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13.2 Getting the Canonical System Type
The following macros make the system type available to `configure'
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. Note: 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
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 Note: Hosts and Cross-Compilation, for more.
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