1CPP(1) GNU CPP(1)
2
6 cpp - The C Preprocessor
7
9 cpp [-Dmacro[=defn]...] [-Umacro]
10 [-Idir...] [-iquotedir...]
11 [-Wwarn...]
12 [-M⎪-MM] [-MG] [-MF filename]
13 [-MP] [-MQ target...]
14 [-MT target...]
15 [-P] [-fno-working-directory]
16 [-x language] [-std=standard]
17 infile outfile
18 Only the most useful options are listed here; see below for the remain‐
20 der.
21
23 The C preprocessor, often known as cpp, is a macro processor that is
24 used automatically by the C compiler to transform your program before
25 compilation. It is called a macro processor because it allows you to
26 define macros, which are brief abbreviations for longer constructs.
27 The C preprocessor is intended to be used only with C, C++, and Objec‐
29 tive-C source code. In the past, it has been abused as a general text
30 processor. It will choke on input which does not obey C's lexical
31 rules. For example, apostrophes will be interpreted as the beginning
32 of character constants, and cause errors. Also, you cannot rely on it
33 preserving characteristics of the input which are not significant to
34 C-family languages. If a Makefile is preprocessed, all the hard tabs
35 will be removed, and the Makefile will not work.
36 Having said that, you can often get away with using cpp on things which
38 are not C. Other Algol-ish programming languages are often safe (Pas‐
39 cal, Ada, etc.) So is assembly, with caution. -traditional-cpp mode
40 preserves more white space, and is otherwise more permissive. Many of
41 the problems can be avoided by writing C or C++ style comments instead
42 of native language comments, and keeping macros simple.
43 Wherever possible, you should use a preprocessor geared to the language
45 you are writing in. Modern versions of the GNU assembler have macro
46 facilities. Most high level programming languages have their own con‐
47 ditional compilation and inclusion mechanism. If all else fails, try a
48 true general text processor, such as GNU M4.
49 C preprocessors vary in some details. This manual discusses the GNU C
51 preprocessor, which provides a small superset of the features of ISO
52 Standard C. In its default mode, the GNU C preprocessor does not do a
53 few things required by the standard. These are features which are
54 rarely, if ever, used, and may cause surprising changes to the meaning
55 of a program which does not expect them. To get strict ISO Standard C,
56 you should use the -std=c89 or -std=c99 options, depending on which
57 version of the standard you want. To get all the mandatory diagnos‐
58 tics, you must also use -pedantic.
59 This manual describes the behavior of the ISO preprocessor. To mini‐
61 mize gratuitous differences, where the ISO preprocessor's behavior does
62 not conflict with traditional semantics, the traditional preprocessor
63 should behave the same way. The various differences that do exist are
64 detailed in the section Traditional Mode.
65 For clarity, unless noted otherwise, references to CPP in this manual
67 refer to GNU CPP.
68
70 The C preprocessor expects two file names as arguments, infile and out‐
71 file. The preprocessor reads infile together with any other files it
72 specifies with #include. All the output generated by the combined
73 input files is written in outfile.
74 Either infile or outfile may be -, which as infile means to read from
76 standard input and as outfile means to write to standard output. Also,
77 if either file is omitted, it means the same as if - had been specified
78 for that file.
79 Unless otherwise noted, or the option ends in =, all options which take
81 an argument may have that argument appear either immediately after the
82 option, or with a space between option and argument: -Ifoo and -I foo
83 have the same effect.
84 Many options have multi-letter names; therefore multiple single-letter
86 options may not be grouped: -dM is very different from -d -M.
87 -D name
89 Predefine name as a macro, with definition 1.
90 -D name=definition
92 The contents of definition are tokenized and processed as if they
93 appeared during translation phase three in a #define directive. In
94 particular, the definition will be truncated by embedded newline
95 characters.
96 If you are invoking the preprocessor from a shell or shell-like
98 program you may need to use the shell's quoting syntax to protect
99 characters such as spaces that have a meaning in the shell syntax.
100 If you wish to define a function-like macro on the command line,
102 write its argument list with surrounding parentheses before the
103 equals sign (if any). Parentheses are meaningful to most shells,
104 so you will need to quote the option. With sh and csh,
105 -D'name(args...)=definition' works.
