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.
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
:/special/include, that has the same
effect as ‘-I. -I/special/include’.
See also Search Path.
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.