File: autoconf.info,  Node: Site Defaults,  Prev: Transforming Names,  Up: Site Configuration

15.8 Setting Site Defaults
==========================

Autoconf-generated ‘configure’ scripts allow your site to provide
default values for some configuration values.  You do this by creating
site- and system-wide initialization files.

   If the environment variable ‘CONFIG_SITE’ is set, ‘configure’ uses
its value as a space-separated list of shell scripts to read; it is
recommended that these be absolute file names.  Otherwise, it reads the
shell script ‘PREFIX/share/config.site’ if it exists, then
‘PREFIX/etc/config.site’ if it exists.  Thus, settings in
machine-specific files override those in machine-independent ones in
case of conflict.

   Site files can be arbitrary shell scripts, but only certain kinds of
code are really appropriate to be in them.  Because ‘configure’ reads
any cache file after it has read any site files, a site file can define
a default cache file to be shared between all Autoconf-generated
‘configure’ scripts run on that system (*note Cache Files::).  If you
set a default cache file in a site file, it is a good idea to also set
the output variable ‘CC’ in that site file, because the cache file is
only valid for a particular compiler, but many systems have several
available.

   You can examine or override the value set by a command line option to
‘configure’ in a site file; options set shell variables that have the
same names as the options, with any dashes turned into underscores.  The
exceptions are that ‘--without-’ and ‘--disable-’ options are like
giving the corresponding ‘--with-’ or ‘--enable-’ option and the value
‘no’.  Thus, ‘--cache-file=localcache’ sets the variable ‘cache_file’ to
the value ‘localcache’; ‘--enable-warnings=no’ or ‘--disable-warnings’
sets the variable ‘enable_warnings’ to the value ‘no’; ‘--prefix=/usr’
sets the variable ‘prefix’ to the value ‘/usr’; etc.

   Site files are also good places to set default values for other
output variables, such as ‘CFLAGS’, if you need to give them non-default
values: anything you would normally do, repetitively, on the command
line.  If you use non-default values for PREFIX or EXEC_PREFIX (wherever
you locate the site file), you can set them in the site file if you
specify it with the ‘CONFIG_SITE’ environment variable.

   You can set some cache values in the site file itself.  Doing this is
useful if you are cross-compiling, where it is impossible to check
features that require running a test program.  You could "prime the
cache" by setting those values correctly for that system in
‘PREFIX/etc/config.site’.  To find out the names of the cache variables
you need to set, see the documentation of the respective Autoconf macro.
If the variables or their semantics are undocumented, you may need to
look for shell variables with ‘_cv_’ in their names in the affected
‘configure’ scripts, or in the Autoconf M4 source code for those macros;
but in that case, their name or semantics may change in a future
Autoconf version.

   The cache file is careful to not override any variables set in the
site files.  Similarly, you should not override command-line options in
the site files.  Your code should check that variables such as ‘prefix’
and ‘cache_file’ have their default values (as set near the top of
‘configure’) before changing them.

   Here is a sample file ‘/usr/share/local/gnu/share/config.site’.  The
command ‘configure --prefix=/usr/share/local/gnu’ would read this file
(if ‘CONFIG_SITE’ is not set to a different file).

     # /usr/share/local/gnu/share/config.site for configure
     #
     # Change some defaults.
     test "$prefix" = NONE && prefix=/usr/share/local/gnu
     test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu
     test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
     test "$localstatedir" = '${prefix}/var' && localstatedir=/var
     test "$runstatedir" = '${localstatedir}/run' && runstatedir=/run

     # Give Autoconf 2.x generated configure scripts a shared default
     # cache file for feature test results, architecture-specific.
     if test "$cache_file" = /dev/null; then
       cache_file="$prefix/var/config.cache"
       # A cache file is only valid for one C compiler.
       CC=gcc
     fi

   Another use of ‘config.site’ is for priming the directory variables
in a manner consistent with the Filesystem Hierarchy Standard (FHS).
Once the following file is installed at ‘/usr/share/config.site’, a user
can execute simply ‘./configure --prefix=/usr’ to get all the
directories chosen in the locations recommended by FHS.

     # /usr/share/config.site for FHS defaults when installing below /usr,
     # and the respective settings were not changed on the command line.
     if test "$prefix" = /usr; then
       test "$sysconfdir" = '${prefix}/etc' && sysconfdir=/etc
       test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
       test "$localstatedir" = '${prefix}/var' && localstatedir=/var
     fi

   Likewise, on platforms where 64-bit libraries are built by default,
then installed in ‘/usr/local/lib64’ instead of ‘/usr/local/lib’, it is
appropriate to install ‘/usr/local/share/config.site’:

     # /usr/local/share/config.site for platforms that prefer
     # the directory /usr/local/lib64 over /usr/local/lib.
     test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'