Re: When crosscompiling, does configure really need --build?

[Akim Demaille]

| Also, I *do* give the --host flag (I must have been unclear), yet I get
| the warning:
|   "configure: WARNING: If you wanted to set the --build type, don't use
|   --host. If a cross compiler is detected then cross compile mode will be 
used."
| 
| Why?

I must have been unclear myself: this warning is left at the intention
of the people who were using the old interface.  That's why I was
referring you to the documentation.

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