CPP(1)

✨ CPP(1) — The C Preprocessor ✨

GNU manual page, bestie. Part one. Let's get into it. 💅

💖 NAME 💖

cpp - The C Preprocessor

🌈 SYNOPSIS 🌈

cpp [-Dmacro[=defn]...] [-Umacro]
    [-Idir...] [-iquotedir...]
    [-M|-MM] [-MG] [-MF filename]
    [-MP] [-MQ target...]
    [-MT target...]
    infile [[-o] outfile]

Okay so only the most useful options are given above; see below for a more complete list of preprocessor-specific options. Also, cpp accepts most gcc driver options, which are not listed here. Refer to the GCC documentation for details. ✨

🦋 DESCRIPTION 🦋

The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C compiler to transform your program before compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs. iconic behavior, honestly.

The C preprocessor is intended to be used only with C, C++, and Objective-C source code. In the past, it has been abused as a general text processor. It will choke on input which does not obey C's lexical rules. For example, apostrophes will be interpreted as the beginning of character constants, and cause errors. Also, you cannot rely on it preserving characteristics of the input which are not significant to C-family languages. If a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile will not work. not the Makefile, bestie 😭

Having said that, you can often get away with using cpp on things which are not C. Other Algol-ish programming languages are often safe (Ada, etc.) So is assembly, with caution. -traditional-cpp mode preserves more white space, and is otherwise more permissive. Many of the problems can be avoided by writing C or C++ style comments instead of native language comments, and keeping macros simple.

Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern versions of the GNU assembler have macro facilities. Most high level programming languages have their own conditional compilation and inclusion mechanism. If all else fails, try a true general text processor, such as GNU M4.

C preprocessors vary in some details. This manual discusses the GNU C preprocessor, which provides a small superset of the features of ISO Standard C. In its default mode, the GNU C preprocessor does not do a few things required by the standard. These are features which are rarely, if ever, used, and may cause surprising changes to the meaning of a program which does not expect them. To get strict ISO Standard C, you should use the -std=c90, -std=c99, -std=c11 or -std=c17 options, depending on which version of the standard you want. To get all the mandatory diagnostics, you must also use -pedantic. periodt.

This manual describes the behavior of the ISO preprocessor. To minimize gratuitous differences, where the ISO preprocessor's behavior does not conflict with traditional semantics, the traditional preprocessor should behave the same way. The various differences that do exist are detailed in the section Traditional Mode.

For clarity, unless noted otherwise, references to CPP in this manual refer to GNU CPP.

👑 OPTIONS 👑

The cpp command expects two file names as arguments, infile and outfile. The preprocessor reads infile together with any other files it specifies with #include. All the output generated by the combined input files is written in outfile.

Either infile or outfile may be -, which as infile means to read from standard input and as outfile means to write to standard output. If either file is omitted, it means the same as if - had been specified for that file. You can also use the -o outfile option to specify the output file.

Unless otherwise noted, or the option ends in =, all options which take an argument may have that argument appear either immediately after the option, or with a space between option and argument: -Ifoo and -I foo have the same effect.

Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dM is very different from -d -M.

Now the options, one by one, no skips:

• -D name — Predefine name as a macro, with definition 1.
• -D 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 — Cancel any previous definition of name, either built in or provided with a -D option.
• -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 — 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 — 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. 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 — 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 — 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 — -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, 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 — literally exactly like -MD bestie, except it only names the user header files. System header files? Not invited to the party. 💅
• -fpreprocessed — tells the preprocessor "girl, this file is already preprocessed, we're good." 🫶 What does that shut off? Macro expansion, trigraph conversion, escaped newline splicing, and most directive processing. But it still recognizes and removes comments, so you can hand it a file that was preprocessed with -C and the compiler won't freak out. In this mode the integrated preprocessor is lowkey just a tokenizer for the front ends. ✨

-fpreprocessed is implicit if the input file ends in .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files made by -save-temps. iykyk 🎀

• -fdirectives-only — when preprocessing, it handles directives but does not expand macros.

Its whole vibe depends on the -E and -fpreprocessed options. Let's break it down: 👑

• With -E, preprocessing is limited to handling directives like "#define", "#ifdef", and "#error". Other preprocessor operations — macro expansion, trigraph conversion — are not performed. Bonus: the -dD option gets implicitly enabled.
• With -fpreprocessed, predefinition of command-line and most builtin macros is disabled. Macros like "__LINE__" that 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. That enables full preprocessing of files previously preprocessed with "-E -fdirectives-only". 🌈

• -fdollars-in-identifiers — accept $ in identifiers. That's it, that's the tweet. 💸
• -fextended-identifiers — accept universal character names and extended characters in identifiers. 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 — sets the max depth of the nested #include. The default is 200. Don't go too deep, bestie 🌊
• -ftabstop=width — sets the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even when tabs show up 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] — tracks locations of tokens across macro expansions. This lets the compiler emit diagnostics about the current macro expansion stack when a compilation error happens inside a macro expansion. Heads up: using this makes the preprocessor and compiler eat more memory. 😭

The level parameter picks how precise the token location tracking is, so you can dial down the memory consumption if you need to:

• 0 — de-activates this option.
• 1 — tracks token locations in a degraded mode for minimal memory overhead. In this mode, all tokens resulting from the expansion of an argument of a function-like macro have the same location.
• 2 — tracks token locations completely. This one is the most memory hungry. 🍓

