize `#error' or 3418 `#pragma', and may not recognize `#elif'. CPP supports all the 3419 directives in traditional mode that it supports in ISO mode, 3420 including extensions, with the exception that the effects of 3421 `#pragma GCC poison' are undefined. 3422 3423 * __STDC__ is not defined. 3424 3425 * If you use digraphs the behavior is undefined. 3426 3427 * If a line that looks like a directive appears within macro 3428 arguments, the behavior is undefined. 3429 3430 3431 3432 File: cpp.info, Node: Traditional warnings, Prev: Traditional miscellany, Up: Traditional Mode 3433 3434 10.4 Traditional warnings 3435 ========================= 3436 3437 You can request warnings about features that did not exist, or worked 3438 differently, in traditional C with the `-Wtraditional' option. GCC 3439 does not warn about features of ISO C which you must use when you are 3440 using a conforming compiler, such as the `#' and `##' operators. 3441 3442 Presently `-Wtraditional' warns about: 3443 3444 * Macro parameters that appear within string literals in the macro 3445 body. In traditional C macro replacement takes place within 3446 string literals, but does not in ISO C. 3447 3448 * In traditional C, some preprocessor directives did not exist. 3449 Traditional preprocessors would only consider a line to be a 3450 directive if the `#' appeared in column 1 on the line. Therefore 3451 `-Wtraditional' warns about directives that traditional C 3452 understands but would ignore because the `#' does not appear as the 3453 first character on the line. It also suggests you hide directives 3454 like `#pragma' not understood by traditional C by indenting them. 3455 Some traditional implementations would not recognize `#elif', so it 3456 suggests avoiding it altogether. 3457 3458 * A function-like macro that appears without an argument list. In 3459 some traditional preprocessors this was an error. In ISO C it 3460 merely means that the macro is not expanded. 3461 3462 * The unary plus operator. This did not exist in traditional C. 3463 3464 * The `U' and `LL' integer constant suffixes, which were not 3465 available in traditional C. (Traditional C does support the `L' 3466 suffix for simple long integer constants.) You are not warned 3467 about uses of these suffixes in macros defined in system headers. 3468 For instance, `UINT_MAX' may well be defined as `4294967295U', but 3469 you will not be warned if you use `UINT_MAX'. 3470 3471 You can usually avoid the warning, and the related warning about 3472 constants which are so large that they are unsigned, by writing the 3473 integer constant in question in hexadecimal, with no U suffix. 3474 Take care, though, because this gives the wrong result in exotic 3475 cases. 3476 3477 3478 File: cpp.info, Node: Implementation Details, Next: Invocation, Prev: Traditional Mode, Up: Top 3479 3480 11 Implementation Details 3481 ************************* 3482 3483 Here we document details of how the preprocessor's implementation 3484 affects its user-visible behavior. You should try to avoid undue 3485 reliance on behavior described here, as it is possible that it will 3486 change subtly in future implementations. 3487 3488 Also documented here are obsolete features and changes from previous 3489 versions of CPP. 3490 3491 * Menu: 3492 3493 * Implementation-defined behavior:: 3494 * Implementation limits:: 3495 * Obsolete Features:: 3496 * Differences from previous versions:: 3497 3498 3499 File: cpp.info, Node: Implementation-defined behavior, Next: Implementation limits, Up: Implementation Details 3500 3501 11.1 Implementation-defined behavior 3502 ==================================== 3503 3504 This is how CPP behaves in all the cases which the C standard describes 3505 as "implementation-defined". This term means that the implementation 3506 is free to do what it likes, but must document its choice and stick to 3507 it. 3508 3509 * The mapping of physical source file multi-byte characters to the 3510 execution character set. 3511 3512 Currently, CPP requires its input to be ASCII or UTF-8. The 3513 execution character set may be controlled by the user, with the 3514 `-fexec-charset' and `-fwide-exec-charset' options. 3515 3516 * Identifier characters. 3517 3518 The C and C++ standards allow identifiers to be composed of `_' 3519 and the alphanumeric characters. C++ and C99 also allow universal 3520 character names, and C99 further permits implementation-defined 3521 characters. GCC currently only permits universal character names 3522 if `-fextended-identifiers' is used, because the implementation of 3523 universal character names in identifiers is experimental. 3524 3525 GCC allows the `$' character in identifiers as an extension for 3526 most targets. This is true regardless of the `std=' switch, since 3527 this extension cannot conflict with standards-conforming programs. 3528 When preprocessing assembler, however, dollars are not identifier 3529 characters by default. 3530 3531 Currently the targets that by default do not permit `$' are AVR, 3532 IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 3533 and BeOS operating systems. 3534 3535 You can override the default with `-fdollars-in-identifiers' or 3536 `fno-dollars-in-identifiers'. *Note fdollars-in-identifiers::. 3537 3538 * Non-empty sequences of whitespace characters. 3539 3540 In textual output, each whitespace sequence is collapsed to a 3541 single space. For aesthetic reasons, the first token on each 3542 non-directive line of output is preceded with sufficient spaces 3543 that it appears in the same column as it did in the original 3544 source file. 3545 3546 * The numeric value of character constants in preprocessor 3547 expressions. 3548 3549 The preprocessor and compiler interpret character constants in the 3550 same way; i.e. escape sequences such as `\a' are given the values 3551 they would have on the target machine. 3552 3553 The compiler values a multi-character character constant a 3554 character at a time, shifting the previous value left by the 3555 number of bits per target character, and then or-ing in the 3556 bit-pattern of the new character truncated to the width of a 3557 target character. The final bit-pattern is given type `int', and 3558 is therefore signed, regardless of whether single characters are 3559 signed or not (a slight change from versions 3.1 and earlier of 3560 GCC). If there are more characters in the constant than would fit 3561 in the target `int' the compiler issues a warning, and the excess 3562 leading characters are ignored. 