Preprocessor Options (Using the GNU Compiler Collection (GCC))

### 3.14 Options Controlling the Preprocessor ¶

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the `-E` option, nothing is done except preprocessing. Some of these options make sense only together with `-E` because they cause the preprocessor output to be unsuitable for actual compilation.

In addition to the options listed here, there are a number of options to control search paths for include files documented in Options for Directory Search. Options to control preprocessor diagnostics are listed in Options to Request or Suppress Warnings.

`-D name` ¶
`--define-macro=name`
`--define-macro name`

  Predefine name as a macro, with definition `1`.

`-D name=definition`
`--define-macro=name=definition`
`--define-macro name=definition`

  The contents of definition are tokenized and processed as if they appeared during translation phase three in a ‘`#define`’ directive. In particular, the definition is truncated by embedded newline characters.

  If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell’s quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

  If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With `sh` and `csh`, `-D'name(args…)=definition'` works.

  `-D` and `-U` options are processed in the order they are given on the command line. All `-imacros file` and `-include file` options are processed after all `-D` and `-U` options.

`-U name` ¶
`--undefine-macro=name`
`--undefine-macro name`

  Cancel any previous definition of name, either built in or provided with a `-D` option.

`-include file` ¶
`--include=file`
`--include file`

  Process file as if `#include "file"` appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor’s working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the `#include "…"` search chain as normal.

  If multiple `-include` options are given, the files are included in the order they appear on the command line.

`-imacros file` ¶
`--imacros=file`
`--imacros file`

  Exactly like `-include`, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations.

  All files specified by `-imacros` are processed before all files specified by `-include`.

`-undef` ¶

  Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

`-pthread` ¶

  Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.

`-M` ¶
`--dependencies`

  Instead of outputting the result of preprocessing, output a rule suitable for `make` describing the dependencies of the main source file. The preprocessor outputs one `make` rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from `-include` or `-imacros` command-line options.

  Unless specified explicitly (with `-MT` or `-MQ`), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using ‘`\`’-newline. The rule has no commands.

  This option does not suppress the preprocessor’s debug output, such as `-dM`. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with `-MF`, or use an environment variable like `DEPENDENCIES_OUTPUT` (see Environment Variables Affecting GCC). Debug output is still sent to the regular output stream as normal.

  Passing `-M` to the driver implies `-E`, and suppresses warnings with an implicit `-w`.

`-MM` ¶
`--user-dependencies`

  Like `-M` but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

  This implies that the choice of angle brackets or double quotes in an ‘`#include`’ directive does not in itself determine whether that header appears in `-MM` dependency output.

`-MF file` ¶

  When used with `-M` or `-MM`, specifies a file to write the dependencies to. If no `-MF` switch is given the preprocessor sends the rules to the same place it would send preprocessed output.

  When used with the driver options `-MD` or `-MMD`, `-MF` overrides the default dependency output file.

  If file is `-`, then the dependencies are written to `stdout`.

`-MG` ¶
`--print-missing-file-dependencies`

  In conjunction with an option such as `-M` requesting dependency generation, `-MG` assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the `#include` directive without prepending any path. `-MG` also suppresses preprocessed output, as a missing header file renders this useless.

  This feature is used in automatic updating of makefiles.

`-Mno-modules` ¶

  Disable dependency generation for compiled module interfaces.

`-MP` ¶

  This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors `make` gives if you remove header files without updating the `Makefile` to match.

  This is typical output:

      test.o: test.c test.h

      test.h:

`-MT target` ¶

  Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as ‘`.c`’, and appends the platform’s usual object suffix. The result is the target.

  An `-MT` option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to `-MT`, or use multiple `-MT` options.

  For example, `-MT '$(objpfx)foo.o'` might give

      $(objpfx)foo.o: foo.c

`-MQ target` ¶

  Same as `-MT`, but it quotes any characters which are special to Make. `-MQ '$(objpfx)foo.o'` gives

      $$(objpfx)foo.o: foo.c

  The default target is automatically quoted, as if it were given with `-MQ`.