106 -D and -U options are processed in the order they are given on the
108 command line. All -imacros file and -include file options are pro‐
109 cessed after all -D and -U options.
110 -U name
112 Cancel any previous definition of name, either built in or provided
113 with a -D option.
114 -undef
116 Do not predefine any system-specific or GCC-specific macros. The
117 standard predefined macros remain defined.
118 -I dir
120 Add the directory dir to the list of directories to be searched for
121 header files.
122 Directories named by -I are searched before the standard system
124 include directories. If the directory dir is a standard system
125 include directory, the option is ignored to ensure that the default
126 search order for system directories and the special treatment of
127 system headers are not defeated .
128 -o file
130 Write output to file. This is the same as specifying file as the
131 second non-option argument to cpp. gcc has a different interpreta‐
132 tion of a second non-option argument, so you must use -o to specify
133 the output file.
134 -Wall
136 Turns on all optional warnings which are desirable for normal code.
137 At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warn‐
138 ing about integer promotion causing a change of sign in "#if"
139 expressions. Note that many of the preprocessor's warnings are on
140 by default and have no options to control them.
141 -Wcomment
143 -Wcomments
144 Warn whenever a comment-start sequence /* appears in a /* comment,
145 or whenever a backslash-newline appears in a // comment. (Both
146 forms have the same effect.)
147 -Wtrigraphs
149 Most trigraphs in comments cannot affect the meaning of the pro‐
150 gram. However, a trigraph that would form an escaped newline (??/
151 at the end of a line) can, by changing where the comment begins or
152 ends. Therefore, only trigraphs that would form escaped newlines
153 produce warnings inside a comment.
154 This option is implied by -Wall. If -Wall is not given, this
156 option is still enabled unless trigraphs are enabled. To get tri‐
157 graph conversion without warnings, but get the other -Wall warn‐
158 ings, use -trigraphs -Wall -Wno-trigraphs.
159 -Wtraditional
161 Warn about certain constructs that behave differently in tradi‐
162 tional and ISO C. Also warn about ISO C constructs that have no
163 traditional C equivalent, and problematic constructs which should
164 be avoided.
165 -Wimport
167 Warn the first time #import is used.
168 -Wundef
170 Warn whenever an identifier which is not a macro is encountered in
171 an #if directive, outside of defined. Such identifiers are
172 replaced with zero.
173 -Wunused-macros
175 Warn about macros defined in the main file that are unused. A
176 macro is used if it is expanded or tested for existence at least
177 once. The preprocessor will also warn if the macro has not been
178 used at the time it is redefined or undefined.
179 Built-in macros, macros defined on the command line, and macros
181 defined in include files are not warned about.
182 Note: If a macro is actually used, but only used in skipped condi‐
184 tional blocks, then CPP will report it as unused. To avoid the
185 warning in such a case, you might improve the scope of the macro's
186 definition by, for example, moving it into the first skipped block.
187 Alternatively, you could provide a dummy use with something like:
188 #if defined the_macro_causing_the_warning
190 #endif
191 -Wendif-labels
193 Warn whenever an #else or an #endif are followed by text. This
194 usually happens in code of the form
195 #if FOO
197 ...
198 #else FOO
199 ...
200 #endif FOO
201 The second and third "FOO" should be in comments, but often are not
203 in older programs. This warning is on by default.
204 -Werror
206 Make all warnings into hard errors. Source code which triggers
207 warnings will be rejected.
208 -Wsystem-headers
210 Issue warnings for code in system headers. These are normally
211 unhelpful in finding bugs in your own code, therefore suppressed.
212 If you are responsible for the system library, you may want to see
213 them.
214 -w Suppress all warnings, including those which GNU CPP issues by
216 default.
217 -pedantic
219 Issue all the mandatory diagnostics listed in the C standard. Some
220 of them are left out by default, since they trigger frequently on
221 harmless code.
222 -pedantic-errors
224 Issue all the mandatory diagnostics, and make all mandatory diag‐
225 nostics into errors. This includes mandatory diagnostics that GCC
226 issues without -pedantic but treats as warnings.
227 -M Instead of outputting the result of preprocessing, output a rule
229 suitable for make describing the dependencies of the main source
230 file. The preprocessor outputs one make rule containing the object
231 file name for that source file, a colon, and the names of all the
232 included files, including those coming from -include or -imacros
233 command line options.
