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
gcc in GNU Compiler Collection (GCC) Internals.
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories
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.
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 are otherwise 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.
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
GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the documentation
of this option for more details.
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.
GCC_EXEC_PREFIX is not set, GCC attempts to figure out
an appropriate prefix to use based on the pathname it is 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
prefix is the prefix to
the installed compiler. In many cases
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 searches
foo/bar just before it searches the standard directory
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 cannot find the
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 cannot find them using
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:
Recognize JIS characters.
Recognize SJIS characters.
Recognize EUCJP characters.
LANG is not defined, or if it has some other value, then the
mbtowc as defined by the default locale to
recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the preprocessor.
Each variable’s value is a list of directories separated by a special
character, much like
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
effect as ‘
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
target as the target name.
In other words, this environment variable is equivalent to combining
(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.
If this variable is set, its value specifies a UNIX timestamp to be
used in replacement of the current date and time in the
__TIME__ macros, so that the embedded timestamps become
The value of
SOURCE_DATE_EPOCH must be a UNIX timestamp,
defined as the number of seconds (excluding leap seconds) since
01 Jan 1970 00:00:00 represented in ASCII; identical to the output of
date +%s’ on GNU/Linux and other systems that support the
%s extension in the
The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process.