3563 3564 For example, `'ab'' for a target with an 8-bit `char' would be 3565 interpreted as 3566 `(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and 3567 `'\234a'' as 3568 `(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'. 3569 3570 * Source file inclusion. 3571 3572 For a discussion on how the preprocessor locates header files, 3573 *Note Include Operation::. 3574 3575 * Interpretation of the filename resulting from a macro-expanded 3576 `#include' directive. 3577 3578 *Note Computed Includes::. 3579 3580 * Treatment of a `#pragma' directive that after macro-expansion 3581 results in a standard pragma. 3582 3583 No macro expansion occurs on any `#pragma' directive line, so the 3584 question does not arise. 3585 3586 Note that GCC does not yet implement any of the standard pragmas. 3587 3588 3589 3590 File: cpp.info, Node: Implementation limits, Next: Obsolete Features, Prev: Implementation-defined behavior, Up: Implementation Details 3591 3592 11.2 Implementation limits 3593 ========================== 3594 3595 CPP has a small number of internal limits. This section lists the 3596 limits which the C standard requires to be no lower than some minimum, 3597 and all the others known. It is intended that there should be as few 3598 limits as possible. If you encounter an undocumented or inconvenient 3599 limit, please report that as a bug. *Note Reporting Bugs: (gcc)Bugs. 3600 3601 Where we say something is limited "only by available memory", that 3602 means that internal data structures impose no intrinsic limit, and space 3603 is allocated with `malloc' or equivalent. The actual limit will 3604 therefore depend on many things, such as the size of other things 3605 allocated by the compiler at the same time, the amount of memory 3606 consumed by other processes on the same computer, etc. 3607 3608 * Nesting levels of `#include' files. 3609 3610 We impose an arbitrary limit of 200 levels, to avoid runaway 3611 recursion. The standard requires at least 15 levels. 3612 3613 * Nesting levels of conditional inclusion. 3614 3615 The C standard mandates this be at least 63. CPP is limited only 3616 by available memory. 3617 3618 * Levels of parenthesized expressions within a full expression. 3619 3620 The C standard requires this to be at least 63. In preprocessor 3621 conditional expressions, it is limited only by available memory. 3622 3623 * Significant initial characters in an identifier or macro name. 3624 3625 The preprocessor treats all characters as significant. The C 3626 standard requires only that the first 63 be significant. 3627 3628 * Number of macros simultaneously defined in a single translation 3629 unit. 3630 3631 The standard requires at least 4095 be possible. CPP is limited 3632 only by available memory. 3633 3634 * Number of parameters in a macro definition and arguments in a 3635 macro call. 3636 3637 We allow `USHRT_MAX', which is no smaller than 65,535. The minimum 3638 required by the standard is 127. 3639 3640 * Number of characters on a logical source line. 3641 3642 The C standard requires a minimum of 4096 be permitted. CPP places 3643 no limits on this, but you may get incorrect column numbers 3644 reported in diagnostics for lines longer than 65,535 characters. 3645 3646 * Maximum size of a source file. 3647 3648 The standard does not specify any lower limit on the maximum size 3649 of a source file. GNU cpp maps files into memory, so it is 3650 limited by the available address space. This is generally at 3651 least two gigabytes. Depending on the operating system, the size 3652 of physical memory may or may not be a limitation. 3653 3654 3655 3656 File: cpp.info, Node: Obsolete Features, Next: Differences from previous versions, Prev: Implementation limits, Up: Implementation Details 3657 3658 11.3 Obsolete Features 3659 ====================== 3660 3661 CPP has a number of features which are present mainly for compatibility 3662 with older programs. We discourage their use in new code. In some 3663 cases, we plan to remove the feature in a future version of GCC. 3664 3665 * Menu: 3666 3667 * Assertions:: 3668 * Obsolete once-only headers:: 3669 3670 3671 File: cpp.info, Node: Assertions, Next: Obsolete once-only headers, Up: Obsolete Features 3672 3673 11.3.1 Assertions 3674 ----------------- 3675 3676 "Assertions" are a deprecated alternative to macros in writing 3677 conditionals to test what sort of computer or system the compiled 3678 program will run on. Assertions are usually predefined, but you can 3679 define them with preprocessing directives or command-line options. 3680 3681 Assertions were intended to provide a more systematic way to describe 3682 the compiler's target system. However, in practice they are just as 3683 unpredictable as the system-specific predefined macros. In addition, 3684 they are not part of any standard, and only a few compilers support 3685 them. Therefore, the use of assertions is *less* portable than the use 3686 of system-specific predefined macros. We recommend you do not use them 3687 at all. 3688 3689 An assertion looks like this: 3690 3691 #PREDICATE (ANSWER) 3692 3693 PREDICATE must be a single identifier. ANSWER can be any sequence of 3694 tokens; all characters are significant except for leading and trailing 3695 whitespace, and differences in internal whitespace sequences are 3696 ignored. (This is similar to the rules governing macro redefinition.) 3697 Thus, `(x + y)' is different from `(x+y)' but equivalent to 3698 `( x + y )'. Parentheses do not nest inside an answer. 3699 3700 To test an assertion, you write it in an `#if'. For example, this 3701 conditional succeeds if either `vax' or `ns16000' has been asserted as 3702 an answer for `machine'. 3703 3704 #if #machine (vax) || #machine (ns16000) 3705 3706 You can test whether _any_ answer is asserted for a predicate by 3707 omitting the answer in the conditional: 3708 3709 #if #machine 3710 3711 Assertions are made with the `#assert' directive. Its sole argument 3712 is the assertion to make, without the leading `#' that identifies 3713 assertions in conditionals. 