Node: Environment Variables, Next: Precompiled Headers, Previous: Code Gen Options, Up: Invoking GCC
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
-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
LC_MESSAGESif 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_UKfor English in the United Kingdom.
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.
LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
LC_ALL environment variable is set, it overrides the value
LC_MESSAGES default to the value of the
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
TMPDIRis 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.
GCC_EXEC_PREFIXis 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.
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
/lib/gcc/ where prefix is the value
prefix when you ran the
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
(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
These alternate directories are searched first; the standard directories
COMPILER_PATHis 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
LIBRARY_PATHis 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
-loption (but directories specified with
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, 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
-I, but after any paths given with
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
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
, 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
(see Preprocessor Options),
with an optional
-MT switch too.
DEPENDENCIES_OUTPUT(see above), except that system header files are not ignored, so it implies
-MM. However, the dependence on the main input file is omitted. See Preprocessor Options.