`-MD` ¶
`--write-dependencies`

  `-MD` is equivalent to `-M -MF file`, except that `-E` is not implied. The driver determines file based on whether an `-o` option is given. If it is, the driver uses its argument but with a suffix of `.d`, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a `.d` suffix.

  If `-MD` is used in conjunction with `-E`, any `-o` switch is understood to specify the dependency output file (see -MF), but if used without `-E`, each `-o` is understood to specify a target object file.

  Since `-E` is not implied, `-MD` can be used to generate a dependency output file as a side effect of the compilation process.

`-MMD` ¶
`--write-user-dependencies`

  Like `-MD` except mention only user header files, not system header files.

`-fpreprocessed` ¶

  Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with `-C` to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

  `-fpreprocessed` is implicit if the input file has one of the extensions ‘`.i`’, ‘`.ii`’ or ‘`.mi`’. These are the extensions that GCC uses for preprocessed files created by `-save-temps`.

`-fdirectives-only` ¶

  When preprocessing, handle directives, but do not expand macros.

  The option’s behavior depends on the `-E` and `-fpreprocessed` options.

  With `-E`, preprocessing is limited to the handling of directives such as `#define`, `#ifdef`, and `#error`. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the `-dD` option is implicitly enabled.

  With `-fpreprocessed`, predefinition of command line and most builtin macros is disabled. Macros such as `__LINE__`, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with `-E -fdirectives-only`.

  With both `-E` and `-fpreprocessed`, the rules for `-fpreprocessed` take precedence. This enables full preprocessing of files previously preprocessed with `-E -fdirectives-only`.

`-fdollars-in-identifiers` ¶

  Accept ‘`$`’ in identifiers.

`-fextended-identifiers` ¶

  Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.

`-fno-canonical-system-headers` ¶

  When preprocessing, do not shorten system header paths with canonicalization.

`-fmax-include-depth=depth` ¶

  Set the maximum depth of the nested #include. The default is 200.

`-fsearch-include-path[=kind]` ¶

  Look for input files on the #include path, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:

      g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc

  kind defaults to ‘`user`’, which looks on the `#include "…"` search path; you can also explicitly specify ‘`system`’ for the `#include <…>` search path.

`-ftabstop=width` ¶

  Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.

`-ftrack-macro-expansion[=level]` ¶

  Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value ‘`0`’ of level de-activates this option. Value ‘`1`’ tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value ‘`2`’ tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is ‘`2`’.

  Note that `-ftrack-macro-expansion=2` is activated by default.

`-fmacro-prefix-map=old=new` ¶

  When preprocessing files residing in directory `old`, expand the `__FILE__` and `__BASE_FILE__` macros as if the files resided in directory `new` instead. This can be used to change an absolute path to a relative path by using `.` for new which can result in more reproducible builds that are location independent. This option also affects `__builtin_FILE()` during compilation. See also `-ffile-prefix-map` and `-fcanon-prefix-map`.

`-fexec-charset=charset` ¶

  Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system’s `iconv` library routine.

`-fwide-exec-charset=charset` ¶

  Set the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of `wchar_t` and the big-endian or little-endian byte order being used for code generation. As with `-fexec-charset`, charset can be any encoding supported by the system’s `iconv` library routine; however, you will have problems with encodings that do not fit exactly in `wchar_t`.

`-finput-charset=charset` ¶

  Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. The default is UTF-8. charset can be any encoding supported by the system’s `iconv` library routine.

  If the input character set is UTF-8, warnings about ill-formed code unit sequences are issued if `-Winvalid-utf8` is enabled. Otherwise no diagnostics are issued when the input character set matches the execution character set. If they are different, ill-formed code unit sequences result in an error during transcoding to the execution character set.