3714 3715 #assert PREDICATE (ANSWER) 3716 3717 You may make several assertions with the same predicate and different 3718 answers. Subsequent assertions do not override previous ones for the 3719 same predicate. All the answers for any given predicate are 3720 simultaneously true. 3721 3722 Assertions can be canceled with the `#unassert' directive. It has 3723 the same syntax as `#assert'. In that form it cancels only the answer 3724 which was specified on the `#unassert' line; other answers for that 3725 predicate remain true. You can cancel an entire predicate by leaving 3726 out the answer: 3727 3728 #unassert PREDICATE 3729 3730 In either form, if no such assertion has been made, `#unassert' has no 3731 effect. 3732 3733 You can also make or cancel assertions using command line options. 3734 *Note Invocation::. 3735 3736 3737 File: cpp.info, Node: Obsolete once-only headers, Prev: Assertions, Up: Obsolete Features 3738 3739 11.3.2 Obsolete once-only headers 3740 --------------------------------- 3741 3742 CPP supports two more ways of indicating that a header file should be 3743 read only once. Neither one is as portable as a wrapper `#ifndef', and 3744 we recommend you do not use them in new programs. 3745 3746 In the Objective-C language, there is a variant of `#include' called 3747 `#import' which includes a file, but does so at most once. If you use 3748 `#import' instead of `#include', then you don't need the conditionals 3749 inside the header file to prevent multiple inclusion of the contents. 3750 GCC permits the use of `#import' in C and C++ as well as Objective-C. 3751 However, it is not in standard C or C++ and should therefore not be 3752 used by portable programs. 3753 3754 `#import' is not a well designed feature. It requires the users of 3755 a header file to know that it should only be included once. It is much 3756 better for the header file's implementor to write the file so that users 3757 don't need to know this. Using a wrapper `#ifndef' accomplishes this 3758 goal. 3759 3760 In the present implementation, a single use of `#import' will 3761 prevent the file from ever being read again, by either `#import' or 3762 `#include'. You should not rely on this; do not use both `#import' and 3763 `#include' to refer to the same header file. 3764 3765 Another way to prevent a header file from being included more than 3766 once is with the `#pragma once' directive. If `#pragma once' is seen 3767 when scanning a header file, that file will never be read again, no 3768 matter what. 3769 3770 `#pragma once' does not have the problems that `#import' does, but 3771 it is not recognized by all preprocessors, so you cannot rely on it in 3772 a portable program. 3773 3774 3775 File: cpp.info, Node: Differences from previous versions, Prev: Obsolete Features, Up: Implementation Details 3776 3777 11.4 Differences from previous versions 3778 ======================================= 3779 3780 This section details behavior which has changed from previous versions 3781 of CPP. We do not plan to change it again in the near future, but we 3782 do not promise not to, either. 3783 3784 The "previous versions" discussed here are 2.95 and before. The 3785 behavior of GCC 3.0 is mostly the same as the behavior of the widely 3786 used 2.96 and 2.97 development snapshots. Where there are differences, 3787 they generally represent bugs in the snapshots. 3788 3789 * -I- deprecated 3790 3791 This option has been deprecated in 4.0. `-iquote' is meant to 3792 replace the need for this option. 3793 3794 * Order of evaluation of `#' and `##' operators 3795 3796 The standard does not specify the order of evaluation of a chain of 3797 `##' operators, nor whether `#' is evaluated before, after, or at 3798 the same time as `##'. You should therefore not write any code 3799 which depends on any specific ordering. It is possible to 3800 guarantee an ordering, if you need one, by suitable use of nested 3801 macros. 3802 3803 An example of where this might matter is pasting the arguments `1', 3804 `e' and `-2'. This would be fine for left-to-right pasting, but 3805 right-to-left pasting would produce an invalid token `e-2'. 3806 3807 GCC 3.0 evaluates `#' and `##' at the same time and strictly left 3808 to right. Older versions evaluated all `#' operators first, then 3809 all `##' operators, in an unreliable order. 3810 3811 * The form of whitespace between tokens in preprocessor output 3812 3813 *Note Preprocessor Output::, for the current textual format. This 3814 is also the format used by stringification. Normally, the 3815 preprocessor communicates tokens directly to the compiler's 3816 parser, and whitespace does not come up at all. 3817 3818 Older versions of GCC preserved all whitespace provided by the 3819 user and inserted lots more whitespace of their own, because they 3820 could not accurately predict when extra spaces were needed to 3821 prevent accidental token pasting. 3822 3823 * Optional argument when invoking rest argument macros 3824 3825 As an extension, GCC permits you to omit the variable arguments 3826 entirely when you use a variable argument macro. This is 3827 forbidden by the 1999 C standard, and will provoke a pedantic 3828 warning with GCC 3.0. Previous versions accepted it silently. 3829 3830 * `##' swallowing preceding text in rest argument macros 3831 3832 Formerly, in a macro expansion, if `##' appeared before a variable 3833 arguments parameter, and the set of tokens specified for that 3834 argument in the macro invocation was empty, previous versions of 3835 CPP would back up and remove the preceding sequence of 3836 non-whitespace characters (*not* the preceding token). This 3837 extension is in direct conflict with the 1999 C standard and has 3838 been drastically pared back. 3839 3840 In the current version of the preprocessor, if `##' appears between 3841 a comma and a variable arguments parameter, and the variable 3842 argument is omitted entirely, the comma will be removed from the 3843 expansion. If the variable argument is empty, or the token before 3844 `##' is not a comma, then `##' behaves as a normal token paste. 3845 3846 * `#line' and `#include' 3847 3848 The `#line' directive used to change GCC's notion of the 3849 "directory containing the current file", used by `#include' with a 3850 double-quoted header file name. In 3.0 and later, it does not. 3851 *Note Line Control::, for further explanation. 