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3.16: 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/machine/version/' where prefix is the value of prefix when you ran the `configure' script and machine and version are the configuration name and version number of GNU CC, respectively.

Other prefixes specified with `-B' take precedence over this prefix.

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. 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.