234 Unless specified explicitly (with -MT or -MQ), the object file name
236 consists of the basename of the source file with any suffix
237 replaced with object file suffix. If there are many included files
238 then the rule is split into several lines using \-newline. The
239 rule has no commands.
240 This option does not suppress the preprocessor's debug output, such
242 as -dM. To avoid mixing such debug output with the dependency
243 rules you should explicitly specify the dependency output file with
244 -MF, or use an environment variable like DEPENDENCIES_OUTPUT.
245 Debug output will still be sent to the regular output stream as
246 normal.
247 Passing -M to the driver implies -E, and suppresses warnings with
249 an implicit -w.
250 -MM Like -M but do not mention header files that are found in system
252 header directories, nor header files that are included, directly or
253 indirectly, from such a header.
254 This implies that the choice of angle brackets or double quotes in
256 an #include directive does not in itself determine whether that
257 header will appear in -MM dependency output. This is a slight
258 change in semantics from GCC versions 3.0 and earlier.
259 -MF file
261 When used with -M or -MM, specifies a file to write the dependen‐
262 cies to. If no -MF switch is given the preprocessor sends the
263 rules to the same place it would have sent preprocessed output.
264 When used with the driver options -MD or -MMD, -MF overrides the
266 default dependency output file.
267 -MG In conjunction with an option such as -M requesting dependency gen‐
269 eration, -MG assumes missing header files are generated files and
270 adds them to the dependency list without raising an error. The
271 dependency filename is taken directly from the "#include" directive
272 without prepending any path. -MG also suppresses preprocessed out‐
273 put, as a missing header file renders this useless.
274 This feature is used in automatic updating of makefiles.
276 -MP This option instructs CPP to add a phony target for each dependency
278 other than the main file, causing each to depend on nothing. These
279 dummy rules work around errors make gives if you remove header
280 files without updating the Makefile to match.
281 This is typical output:
283 test.o: test.c test.h
285 test.h:
287 -MT target
289 Change the target of the rule emitted by dependency generation. By
290 default CPP takes the name of the main input file, including any
291 path, deletes any file suffix such as .c, and appends the plat‐
292 form's usual object suffix. The result is the target.
293 An -MT option will set the target to be exactly the string you
295 specify. If you want multiple targets, you can specify them as a
296 single argument to -MT, or use multiple -MT options.
297 For example, -MT '$(objpfx)foo.o' might give
299 $(objpfx)foo.o: foo.c
301 -MQ target
303 Same as -MT, but it quotes any characters which are special to
304 Make. -MQ '$(objpfx)foo.o' gives
305 $$(objpfx)foo.o: foo.c
307 The default target is automatically quoted, as if it were given
309 with -MQ.
310 -MD -MD is equivalent to -M -MF file, except that -E is not implied.
312 The driver determines file based on whether an -o option is given.
313 If it is, the driver uses its argument but with a suffix of .d,
314 otherwise it take the basename of the input file and applies a .d
315 suffix.
316 If -MD is used in conjunction with -E, any -o switch is understood
318 to specify the dependency output file (but @pxref{dashMF,,-MF}),
319 but if used without -E, each -o is understood to specify a target
320 object file.
321 Since -E is not implied, -MD can be used to generate a dependency
323 output file as a side-effect of the compilation process.
324 -MMD
326 Like -MD except mention only user header files, not system header
327 files.
328 -x c
330 -x c++
331 -x objective-c
332 -x assembler-with-cpp
333 Specify the source language: C, C++, Objective-C, or assembly.
334 This has nothing to do with standards conformance or extensions; it
335 merely selects which base syntax to expect. If you give none of
336 these options, cpp will deduce the language from the extension of
337 the source file: .c, .cc, .m, or .S. Some other common extensions
338 for C++ and assembly are also recognized. If cpp does not recog‐
339 nize the extension, it will treat the file as C; this is the most
340 generic mode.
341 Note: Previous versions of cpp accepted a -lang option which
343 selected both the language and the standards conformance level.
344 This option has been removed, because it conflicts with the -l
345 option.
346 -std=standard
348 -ansi
349 Specify the standard to which the code should conform. Currently
350 CPP knows about C and C++ standards; others may be added in the
351 future.