3852 3853 * Syntax of `#line' 3854 3855 In GCC 2.95 and previous, the string constant argument to `#line' 3856 was treated the same way as the argument to `#include': backslash 3857 escapes were not honored, and the string ended at the second `"'. 3858 This is not compliant with the C standard. In GCC 3.0, an attempt 3859 was made to correct the behavior, so that the string was treated 3860 as a real string constant, but it turned out to be buggy. In 3.1, 3861 the bugs have been fixed. (We are not fixing the bugs in 3.0 3862 because they affect relatively few people and the fix is quite 3863 invasive.) 3864 3865 3866 3867 File: cpp.info, Node: Invocation, Next: Environment Variables, Prev: Implementation Details, Up: Top 3868 3869 12 Invocation 3870 ************* 3871 3872 Most often when you use the C preprocessor you will not have to invoke 3873 it explicitly: the C compiler will do so automatically. However, the 3874 preprocessor is sometimes useful on its own. All the options listed 3875 here are also acceptable to the C compiler and have the same meaning, 3876 except that the C compiler has different rules for specifying the output 3877 file. 3878 3879 _Note:_ Whether you use the preprocessor by way of `gcc' or `cpp', 3880 the "compiler driver" is run first. This program's purpose is to 3881 translate your command into invocations of the programs that do the 3882 actual work. Their command line interfaces are similar but not 3883 identical to the documented interface, and may change without notice. 3884 3885 The C preprocessor expects two file names as arguments, INFILE and 3886 OUTFILE. The preprocessor reads INFILE together with any other files 3887 it specifies with `#include'. All the output generated by the combined 3888 input files is written in OUTFILE. 3889 3890 Either INFILE or OUTFILE may be `-', which as INFILE means to read 3891 from standard input and as OUTFILE means to write to standard output. 3892 Also, if either file is omitted, it means the same as if `-' had been 3893 specified for that file. 3894 3895 Unless otherwise noted, or the option ends in `=', all options which 3896 take an argument may have that argument appear either immediately after 3897 the option, or with a space between option and argument: `-Ifoo' and 3898 `-I foo' have the same effect. 3899 3900 Many options have multi-letter names; therefore multiple 3901 single-letter options may _not_ be grouped: `-dM' is very different from 3902 `-d -M'. 3903 3904 `-D NAME' 3905 Predefine NAME as a macro, with definition `1'. 3906 3907 `-D NAME=DEFINITION' 3908 The contents of DEFINITION are tokenized and processed as if they 3909 appeared during translation phase three in a `#define' directive. 3910 In particular, the definition will be truncated by embedded 3911 newline characters. 3912 3913 If you are invoking the preprocessor from a shell or shell-like 3914 program you may need to use the shell's quoting syntax to protect 3915 characters such as spaces that have a meaning in the shell syntax. 3916 3917 If you wish to define a function-like macro on the command line, 3918 write its argument list with surrounding parentheses before the 3919 equals sign (if any). Parentheses are meaningful to most shells, 3920 so you will need to quote the option. With `sh' and `csh', 3921 `-D'NAME(ARGS...)=DEFINITION'' works. 3922 3923 `-D' and `-U' options are processed in the order they are given on 3924 the command line. All `-imacros FILE' and `-include FILE' options 3925 are processed after all `-D' and `-U' options. 3926 3927 `-U NAME' 3928 Cancel any previous definition of NAME, either built in or 3929 provided with a `-D' option. 3930 3931 `-undef' 3932 Do not predefine any system-specific or GCC-specific macros. The 3933 standard predefined macros remain defined. *Note Standard 3934 Predefined Macros::. 3935 3936 `-I DIR' 3937 Add the directory DIR to the list of directories to be searched 3938 for header files. *Note Search Path::. Directories named by `-I' 3939 are searched before the standard system include directories. If 3940 the directory DIR is a standard system include directory, the 3941 option is ignored to ensure that the default search order for 3942 system directories and the special treatment of system headers are 3943 not defeated (*note System Headers::) . If DIR begins with `=', 3944 then the `=' will be replaced by the sysroot prefix; see 3945 `--sysroot' and `-isysroot'. 3946 3947 `-o FILE' 3948 Write output to FILE. This is the same as specifying FILE as the 3949 second non-option argument to `cpp'. `gcc' has a different 3950 interpretation of a second non-option argument, so you must use 3951 `-o' to specify the output file. 3952 3953 `-Wall' 3954 Turns on all optional warnings which are desirable for normal code. 3955 At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a 3956 warning about integer promotion causing a change of sign in `#if' 3957 expressions. Note that many of the preprocessor's warnings are on 3958 by default and have no options to control them. 3959 3960 `-Wcomment' 3961 `-Wcomments' 3962 Warn whenever a comment-start sequence `/*' appears in a `/*' 3963 comment, or whenever a backslash-newline appears in a `//' comment. 3964 (Both forms have the same effect.) 3965 3966 `-Wtrigraphs' 3967 Most trigraphs in comments cannot affect the meaning of the 3968 program. However, a trigraph that would form an escaped newline 3969 (`??/' at the end of a line) can, by changing where the comment 3970 begins or ends. Therefore, only trigraphs that would form escaped 3971 newlines produce warnings inside a comment. 3972 3973 This option is implied by `-Wall'. If `-Wall' is not given, this 3974 option is still enabled unless trigraphs are enabled. To get 3975 trigraph conversion without warnings, but get the other `-Wall' 3976 warnings, use `-trigraphs -Wall -Wno-trigraphs'. 3977 3978 `-Wtraditional' 3979 Warn about certain constructs that behave differently in 3980 traditional and ISO C. Also warn about ISO C constructs that have 3981 no traditional C equivalent, and problematic constructs which 3982 should be avoided. *Note Traditional Mode::. 