File: autoconf.info,  Node: Hosts and Cross-Compilation,  Next: AC_LIBOBJ vs LIBOBJS,  Prev: New Macros,  Up: Autoconf 2.13

18.6.3 Hosts and Cross-Compilation
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Based on the experience of compiler writers, and after long public
debates, many aspects of the cross-compilation chain have changed:

   − the relationship between the build, host, and target architecture
     types,

   − the command line interface for specifying them to ‘configure’,

   − the variables defined in ‘configure’,

   − the enabling of cross-compilation mode.


   The relationship between build, host, and target have been cleaned
up: the chain of default is now simply: target defaults to host, host to
build, and build to the result of ‘config.guess’.  Nevertheless, in
order to ease the transition from 2.13 to 2.50, the following transition
scheme is implemented.  _Do not rely on it_, as it will be completely
disabled in a couple of releases (we cannot keep it, as it proves to
cause more problems than it cures).

   They all default to the result of running ‘config.guess’, unless you
specify either ‘--build’ or ‘--host’.  In this case, the default becomes
the system type you specified.  If you specify both, and they're
different, ‘configure’ enters cross compilation mode, so it doesn't run
any tests that require execution.

   Hint: if you mean to override the result of ‘config.guess’, prefer
‘--build’ over ‘--host’.


   For backward compatibility, ‘configure’ accepts a system type as an
option by itself.  Such an option overrides the defaults for build,
host, and target system types.  The following configure statement
configures a cross toolchain that runs on NetBSD/alpha but generates
code for GNU Hurd/sparc, which is also the build platform.

     ./configure --host=alpha-netbsd sparc-gnu


   In Autoconf 2.13 and before, the variables ‘build’, ‘host’, and
‘target’ had a different semantics before and after the invocation of
‘AC_CANONICAL_BUILD’ etc.  Now, the argument of ‘--build’ is strictly
copied into ‘build_alias’, and is left empty otherwise.  After the
‘AC_CANONICAL_BUILD’, ‘build’ is set to the canonicalized build type.
To ease the transition, before, its contents is the same as that of
‘build_alias’.  Do _not_ rely on this broken feature.

   For consistency with the backward compatibility scheme exposed above,
when ‘--host’ is specified but ‘--build’ isn't, the build system is
assumed to be the same as ‘--host’, and ‘build_alias’ is set to that
value.  Eventually, this historically incorrect behavior will go away.


   The former scheme to enable cross-compilation proved to cause more
harm than good, in particular, it used to be triggered too easily,
leaving regular end users puzzled in front of cryptic error messages.
‘configure’ could even enter cross-compilation mode only because the
compiler was not functional.  This is mainly because ‘configure’ used to
try to detect cross-compilation, instead of waiting for an explicit flag
from the user.

   Now, ‘configure’ enters cross-compilation mode if and only if
‘--host’ is passed.

   That's the short documentation.  To ease the transition between 2.13
and its successors, a more complicated scheme is implemented.  _Do not
rely on the following_, as it will be removed in the near future.

   If you specify ‘--host’, but not ‘--build’, when ‘configure’ performs
the first compiler test it tries to run an executable produced by the
compiler.  If the execution fails, it enters cross-compilation mode.
This is fragile.  Moreover, by the time the compiler test is performed,
it may be too late to modify the build-system type: other tests may have
already been performed.  Therefore, whenever you specify ‘--host’, be
sure to specify ‘--build’ too.

     ./configure --build=x86_64-pc-linux-gnu --host=x86_64-w64-mingw64

enters cross-compilation mode.  The former interface, which consisted in
setting the compiler to a cross-compiler without informing ‘configure’
is obsolete.  For instance, ‘configure’ fails if it can't run the code
generated by the specified compiler if you configure as follows:

     ./configure CC=x86_64-w64-mingw64-gcc