352 standard may be one of:
354 "iso9899:1990"
356 "c89"
357 The ISO C standard from 1990. c89 is the customary shorthand
358 for this version of the standard.
359 The -ansi option is equivalent to -std=c89.
361 "iso9899:199409"
363 The 1990 C standard, as amended in 1994.
364 "iso9899:1999"
366 "c99"
367 "iso9899:199x"
368 "c9x"
369 The revised ISO C standard, published in December 1999. Before
370 publication, this was known as C9X.
371 "gnu89"
373 The 1990 C standard plus GNU extensions. This is the default.
374 "gnu99"
376 "gnu9x"
377 The 1999 C standard plus GNU extensions.
378 "c++98"
380 The 1998 ISO C++ standard plus amendments.
381 "gnu++98"
383 The same as -std=c++98 plus GNU extensions. This is the
384 default for C++ code.
385 -I- Split the include path. Any directories specified with -I options
387 before -I- are searched only for headers requested with
388 "#include "file""; they are not searched for "#include <file>". If
389 additional directories are specified with -I options after the -I-,
390 those directories are searched for all #include directives.
391 In addition, -I- inhibits the use of the directory of the current
393 file directory as the first search directory for "#include "file"".
394 This option has been deprecated.
396 -nostdinc
398 Do not search the standard system directories for header files.
399 Only the directories you have specified with -I options (and the
400 directory of the current file, if appropriate) are searched.
401 -nostdinc++
403 Do not search for header files in the C++-specific standard direc‐
404 tories, but do still search the other standard directories. (This
405 option is used when building the C++ library.)
406 -include file
408 Process file as if "#include "file"" appeared as the first line of
409 the primary source file. However, the first directory searched for
410 file is the preprocessor's working directory instead of the direc‐
411 tory containing the main source file. If not found there, it is
412 searched for in the remainder of the "#include "..."" search chain
413 as normal.
414 If multiple -include options are given, the files are included in
416 the order they appear on the command line.
417 -imacros file
419 Exactly like -include, except that any output produced by scanning
420 file is thrown away. Macros it defines remain defined. This
421 allows you to acquire all the macros from a header without also
422 processing its declarations.
423 All files specified by -imacros are processed before all files
425 specified by -include.
426 -idirafter dir
428 Search dir for header files, but do it after all directories speci‐
429 fied with -I and the standard system directories have been
430 exhausted. dir is treated as a system include directory.
431 -iprefix prefix
433 Specify prefix as the prefix for subsequent -iwithprefix options.
434 If the prefix represents a directory, you should include the final
435 /.
436 -iwithprefix dir
438 -iwithprefixbefore dir
439 Append dir to the prefix specified previously with -iprefix, and
440 add the resulting directory to the include search path. -iwithpre‐
441 fixbefore puts it in the same place -I would; -iwithprefix puts it
442 where -idirafter would.
443 -isysroot dir
445 This option is like the --sysroot option, but applies only to
446 header files. See the --sysroot option for more information.
447 -isystem dir
449 Search dir for header files, after all directories specified by -I
450 but before the standard system directories. Mark it as a system
451 directory, so that it gets the same special treatment as is applied
452 to the standard system directories.
453 -iquote dir
455 Search dir only for header files requested with "#include "file"";
456 they are not searched for "#include <file>", before all directories
457 specified by -I and before the standard system directories.
458 -fdollars-in-identifiers
460 Accept $ in identifiers.
461 -fextended-identifiers
463 Accept universal character names in identifiers. This option is
464 experimental; in a future version of GCC, it will be enabled by
465 default for C99 and C++.
466 -fpreprocessed
468 Indicate to the preprocessor that the input file has already been
469 preprocessed. This suppresses things like macro expansion, tri‐
470 graph conversion, escaped newline splicing, and processing of most
471 directives. The preprocessor still recognizes and removes com‐
472 ments, so that you can pass a file preprocessed with -C to the com‐
473 piler without problems. In this mode the integrated preprocessor
474 is little more than a tokenizer for the front ends.
475 -fpreprocessed is implicit if the input file has one of the exten‐
477 sions .i, .ii or .mi. These are the extensions that GCC uses for
478 preprocessed files created by -save-temps.