3983 3984 `-Wimport' 3985 Warn the first time `#import' is used. 3986 3987 `-Wundef' 3988 Warn whenever an identifier which is not a macro is encountered in 3989 an `#if' directive, outside of `defined'. Such identifiers are 3990 replaced with zero. 3991 3992 `-Wunused-macros' 3993 Warn about macros defined in the main file that are unused. A 3994 macro is "used" if it is expanded or tested for existence at least 3995 once. The preprocessor will also warn if the macro has not been 3996 used at the time it is redefined or undefined. 3997 3998 Built-in macros, macros defined on the command line, and macros 3999 defined in include files are not warned about. 4000 4001 _Note:_ If a macro is actually used, but only used in skipped 4002 conditional blocks, then CPP will report it as unused. To avoid 4003 the warning in such a case, you might improve the scope of the 4004 macro's definition by, for example, moving it into the first 4005 skipped block. Alternatively, you could provide a dummy use with 4006 something like: 4007 4008 #if defined the_macro_causing_the_warning 4009 #endif 4010 4011 `-Wendif-labels' 4012 Warn whenever an `#else' or an `#endif' are followed by text. 4013 This usually happens in code of the form 4014 4015 #if FOO 4016 ... 4017 #else FOO 4018 ... 4019 #endif FOO 4020 4021 The second and third `FOO' should be in comments, but often are not 4022 in older programs. This warning is on by default. 4023 4024 `-Werror' 4025 Make all warnings into hard errors. Source code which triggers 4026 warnings will be rejected. 4027 4028 `-Wsystem-headers' 4029 Issue warnings for code in system headers. These are normally 4030 unhelpful in finding bugs in your own code, therefore suppressed. 4031 If you are responsible for the system library, you may want to see 4032 them. 4033 4034 `-w' 4035 Suppress all warnings, including those which GNU CPP issues by 4036 default. 4037 4038 `-pedantic' 4039 Issue all the mandatory diagnostics listed in the C standard. 4040 Some of them are left out by default, since they trigger 4041 frequently on harmless code. 4042 4043 `-pedantic-errors' 4044 Issue all the mandatory diagnostics, and make all mandatory 4045 diagnostics into errors. This includes mandatory diagnostics that 4046 GCC issues without `-pedantic' but treats as warnings. 4047 4048 `-M' 4049 Instead of outputting the result of preprocessing, output a rule 4050 suitable for `make' describing the dependencies of the main source 4051 file. The preprocessor outputs one `make' rule containing the 4052 object file name for that source file, a colon, and the names of 4053 all the included files, including those coming from `-include' or 4054 `-imacros' command line options. 4055 4056 Unless specified explicitly (with `-MT' or `-MQ'), the object file 4057 name consists of the name of the source file with any suffix 4058 replaced with object file suffix and with any leading directory 4059 parts removed. If there are many included files then the rule is 4060 split into several lines using `\'-newline. The rule has no 4061 commands. 4062 4063 This option does not suppress the preprocessor's debug output, 4064 such as `-dM'. To avoid mixing such debug output with the 4065 dependency rules you should explicitly specify the dependency 4066 output file with `-MF', or use an environment variable like 4067 `DEPENDENCIES_OUTPUT' (*note Environment Variables::). Debug 4068 output will still be sent to the regular output stream as normal. 4069 4070 Passing `-M' to the driver implies `-E', and suppresses warnings 4071 with an implicit `-w'. 4072 4073 `-MM' 4074 Like `-M' but do not mention header files that are found in system 4075 header directories, nor header files that are included, directly 4076 or indirectly, from such a header. 4077 4078 This implies that the choice of angle brackets or double quotes in 4079 an `#include' directive does not in itself determine whether that 4080 header will appear in `-MM' dependency output. This is a slight 4081 change in semantics from GCC versions 3.0 and earlier. 4082 4083 `-MF FILE' 4084 When used with `-M' or `-MM', specifies a file to write the 4085 dependencies to. If no `-MF' switch is given the preprocessor 4086 sends the rules to the same place it would have sent preprocessed 4087 output. 4088 4089 When used with the driver options `-MD' or `-MMD', `-MF' overrides 4090 the default dependency output file. 4091 4092 `-MG' 4093 In conjunction with an option such as `-M' requesting dependency 4094 generation, `-MG' assumes missing header files are generated files 4095 and adds them to the dependency list without raising an error. 4096 The dependency filename is taken directly from the `#include' 4097 directive without prepending any path. `-MG' also suppresses 4098 preprocessed output, as a missing header file renders this useless. 4099 4100 This feature is used in automatic updating of makefiles. 4101 4102 `-MP' 4103 This option instructs CPP to add a phony target for each dependency 4104 other than the main file, causing each to depend on nothing. These 4105 dummy rules work around errors `make' gives if you remove header 4106 files without updating the `Makefile' to match. 4107 4108 This is typical output: 4109 4110 test.o: test.c test.h 4111 4112 test.h: 4113 4114 `-MT TARGET' 4115 Change the target of the rule emitted by dependency generation. By 4116 default CPP takes the name of the main input file, deletes any 4117 directory components and any file suffix such as `.c', and appends 4118 the platform's usual object suffix. The result is the target. 4119 4120 An `-MT' option will set the target to be exactly the string you 4121 specify. If you want multiple targets, you can specify them as a 4122 single argument to `-MT', or use multiple `-MT' options. 4123 4124 For example, `-MT '$(objpfx)foo.o'' might give 4125 4126 $(objpfx)foo.o: foo.c 4127 4128 `-MQ TARGET' 4129 Same as `-MT', but it quotes any characters which are special to 4130 Make. `-MQ '$(objpfx)foo.o'' gives 4131 4132 $$(objpfx)foo.o: foo.c 4133 4134 The default target is automatically quoted, as if it were given 4135 with `-MQ'. 4136 4137 `-MD' 4138 `-MD' is equivalent to `-M -MF FILE', except that `-E' is not 4139 implied. The driver determines FILE based on whether an `-o' 4140 option is given. If it is, the driver uses its argument but with 4141 a suffix of `.d', otherwise it takes the name of the input file, 4142 removes any directory components and suffix, and applies a `.d' 4143 suffix. 