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.
