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13.2 Getting the Canonical System Type
The following macros make the system type available to configure
scripts.
The variables ‘build_alias’, ‘host_alias’, and
‘target_alias’ are always exactly the arguments of --build,
--host, and --target; in particular, they are left empty
if the user did not use them, even if the corresponding
AC_CANONICAL macro was run. Any configure script may use these
variables anywhere. These are the variables that should be used when in
interaction with the user.
If you need to recognize some special environments based on their system
type, run the following macros to get canonical system names. These
variables are not set before the macro call.
If you use these macros, you must distribute config.guess and
config.sub along with your source code. See Output, for
information about the AC_CONFIG_AUX_DIR macro which you can use
to control in which directory configure looks for those scripts.
— Macro: AC_CANONICAL_BUILD
Compute the canonical build-system type variable, build, and its
three individual parts build_cpu, build_vendor, and
build_os.
If --build was specified, then build is the
canonicalization of build_alias by config.sub,
otherwise it is determined by the shell script config.guess.
— Macro: AC_CANONICAL_HOST
Compute the canonical host-system type variable, host, and its
three individual parts host_cpu, host_vendor, and
host_os.
If --host was specified, then host is the
canonicalization of host_alias by config.sub,
otherwise it defaults to build.
— Macro: AC_CANONICAL_TARGET
Compute the canonical target-system type variable, target, and its
three individual parts target_cpu, target_vendor, and
target_os.
If --target was specified, then target is the
canonicalization of target_alias by config.sub,
otherwise it defaults to host.
Note that there can be artifacts due to the backward compatibility
code. See See Hosts and Cross-Compilation, for more.
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