4144 4145 If `-MD' is used in conjunction with `-E', any `-o' switch is 4146 understood to specify the dependency output file (*note -MF: 4147 dashMF.), but if used without `-E', each `-o' is understood to 4148 specify a target object file. 4149 4150 Since `-E' is not implied, `-MD' can be used to generate a 4151 dependency output file as a side-effect of the compilation process. 4152 4153 `-MMD' 4154 Like `-MD' except mention only user header files, not system 4155 header files. 4156 4157 `-x c' 4158 `-x c++' 4159 `-x objective-c' 4160 `-x assembler-with-cpp' 4161 Specify the source language: C, C++, Objective-C, or assembly. 4162 This has nothing to do with standards conformance or extensions; 4163 it merely selects which base syntax to expect. If you give none 4164 of these options, cpp will deduce the language from the extension 4165 of the source file: `.c', `.cc', `.m', or `.S'. Some other common 4166 extensions for C++ and assembly are also recognized. If cpp does 4167 not recognize the extension, it will treat the file as C; this is 4168 the most generic mode. 4169 4170 _Note:_ Previous versions of cpp accepted a `-lang' option which 4171 selected both the language and the standards conformance level. 4172 This option has been removed, because it conflicts with the `-l' 4173 option. 4174 4175 `-std=STANDARD' 4176 `-ansi' 4177 Specify the standard to which the code should conform. Currently 4178 CPP knows about C and C++ standards; others may be added in the 4179 future. 4180 4181 STANDARD may be one of: 4182 `iso9899:1990' 4183 `c89' 4184 The ISO C standard from 1990. `c89' is the customary 4185 shorthand for this version of the standard. 4186 4187 The `-ansi' option is equivalent to `-std=c89'. 4188 4189 `iso9899:199409' 4190 The 1990 C standard, as amended in 1994. 4191 4192 `iso9899:1999' 4193 `c99' 4194 `iso9899:199x' 4195 `c9x' 4196 The revised ISO C standard, published in December 1999. 4197 Before publication, this was known as C9X. 4198 4199 `gnu89' 4200 The 1990 C standard plus GNU extensions. This is the default. 4201 4202 `gnu99' 4203 `gnu9x' 4204 The 1999 C standard plus GNU extensions. 4205 4206 `c++98' 4207 The 1998 ISO C++ standard plus amendments. 4208 4209 `gnu++98' 4210 The same as `-std=c++98' plus GNU extensions. This is the 4211 default for C++ code. 4212 4213 `-I-' 4214 Split the include path. Any directories specified with `-I' 4215 options before `-I-' are searched only for headers requested with 4216 `#include "FILE"'; they are not searched for `#include <FILE>'. 4217 If additional directories are specified with `-I' options after 4218 the `-I-', those directories are searched for all `#include' 4219 directives. 4220 4221 In addition, `-I-' inhibits the use of the directory of the current 4222 file directory as the first search directory for `#include "FILE"'. 4223 *Note Search Path::. This option has been deprecated. 4224 4225 `-nostdinc' 4226 Do not search the standard system directories for header files. 4227 Only the directories you have specified with `-I' options (and the 4228 directory of the current file, if appropriate) are searched. 4229 4230 `-nostdinc++' 4231 Do not search for header files in the C++-specific standard 4232 directories, but do still search the other standard directories. 4233 (This option is used when building the C++ library.) 4234 4235 `-include FILE' 4236 Process FILE as if `#include "file"' appeared as the first line of 4237 the primary source file. However, the first directory searched 4238 for FILE is the preprocessor's working directory _instead of_ the 4239 directory containing the main source file. If not found there, it 4240 is searched for in the remainder of the `#include "..."' search 4241 chain as normal. 4242 4243 If multiple `-include' options are given, the files are included 4244 in the order they appear on the command line. 4245 4246 `-imacros FILE' 4247 Exactly like `-include', except that any output produced by 4248 scanning FILE is thrown away. Macros it defines remain defined. 4249 This allows you to acquire all the macros from a header without 4250 also processing its declarations. 4251 4252 All files specified by `-imacros' are processed before all files 4253 specified by `-include'. 4254 4255 `-idirafter DIR' 4256 Search DIR for header files, but do it _after_ all directories 4257 specified with `-I' and the standard system directories have been 4258 exhausted. DIR is treated as a system include directory. If DIR 4259 begins with `=', then the `=' will be replaced by the sysroot 4260 prefix; see `--sysroot' and `-isysroot'. 4261 4262 `-iprefix PREFIX' 4263 Specify PREFIX as the prefix for subsequent `-iwithprefix' 4264 options. If the prefix represents a directory, you should include 4265 the final `/'. 4266 4267 `-iwithprefix DIR' 4268 `-iwithprefixbefore DIR' 4269 Append DIR to the prefix specified previously with `-iprefix', and 4270 add the resulting directory to the include search path. 4271 `-iwithprefixbefore' puts it in the same place `-I' would; 4272 `-iwithprefix' puts it where `-idirafter' would. 4273 4274 `-isysroot DIR' 4275 This option is like the `--sysroot' option, but applies only to 4276 header files. See the `--sysroot' option for more information. 4277 4278 `-imultilib DIR' 4279 Use DIR as a subdirectory of the directory containing 4280 target-specific C++ headers. 4281 4282 `-isystem DIR' 4283 Search DIR for header files, after all directories specified by 4284 `-I' but before the standard system directories. Mark it as a 4285 system directory, so that it gets the same special treatment as is 4286 applied to the standard system directories. *Note System 4287 Headers::. If DIR begins with `=', then the `=' will be replaced 4288 by the sysroot prefix; see `--sysroot' and `-isysroot'. 4289 4290 `-iquote DIR' 4291 Search DIR only for header files requested with `#include "FILE"'; 4292 they are not searched for `#include <FILE>', before all 4293 directories specified by `-I' and before the standard system 4294 directories. *Note Search Path::. If DIR begins with `=', then 4295 the `=' will be replaced by the sysroot prefix; see `--sysroot' 4296 and `-isysroot'. 4297 4298 `-fdirectives-only' 4299 When preprocessing, handle directives, but do not expand macros. 