`-fpch-deps` ¶

  When using precompiled headers (see Using Precompiled Headers), this flag causes the dependency-output flags to also list the files from the precompiled header’s dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used.

`-fpch-preprocess` ¶

  This option allows use of a precompiled header (see Using Precompiled Headers) together with `-E`. It inserts a special `#pragma`, `#pragma GCC pch_preprocess "filename"` in the output to mark the place where the precompiled header was found, and its filename. When `-fpreprocessed` is in use, GCC recognizes this `#pragma` and loads the PCH.

  This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by `-save-temps`.

  You should not write this `#pragma` in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC’s current directory.

`-fworking-directory` ¶

  Enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it’s present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form `-fno-working-directory`. If the `-P` flag is present in the command line, this option has no effect, since no `#line` directives are emitted whatsoever.

`-C` ¶
`--comments`

  Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

  You should be prepared for side effects when using `-C`; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a ‘`#`’.

`-CC` ¶
`--comments-in-macros`

  Do not discard comments, including during macro expansion. This is like `-C`, except that comments contained within macros are also passed through to the output file where the macro is expanded.

  In addition to the side effects of the `-C` option, the `-CC` option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

  The `-CC` option is generally used to support lint comments.

`-P` ¶
`--no-line-commands`

  Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.

`-traditional` ¶
`--traditional`
`-traditional-cpp`
`--traditional-cpp`

  Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.

  Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the `-E` switch, or when invoking CPP explicitly.

`-trigraphs` ¶
`--trigraphs`

  Support ISO C trigraphs. These are three-character sequences, all starting with ‘`??`’, that are defined by ISO C to stand for single characters. For example, ‘`??/`’ stands for ‘`\`’, so ‘`'??/n'`’ is a character constant for a newline.

  The nine trigraphs and their replacements are

      Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
      Replacement:      [    ]    {    }    #    \    ^    |    ~

  By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the `-std` and `-ansi` options.

`-remap` ¶

  Enable special code to work around file systems which only permit very short file names, such as MS-DOS.

`-H` ¶
`--trace-includes`

  Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the ‘`#include`’ stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ‘`...x`’ and a valid one with ‘`...!`’ .

`-dletters` ¶
`--dump=letters`
`--dump letters`

  Says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. See GCC Developer Options, for more information.

  `-dM` ¶
  `--dump=M`

    Instead of the normal output, generate a list of ‘`#define`’ directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file `foo.h`, the command

        touch foo.h; cpp -dM foo.h

    shows all the predefined macros.

    If you use `-dM` without the `-E` option, `-dM` is interpreted as a synonym for `-fdump-rtl-mach`. See (gcc)Developer Options.

  `-dD` ¶
  `--dump=D`

    Like `-dM` except that it outputs both the ‘`#define`’ directives and the result of preprocessing. Both kinds of output go to the standard output file.

  `-dN` ¶
  `--dump=N`

    Like `-dD`, but emit only the macro names, not their expansions.

  `-dI` ¶
  `--dump=I`

    Output ‘`#include`’ directives in addition to the result of preprocessing.

  `-dU` ¶
  `--dump=U`

    Like `-dD` except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and ‘`#undef`’ directives are also output for macros tested but undefined at the time.

`-fdebug-cpp` ¶

  This option is only useful for debugging GCC. When used from CPP or with `-E`, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

  When used from GCC without `-E`, this option has no effect.

`-Wp,option` ¶

  You can use `-Wp,option` to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and `-Wp` forcibly bypasses this phase. The preprocessor’s direct interface is undocumented and subject to change, so whenever possible you should avoid using `-Wp` and let the driver handle the options instead.

`-Xpreprocessor option` ¶

  Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize.

  If you want to pass an option that takes an argument, you must use `-Xpreprocessor` twice, once for the option and once for the argument.

`-no-integrated-cpp` ¶
`--no-integrated-cpp`

  Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (`cc1`, `cc1plus`, or `cc1obj` for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the `-B` or `-wrapper` options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.