Laurent Bercot wrote:
Autotools-based can be good or bad for cross-compiling. The biggest
issue is that lots of people write broken tests that need to run test
programs to get the results they want.
As long as there are differences between systems, build-time tests
will be necessary to check for what the system provides and what
needs working around.
This is the main problem with cross-compilation: build-time tests
only work in the native case. When cross-compiling, there is no way to
automatically figure out what the target flags are.
The best solution would be to have a centralized, worldwide database
of system-dependent flags and options that any cross-compilation process
could search and extract target information from. Until we have that,
the only clean way to cross-compile is to have the user provide target
sysdeps *by hand*.
As far as I am aware, autotools do not make that easy. This essential
part of the cross-compilation process is not integrated into the
autotools architecture, which is why I do not consider them a reliable
system.
Actually it is very easy:
You just need a file that contains the answers to the questions that
configure can't figure out itself. Then you either export all these
values before running configure, or you export CONFIG_SITE=/path/to/file
and configure will read the file.
The file contains lines like
ac_cv_epoll_works=yes
ac_cv_c_littleendian=yes
ac_cv_c_bigendian=no
ac_cv_type_pid_t=yes
ac_cv_type_ptrdiff_t=yes
ac_cv_type_struct_sockaddr_storage=yes
ac_cv_sizeof_char=1
ac_cv_sizeof_short=2
ac_cv_sizeof_int=4
ac_cv_sizeof_long=4
ac_cv_sizeof_long_long=8
ac_cv_sizeof_void_p=4
ac_cv_sizeof_pid_t=4
ac_cv_sizeof_ptrdiff_t=4
ac_cv_sizeof_size_t=4
ac_cv_sizeof_ssize_t=4
ac_cv_sizeof_socklen_t=4
configure will then not run the tests, but use these answers you
provided. As this worldwide database you mention is also created *by
hand*, just by someone else, I consider that essentially the same.
Depending on the target, it may be possible to run the configure scripts
on the target to find out the correct answers, although it may not be
feasible to run the whole compilation on the target.
Note that some of the entries above can and will be detected
automatically by configure just by calling the cross compiler without
running the actual binary.
That covers the tests that come with the autotools. It may be
problematic when people add other tests that don't follow these rules,
but that can't be blamed on autotools.
Ralf
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