4300 4301 The option's behavior depends on the `-E' and `-fpreprocessed' 4302 options. 4303 4304 With `-E', preprocessing is limited to the handling of directives 4305 such as `#define', `#ifdef', and `#error'. Other preprocessor 4306 operations, such as macro expansion and trigraph conversion are 4307 not performed. In addition, the `-dD' option is implicitly 4308 enabled. 4309 4310 With `-fpreprocessed', predefinition of command line and most 4311 builtin macros is disabled. Macros such as `__LINE__', which are 4312 contextually dependent, are handled normally. This enables 4313 compilation of files previously preprocessed with `-E 4314 -fdirectives-only'. 4315 4316 With both `-E' and `-fpreprocessed', the rules for 4317 `-fpreprocessed' take precedence. This enables full preprocessing 4318 of files previously preprocessed with `-E -fdirectives-only'. 4319 4320 `-fdollars-in-identifiers' 4321 Accept `$' in identifiers. *Note Identifier characters::. 4322 4323 `-fextended-identifiers' 4324 Accept universal character names in identifiers. This option is 4325 experimental; in a future version of GCC, it will be enabled by 4326 default for C99 and C++. 4327 4328 `-fpreprocessed' 4329 Indicate to the preprocessor that the input file has already been 4330 preprocessed. This suppresses things like macro expansion, 4331 trigraph conversion, escaped newline splicing, and processing of 4332 most directives. The preprocessor still recognizes and removes 4333 comments, so that you can pass a file preprocessed with `-C' to 4334 the compiler without problems. In this mode the integrated 4335 preprocessor is little more than a tokenizer for the front ends. 4336 4337 `-fpreprocessed' is implicit if the input file has one of the 4338 extensions `.i', `.ii' or `.mi'. These are the extensions that 4339 GCC uses for preprocessed files created by `-save-temps'. 4340 4341 `-ftabstop=WIDTH' 4342 Set the distance between tab stops. This helps the preprocessor 4343 report correct column numbers in warnings or errors, even if tabs 4344 appear on the line. If the value is less than 1 or greater than 4345 100, the option is ignored. The default is 8. 4346 4347 `-fexec-charset=CHARSET' 4348 Set the execution character set, used for string and character 4349 constants. The default is UTF-8. CHARSET can be any encoding 4350 supported by the system's `iconv' library routine. 4351 4352 `-fwide-exec-charset=CHARSET' 4353 Set the wide execution character set, used for wide string and 4354 character constants. The default is UTF-32 or UTF-16, whichever 4355 corresponds to the width of `wchar_t'. As with `-fexec-charset', 4356 CHARSET can be any encoding supported by the system's `iconv' 4357 library routine; however, you will have problems with encodings 4358 that do not fit exactly in `wchar_t'. 4359 4360 `-finput-charset=CHARSET' 4361 Set the input character set, used for translation from the 4362 character set of the input file to the source character set used 4363 by GCC. If the locale does not specify, or GCC cannot get this 4364 information from the locale, the default is UTF-8. This can be 4365 overridden by either the locale or this command line option. 4366 Currently the command line option takes precedence if there's a 4367 conflict. CHARSET can be any encoding supported by the system's 4368 `iconv' library routine. 4369 4370 `-fworking-directory' 4371 Enable generation of linemarkers in the preprocessor output that 4372 will let the compiler know the current working directory at the 4373 time of preprocessing. When this option is enabled, the 4374 preprocessor will emit, after the initial linemarker, a second 4375 linemarker with the current working directory followed by two 4376 slashes. GCC will use this directory, when it's present in the 4377 preprocessed input, as the directory emitted as the current 4378 working directory in some debugging information formats. This 4379 option is implicitly enabled if debugging information is enabled, 4380 but this can be inhibited with the negated form 4381 `-fno-working-directory'. If the `-P' flag is present in the 4382 command line, this option has no effect, since no `#line' 4383 directives are emitted whatsoever. 4384 4385 `-fno-show-column' 4386 Do not print column numbers in diagnostics. This may be necessary 4387 if diagnostics are being scanned by a program that does not 4388 understand the column numbers, such as `dejagnu'. 4389 4390 `-A PREDICATE=ANSWER' 4391 Make an assertion with the predicate PREDICATE and answer ANSWER. 4392 This form is preferred to the older form `-A PREDICATE(ANSWER)', 4393 which is still supported, because it does not use shell special 4394 characters. *Note Assertions::. 4395 4396 `-A -PREDICATE=ANSWER' 4397 Cancel an assertion with the predicate PREDICATE and answer ANSWER. 4398 4399 `-dCHARS' 4400 CHARS is a sequence of one or more of the following characters, 4401 and must not be preceded by a space. Other characters are 4402 interpreted by the compiler proper, or reserved for future 4403 versions of GCC, and so are silently ignored. If you specify 4404 characters whose behavior conflicts, the result is undefined. 4405 4406 `M' 4407 Instead of the normal output, generate a list of `#define' 4408 directives for all the macros defined during the execution of 4409 the preprocessor, including predefined macros. This gives 4410 you a way of finding out what is predefined in your version 4411 of the preprocessor. Assuming you have no file `foo.h', the 4412 command 4413 4414 touch foo.h; cpp -dM foo.h 4415 4416 will show all the predefined macros. 4417 4418 If you use `-dM' without the `-E' option, `-dM' is 4419 interpreted as a synonym for `-fdump-rtl-mach'. *Note 4420 Debugging Options: (gcc)Debugging Options. 4421 4422 `D' 4423 Like `M' except in two respects: it does _not_ include the 4424 predefined macros, and it outputs _both_ the `#define' 4425 directives and the result of preprocessing. Both kinds of 4426 output go to the standard output file. 4427 4428 `N' 4429 Like `D', but emit only the macro names, not their expansions. 4430 4431 `I' 4432 Output `#include' directives in addition to the result of 4433 preprocessing. 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: Assertions. (line 41) 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: Obsolete once-only headers. 