479 -ftabstop=width
481 Set the distance between tab stops. This helps the preprocessor
482 report correct column numbers in warnings or errors, even if tabs
483 appear on the line. If the value is less than 1 or greater than
484 100, the option is ignored. The default is 8.
485 -fexec-charset=charset
487 Set the execution character set, used for string and character con‐
488 stants. The default is UTF-8. charset can be any encoding sup‐
489 ported by the system's "iconv" library routine.
490 -fwide-exec-charset=charset
492 Set the wide execution character set, used for wide string and
493 character constants. The default is UTF-32 or UTF-16, whichever
494 corresponds to the width of "wchar_t". As with -fexec-charset,
495 charset can be any encoding supported by the system's "iconv"
496 library routine; however, you will have problems with encodings
497 that do not fit exactly in "wchar_t".
498 -finput-charset=charset
500 Set the input character set, used for translation from the charac‐
501 ter set of the input file to the source character set used by GCC.
502 If the locale does not specify, or GCC cannot get this information
503 from the locale, the default is UTF-8. This can be overridden by
504 either the locale or this command line option. Currently the com‐
505 mand line option takes precedence if there's a conflict. charset
506 can be any encoding supported by the system's "iconv" library rou‐
507 tine.
508 -fworking-directory
510 Enable generation of linemarkers in the preprocessor output that
511 will let the compiler know the current working directory at the
512 time of preprocessing. When this option is enabled, the preproces‐
513 sor will emit, after the initial linemarker, a second linemarker
514 with the current working directory followed by two slashes. GCC
515 will use this directory, when it's present in the preprocessed
516 input, as the directory emitted as the current working directory in
517 some debugging information formats. This option is implicitly
518 enabled if debugging information is enabled, but this can be inhib‐
519 ited with the negated form -fno-working-directory. If the -P flag
520 is present in the command line, this option has no effect, since no
521 "#line" directives are emitted whatsoever.
522 -fno-show-column
524 Do not print column numbers in diagnostics. This may be necessary
525 if diagnostics are being scanned by a program that does not under‐
526 stand the column numbers, such as dejagnu.
527 -A predicate=answer
529 Make an assertion with the predicate predicate and answer answer.
530 This form is preferred to the older form -A predicate(answer),
531 which is still supported, because it does not use shell special
532 characters.
533 -A -predicate=answer
535 Cancel an assertion with the predicate predicate and answer answer.
536 -dCHARS
538 CHARS is a sequence of one or more of the following characters, and
539 must not be preceded by a space. Other characters are interpreted
540 by the compiler proper, or reserved for future versions of GCC, and
541 so are silently ignored. If you specify characters whose behavior
542 conflicts, the result is undefined.
543 M Instead of the normal output, generate a list of #define direc‐
545 tives for all the macros defined during the execution of the
546 preprocessor, including predefined macros. This gives you a
547 way of finding out what is predefined in your version of the
548 preprocessor. Assuming you have no file foo.h, the command
549 touch foo.h; cpp -dM foo.h
551 will show all the predefined macros.
553 D Like M except in two respects: it does not include the prede‐
555 fined macros, and it outputs both the #define directives and
556 the result of preprocessing. Both kinds of output go to the
557 standard output file.
558 N Like D, but emit only the macro names, not their expansions.
560 I Output #include directives in addition to the result of prepro‐
562 cessing.
563 -P Inhibit generation of linemarkers in the output from the preproces‐
565 sor. This might be useful when running the preprocessor on some‐
566 thing that is not C code, and will be sent to a program which might
567 be confused by the linemarkers.
568 -C Do not discard comments. All comments are passed through to the
570 output file, except for comments in processed directives, which are
571 deleted along with the directive.
572 You should be prepared for side effects when using -C; it causes
574 the preprocessor to treat comments as tokens in their own right.
575 For example, comments appearing at the start of what would be a
576 directive line have the effect of turning that line into an ordi‐
577 nary source line, since the first token on the line is no longer a
578 #.
579 -CC Do not discard comments, including during macro expansion. This is
581 like -C, except that comments contained within macros are also
582 passed through to the output file where the macro is expanded.
583 In addition to the side-effects of the -C option, the -CC option
585 causes all C++-style comments inside a macro to be converted to
586 C-style comments. This is to prevent later use of that macro from
587 inadvertently commenting out the remainder of the source line.
