This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
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 GCC. See Controlling the Compilation Driver gcc (GNU Compiler Collection (GCC) Internals).
These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC 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 GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.
If GCC 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/ where prefix is the prefix to
the installed compiler. In many cases prefix is the value
of prefix when you ran the configure script.
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
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar.
These alternate directories are searched first; the standard directories
come next. If a standard directory begins with the configured
prefix then the value of prefix is replaced by
GCC_EXEC_PREFIX when looking for header files.
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC 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,
GCC tries the directories thus specified when searching for special
linker files, if it can't find them using GCC_EXEC_PREFIX. Linking
using GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with
-L come first).
This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
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
.
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 Preprocessor Options),
with an optional -MT switch too.
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 Preprocessor Options.