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3.19 Environment Variables Affecting 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 `-B', `-I' and `-L' (see section 3.14 Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See section 21.1 Controlling the Compilation Driver, `gcc'.

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_UK' for English in the United Kingdom.

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, GNU CC 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-lib/' where 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-lib' (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.

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).

These environment variables pertain to particular languages. Each variable's value is a colon-separated list of directories, much like PATH. When GCC 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 section 3.11 Options Controlling the Preprocessor), 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.

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.

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.

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