When this option is given with 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 that live in directory old, expand the "__FILE__" and "__BASE_FILE__" macros as if the files lived in directory new instead. You can use this to turn an absolute path into a relative one by setting new to ., which can give you more reproducible builds that are location independent. This option also affects "__builtin_FILE()" during compilation. See also -ffile-prefix-map. 🗺️
• -fexec-charset=charset — sets 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 — sets 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. Just like 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 — sets the input character set, used to translate from the character set of the input file to the source character set GCC uses. If the locale doesn't specify it, or GCC can't get that info from the locale, the default is UTF-8. Either the locale or this command-line option can override it. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine. 🌍
• -fworking-directory — enables generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this is on, 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 you can inhibit it with the negated form -fno-working-directory. And if the -P flag is present on the command line, this option has no effect, since no "#line" directives get emitted whatsoever. 🚫

• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred over the older form -A predicate(answer), which is still supported, because it doesn't use shell special characters. ✨
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer. ❌
• -C — do not discard comments. All comments get 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 makes the preprocessor 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 — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros also get passed through to the output file where the macro is expanded. 🎀

On top of all the side effects of -C, the -CC option makes all C++-style comments inside a macro get converted to C-style comments. Why? To prevent later use of that macro from inadvertently commenting out the rest of the source line. 🙈

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

• -P — inhibits generation of linemarkers in the preprocessor output. This might be useful when you're running the preprocessor on something that isn't C code and it'll be sent to a program that might get confused by the linemarkers. 🌸
• -traditional, -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors.

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 — support ISO C trigraphs. These are three-character sequences, all starting with ??, that ISO C defines to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. 🤯

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

• -remap — enables special code to work around file systems that only permit very short file names, such as MS-DOS. 🕰️
• -H — prints the name of each header file used, on top of all the other normal activities. Each name is indented to show how deep it is in the #include stack. Precompiled header files get printed too, even if they turn out to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . 👑
• -dletters — says to make debugging dumps during compilation as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper, or reserved for future versions of GCC, so they're silently ignored. And if you specify letters whose behavior conflicts? The result is undefined. 😭
  • -dM — 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 to find out what's 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. 🔍

• -dD — like -dM except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — like -dD, but it emits only the macro names, not their expansions. ✂️
• -dI — outputs #include directives in addition to the result of preprocessing.
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, get 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.

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• -I dir, -iquote dir, -isystem dir, -idirafter dir — okay bestie, these four are the ✨include path girlies✨. Each one adds the directory dir to the list of directories that get searched for header files while preprocessing.

If dir starts with = or $SYSROOT, then that = or $SYSROOT gets swapped out for the sysroot prefix — peep --sysroot and -isysroot for the deets. 💅

Directories you set with -iquote only apply to the quote form of the directive, #include "file". Directories you set with -I, -isystem, or -idirafter apply to lookup for both #include "file" and #include <file>.

You can stack any number or combination of these options on the command line to search for header files in several directories, no cap. The lookup order is, in order:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. But listen — you should not use this option to add directories that contain vendor-supplied system header files. Use -isystem for that, okay? 🙈

The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that is applied to the standard system directories. Fancy.

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. Why? To ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. So if you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. Periodt.

• -I- — Split the include path. 🫶 Heads up though: this option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". And there is no way to override this effect of -I-. 😭

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. Don't forget it, bestie.
• -iwithprefix dir, -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. Two sisters, two different vibes. 🎀
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers.
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. Strict queen behavior. 👑
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
• -Wcomment, -Wcomments — Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall.
• -Wtrigraphs — Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

• -Wundef — Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.
• -Wexpansion-to-defined — Warn whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — Warn about macros defined in the main file that are unused. A macro counts as used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

        #if defined the_macro_causing_the_warning
        #endif

• -Wno-endif-labels — Do not warn whenever an #else or an #endif are followed by text. This sometimes happens in older programs with code of the form

        #if FOO
        ...
        #else FOO
        ...
        #endif FOO

The second and third FOO should be in comments. This warning is on by default. 💀

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🌸 ENVIRONMENT 🌸

This section describes the environment variables that affect how CPP operates. You can use them to specify directories or prefixes to use when searching for include files, or to control dependency output.

Note that you can also specify places to search using options such as -I, and control dependency output with options like -M. These take precedence over environment variables, which in turn take precedence over the configuration of GCC. Hierarchy, bestie. 💅

CPATH

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. That special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon. ✨

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include. Iconic. 🦋

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too.

SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. Reproducible builds are so slay. 🔥

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command.

The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process.

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🌈 SEE ALSO 🌈

gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info entries for cpp and gcc.

📜 COPYRIGHT 📜

okay bestie, legal era 💅 this part is short but we're keeping every single word of it, periodt.

Copyright (c) 1987-2022 Free Software Foundation, Inc. 🫶

So here's the tea: permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation. ✨ A copy of the license is included in the man page gfdl(7). Slay.

This manual contains no Invariant Sections. Nope, zero, none, not a single one. 🎀

The Front-Cover Texts are (a) (see below), and the Back-Cover Texts are (b) (see below). Yes, they really do point down like that. Iconic behaviour honestly.

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🌸 (a) The FSF's Front-Cover Text is:

A GNU Manual

🦋 (b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

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lowkey obsessed with how a whole license paragraph can be this wholesome. You have freedom, bestie. Freedom to copy, freedom to modify, freedom to be a GNU main character. 💜

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and that's a wrap on the whole page, fr fr 😭💖 this version of the page is from gcc-12, dated 2024-07-11, and it's literally titled CPP(1). go forth and preprocess something gorgeous, you absolute icon 👑🌈