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Environment Variables Affecting GNU CC
This section describes several environment variables that affect how
GNU CC operates. They work by specifying directories or prefixes to use
when searching for various kinds of files.
Note that you can also specify places to search using options such as
`-B', `-I' and `-L' (see Directory Options.). These take
precedence over places specified using environment variables, which in
turn take precedence over those specified by the configuration of GNU
CC. See Driver.
If `TMPDIR' is set, it specifies the directory to use for temporary
files. GNU CC uses temporary files to hold the output of one
stage of compilation which is to be used as input to the next
stage: for example, the output of the preprocessor, which is the
input to the compiler proper.
If `GCC_EXEC_PREFIX' is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GNU CC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of `GCC_EXEC_PREFIX' is `PREFIX/lib/gcc-lib/'
where PREFIX is the value of `prefix' when you ran the `configure'
Other prefixes specified with `-B' take precedence over this
This prefix is also used for finding files such as `crt0.o' that
are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with
`/usr/local/lib/gcc-lib' (more precisely, with the value of
`GCC_INCLUDE_DIR'), GNU CC tries replacing that beginning with the
specified prefix to produce an alternate directory name. Thus,
with `-Bfoo/', GNU CC will search `foo/bar' where it would
normally search `/usr/local/lib/bar'. These alternate directories
are searched first; the standard directories come next.
The value of `COMPILER_PATH' is a colon-separated list of
directories, much like `PATH'. GNU CC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using `GCC_EXEC_PREFIX'.
The value of `LIBRARY_PATH' is a colon-separated list of
directories, much like `PATH'. When configured as a native
compiler, GNU CC tries the directories thus specified when
searching for special linker files, if it can't find them using
`GCC_EXEC_PREFIX'. Linking using GNU CC also uses these
directories when searching for ordinary libraries for the `-l'
option (but directories specified with `-L' come first).
These environment variables pertain to particular languages. Each
variable's value is a colon-separated list of directories, much
like `PATH'. When GNU CC searches for header files, it tries the
directories listed in the variable for the language you are using,
after the directories specified with `-I' but before the standard
header file directories.
If this variable is set, its value specifies how to output
dependencies for Make based on the header files processed by the
compiler. This output looks much like the output from the `-M'
option (see Preprocessor Options.), but it goes to a separate
file, and is in addition to the usual results of compilation.
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.