[sr #110345] AC_PATH_X and friends should not look in /usr/lib/<various>/include when cross compiling

[Zack Weinberg]

URL:
  <https://savannah.gnu.org/support/?110345>

                 Summary: AC_PATH_X and friends should not look in
/usr/lib/<various>/include when cross compiling
                 Project: Autoconf
            Submitted by: zackw
            Submitted on: Fri 30 Oct 2020 08:46:29 PM UTC
                Category: None
                Priority: 5 - Normal
                Severity: 3 - Normal
                  Status: Confirmed
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: None

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Details:

Reported by Ross Burton on the mailing list:

[Ross:]
AC_PATH_X and friends are unusual in autoconf in that they
*explicitly* look into /usr/ for files (see _AC_PATH_X_DIRECT).
Notable this is the *only* macro in autoconf that explicitly looks
into /usr.

To write the logic out in English: iterate through a long list of
paths in /usr for a X11/Xlib.h file, and if that exists try a compile
test.  Then iterate through the same set of paths after s/include/lib/
looking for anything with a library extension (.a .so .sl .dynlib .dll
.la) and if one is found try a link test.

The big failure case is in cross compilation situations using a
sysroot where the target is binary compatible with the host.  In this
case AC_PATH_X will happily look at /usr and say that yes, X is
available, even if the sysroot doesn't have X.

My preferred solution would be to delete AC_PATH_X and friends and get
everyone to use PKG_CHECK_MODULES(x11), but I can see that isn't a
realistic solution.  Because the failure case above is a real problem
for us, we just delete all of that code.  Does anyone have a better
solution that would continue to work but not be so dramatically
dependent on host assumptions?

[Zack:]
The thing that comes immediately to mind, and which would be feasible
for 2.70, is to not run _AC_PATH_X_DIRECT when cross-compiling.  I
don't think it's unreasonable to require people who want to
cross-compile X11 programs to either have the headers and libraries on
the cross-compiler's default search path, or use the --x-includes and
--x-libraries command line switches.  It might also make sense for
_AC_PATH_X_XMKMF to look for ${ac_tool_prefix}-xmkmf rather than plain
xmkmf, except I don't know if that would break existing cross compile
build farms.




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