5018 (line 10) 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: Assertions. (line 52) 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 525) 5045 * ansi: Invocation. (line 311) 5046 * C: Invocation. (line 577) 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 557) 5055 * DEPENDENCIES_OUTPUT: Environment Variables. 5056 (line 44) 5057 * dI: Invocation. (line 566) 5058 * dM: Invocation. (line 541) 5059 * dN: Invocation. (line 563) 5060 * fdirectives-only: Invocation. (line 433) 5061 * fdollars-in-identifiers: Invocation. (line 455) 5062 * fexec-charset: Invocation. (line 482) 5063 * fextended-identifiers: Invocation. (line 458) 5064 * finput-charset: Invocation. (line 495) 5065 * fno-show-column: Invocation. (line 520) 5066 * fno-working-directory: Invocation. (line 505) 5067 * fpreprocessed: Invocation. (line 463) 5068 * ftabstop: Invocation. (line 476) 5069 * fwide-exec-charset: Invocation. (line 487) 5070 * fworking-directory: Invocation. (line 505) 5071 * H: Invocation. (line 622) 5072 * help: Invocation. (line 614) 5073 * I: Invocation. (line 71) 5074 * I-: Invocation. (line 348) 5075 * idirafter: Invocation. (line 390) 5076 * imacros: Invocation. (line 381) 5077 * imultilib: Invocation. (line 413) 5078 * include: Invocation. (line 370) 5079 * iprefix: Invocation. (line 397) 5080 * iquote: Invocation. (line 425) 5081 * isysroot: Invocation. (line 409) 5082 * isystem: Invocation. (line 417) 5083 * iwithprefix: Invocation. (line 403) 5084 * iwithprefixbefore: Invocation. (line 403) 5085 * M: Invocation. (line 183) 5086 * MD: Invocation. (line 272) 5087 * MF: Invocation. (line 218) 5088 * MG: Invocation. (line 227) 5089 * MM: Invocation. (line 208) 5090 * MMD: Invocation. (line 288) 5091 * MP: Invocation. (line 237) 5092 * MQ: Invocation. (line 263) 5093 * MT: Invocation. (line 249) 5094 * nostdinc: Invocation. (line 360) 5095 * nostdinc++: Invocation. (line 365) 5096 * o: Invocation. (line 82) 5097 * OBJC_INCLUDE_PATH: Environment Variables. 5098 (line 18) 5099 * P: Invocation. (line 570) 5100 * pedantic: Invocation. (line 173) 5101 * pedantic-errors: Invocation. (line 178) 5102 * remap: Invocation. (line 609) 5103 * std=: Invocation. (line 311) 5104 * SUNPRO_DEPENDENCIES: Environment Variables. 5105 (line 60) 5106 * target-help: Invocation. (line 614) 5107 * traditional-cpp: Invocation. (line 602) 5108 * trigraphs: Invocation. (line 606) 5109 * U: Invocation. (line 62) 5110 * undef: Invocation. (line 66) 5111 * v: Invocation. (line 618) 5112 * version: Invocation. (line 631) 5113 * w: Invocation. (line 169) 5114 * Wall: Invocation. (line 88) 5115 * Wcomment: Invocation. (line 96) 5116 * Wcomments: Invocation. (line 96) 5117 * Wendif-labels: Invocation. (line 146) 5118 * Werror: Invocation. (line 159) 5119 * Wimport: Invocation. (line 119) 5120 * Wsystem-headers: Invocation. (line 163) 5121 * Wtraditional: Invocation. (line 113) 5122 * Wtrigraphs: Invocation. (line 101) 5123 * Wundef: Invocation. (line 122) 5124 * Wunused-macros: Invocation. (line 127) 5125 * x: Invocation. (line 295) 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: Assertions. (line 6) 5143 * assertions, canceling: Assertions. (line 52) 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 482) 5149 * character set, input: Invocation. (line 495) 5150 * character set, wide execution: Invocation. (line 487) 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 183) 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 183) 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: Assertions. (line 19) 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: Assertions. (line 30) 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: Top1102 5274 Node: Overview3798 5275 Node: Character sets6619 5276 Ref: Character sets-Footnote-18802 5277 Node: Initial processing8983 5278 Ref: trigraphs10542 5279 Node: Tokenization14744 5280 Ref: Tokenization-Footnote-121880 5281 Node: The preprocessing language21991 5282 Node: Header Files24869 5283 Node: Include Syntax26749 5284 Node: Include Operation28386 5285 Node: Search Path30234 5286 Node: Once-Only Headers33424 5287 Node: Computed Includes35069 5288 Node: Wrapper Headers38213 5289 Node: System Headers40639 5290 Node: Macros42689 5291 Node: Object-like Macros43830 5292 Node: Function-like Macros47420 5293 Node: Macro Arguments49036 5294 Node: Stringification53181 5295 Node: Concatenation56387 5296 Node: Variadic Macros59495 5297 Node: Predefined Macros64282 5298 Node: Standard Predefined Macros64870 5299 Node: Common Predefined Macros70806 5300 Node: System-specific Predefined Macros83493 5301 Node: C++ Named Operators85514 5302 Node: Undefining and Redefining Macros86478 5303 Node: Directives Within Macro Arguments88582 5304 Node: Macro Pitfalls90130 5305 Node: Misnesting90663 5306 Node: Operator Precedence Problems91775 5307 Node: Swallowing the Semicolon93641 5308 Node: Duplication of Side Effects95664 5309 Node: Self-Referential Macros97847 5310 Node: Argument Prescan100256 5311 Node: Newlines in Arguments104010 5312 Node: Conditionals104961 5313 Node: Conditional Uses106791 5314 Node: Conditional Syntax108149 5315 Node: Ifdef108469 5316 Node: If111630 5317 Node: Defined113934 5318 Node: Else115217 5319 Node: Elif115787 5320 Node: Deleted Code117076 5321 Node: Diagnostics118323 5322 Node: Line Control119940 5323 Node: Pragmas123744 5324 Node: Other Directives128014 5325 Node: Preprocessor Output129064 5326 Node: Traditional Mode132265 5327 Node: Traditional lexical analysis133323 5328 Node: Traditional macros135826 5329 Node: Traditional miscellany139628 5330 Node: Traditional warnings140625 5331 Node: Implementation Details142822 5332 Node: Implementation-defined behavior143443 5333 Ref: Identifier characters144176 5334 Node: Implementation limits147261 5335 Node: Obsolete Features149935 5336 Node: Assertions150390 5337 Node: Obsolete once-only headers152931 5338 Node: Differences from previous versions154666 5339 Node: Invocation158874 5340 Ref: Wtrigraphs163326 5341 Ref: dashMF168157 5342 Ref: fdollars-in-identifiers177540 5343 Node: Environment Variables185373 5344 Node: GNU Free Documentation License188339 5345 Node: Index of Directives210772 5346 Node: Option Index212701 5347 Node: Concept Index218885 5348 5349 End Tag Table 5350