# cpp

## NAME



           cpp - The C Preprocessor
    


## SYNOPSIS



           cpp [**-D** _macro_[=_defn_]...] [**-U** _macro_]
               [**-I** _dir_...] [**-iquote** _dir_...]
               [**-M** |**-MM**] [**-MG**] [**-MF** _filename_]
               [**-MP**] [**-MQ** _target_...]
               [**-MT** _target_...]
               _infile_ [[**-o**] _outfile_]
    
           Only the most useful options are given above; see below for a more
           complete list of preprocessor-specific options.  In addition, **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.
    
           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.
    
           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**.
    
           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**.
    
           **-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.
    
           **-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**
               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 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.
    
           **-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**.
    
           **-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 UTF-32 or UTF-16,
               whichever corresponds to the width of "wchar_t".  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.  If the locale does not specify, or GCC cannot
               get this information from the locale, the default is UTF-8.
               This can be overridden by either the locale or this command-
               line option.  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**
               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.
    
           **-A** _predicate_**=**_answer_
               Make an assertion with the predicate _predicate_ and answer
               _answer_.  This form is preferred to the older form **-A**
               _predicate_**(**_answer_**)** , which is still supported, because it does
               not use shell special characters.
    
           **-A -**_predicate_**=**_answer_
               Cancel an assertion with the predicate _predicate_ and answer
               _answer_.
    
           **-C** 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** 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** 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-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 are defined by ISO C 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**
               Enable special code to work around file systems which only
               permit very short file names, such as MS-DOS.
    
           **-H** 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 **...!**.
    
           **-d** _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.
    
               **-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 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.
    
               **-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 emit only the macro names, not their
                   expansions.
    
               **-dI** Output **#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,
                   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.
    
           **-I** _dir_
           **-iquote** _dir_
           **-isystem** _dir_
           **-idirafter** _dir_
               Add the directory _dir_ to the list of directories to be
               searched for header files during preprocessing.
    
               If _dir_ begins with **=** or $SYSROOT, then the **=** or $SYSROOT is
               replaced by the sysroot prefix; see **--sysroot** and **-isysroot**.
    
               Directories specified with **-iquote** apply only to the quote
               form of the directive, "#include "_file""_.  Directories
               specified with **-I** , **-isystem** , or **-idirafter** apply to lookup for
               both the "#include "_file""_ and "#include <_file >"_ directives.
    
               You can specify any number or combination of these options on
               the command line to search for header files in several
               directories.  The lookup order is as follows:
    
               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.  However, you
               should not use this option to add directories that contain
               vendor-supplied system header files; use **-isystem** for that.
    
               The **-isystem** and **-idirafter** options also mark the directory as
               a system directory, so that it gets the same special treatment
               that is applied to the standard system directories.
    
               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.  This is to ensure that GCC's procedure to fix buggy
               system headers and the ordering for the "#include_next"
               directive are not inadvertently changed.  If you really need
               to change the search order for system directories, use the
               **-nostdinc** and/or **-isystem** options.
    
           **-I-** Split the include path.  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""_.  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 **/**.
    
           **-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.
    
           **-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.
    
           **-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 is _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.
    


## 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.
    
           **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.  The 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**.
    
           **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.
    
               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 **@command{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.
    


* * *