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 (see Invocation). 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’.
See also Search Path.
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 (see Invocation), 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.
See Invocation.
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
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.