(autoconf.info.gz) Hosts and Cross-Compilation

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 Hosts and Cross-Compilation
 ---------------------------
 
    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' will enter cross compilation mode, so it
 won't run any tests that require execution.
 
    Hint: if you mean to override the result of `config.guess', prefer
 `--build' over `--host'.  In the future, `--host' will not override the
 name of the build system type.  Whenever you specify `--host', be sure
 to specify `--build' too.
 
 
    For backward compatibility, `configure' will accept a system type as
 an option by itself.  Such an option will override the defaults for
 build, host, and target system types.  The following configure
 statement will configure a cross toolchain that will run on NetBSD/alpha
 but generate 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 will
 be assumed to be the same as `--host', and `build_alias' will be 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 will try to run an executable
 produced by the compiler.  If the execution fails, it will enter
 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=i686-pc-linux-gnu --host=m68k-coff
 
 will enter cross-compilation mode.  The former interface, which
 consisted in setting the compiler to a cross-compiler without informing
 `configure' is obsolete.  For instance, `configure' will fail if it
 can't run the code generated by the specified compiler if you configure
 as follows:
 
      ./configure CC=m68k-coff-gcc
 

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(autoconf.info.gz) AC_LIBOBJ vs LIBOBJS