588 The -CC option is generally used to support lint comments.
590 -traditional-cpp
592 Try to imitate the behavior of old-fashioned C preprocessors, as
593 opposed to ISO C preprocessors.
594 -trigraphs
596 Process trigraph sequences.
597 -remap
599 Enable special code to work around file systems which only permit
600 very short file names, such as MS-DOS.
601 --help
603 --target-help
604 Print text describing all the command line options instead of pre‐
605 processing anything.
606 -v Verbose mode. Print out GNU CPP's version number at the beginning
608 of execution, and report the final form of the include path.
609 -H Print the name of each header file used, in addition to other nor‐
611 mal activities. Each name is indented to show how deep in the
612 #include stack it is. Precompiled header files are also printed,
613 even if they are found to be invalid; an invalid precompiled header
614 file is printed with ...x and a valid one with ...! .
615 -version
617 --version
618 Print out GNU CPP's version number. With one dash, proceed to pre‐
619 process as normal. With two dashes, exit immediately.
620
622 This section describes the environment variables that affect how CPP
623 operates. You can use them to specify directories or prefixes to use
624 when searching for include files, or to control dependency output.
625 Note that you can also specify places to search using options such as
627 -I, and control dependency output with options like -M. These take
628 precedence over environment variables, which in turn take precedence
629 over the configuration of GCC.
630 CPATH
632 C_INCLUDE_PATH
633 CPLUS_INCLUDE_PATH
634 OBJC_INCLUDE_PATH
635 Each variable's value is a list of directories separated by a spe‐
636 cial character, much like PATH, in which to look for header files.
637 The special character, "PATH_SEPARATOR", is target-dependent and
638 determined at GCC build time. For Microsoft Windows-based targets
639 it is a semicolon, and for almost all other targets it is a colon.
640 CPATH specifies a list of directories to be searched as if speci‐
642 fied with -I, but after any paths given with -I options on the com‐
643 mand line. This environment variable is used regardless of which
644 language is being preprocessed.
645 The remaining environment variables apply only when preprocessing
647 the particular language indicated. Each specifies a list of direc‐
648 tories to be searched as if specified with -isystem, but after any
649 paths given with -isystem options on the command line.
650 In all these variables, an empty element instructs the compiler to
652 search its current working directory. Empty elements can appear at
653 the beginning or end of a path. For instance, if the value of
654 CPATH is ":/special/include", that has the same effect as
655 -I. -I/special/include.
656 DEPENDENCIES_OUTPUT
658 If this variable is set, its value specifies how to output depen‐
659 dencies for Make based on the non-system header files processed by
660 the compiler. System header files are ignored in the dependency
661 output.
662 The value of DEPENDENCIES_OUTPUT can be just a file name, in which
664 case the Make rules are written to that file, guessing the target
665 name from the source file name. Or the value can have the form
666 file target, in which case the rules are written to file file using
667 target as the target name.
668 In other words, this environment variable is equivalent to combin‐
670 ing the options -MM and -MF, with an optional -MT switch too.
671 SUNPRO_DEPENDENCIES
673 This variable is the same as DEPENDENCIES_OUTPUT (see above),
674 except that system header files are not ignored, so it implies -M
675 rather than -MM. However, the dependence on the main input file is
676 omitted.
677
679 gpl(7), gfdl(7), fsf-funding(7), gcc(1), as(1), ld(1), and the Info
680 entries for cpp, gcc, and binutils.
681
683 Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
684 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Founda‐
685 tion, Inc.
686 Permission is granted to copy, distribute and/or modify this document
688 under the terms of the GNU Free Documentation License, Version 1.1 or
689 any later version published by the Free Software Foundation. A copy of
690 the license is included in the man page gfdl(7). This manual contains
691 no Invariant Sections. The Front-Cover Texts are (a) (see below), and
692 the Back-Cover Texts are (b) (see below).
693 (a) The FSF's Front-Cover Text is:
695 A GNU Manual
697 (b) The FSF's Back-Cover Text is:
699 You have freedom to copy and modify this GNU Manual, like GNU
701 software. Copies published by the Free Software Foundation raise
702 funds for GNU development.
703gcc-4.1.2 2007-09-25 CPP(1)