[Bug fortran/49138] New: Add /usr/include/fortran/{,gcc-<version>} to the file/module search path

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49138

           Summary: Add /usr/include/fortran/{,gcc-<version>} to the
                    file/module search path
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


There is a serious lack of a default-searched location for Fortran include
files - and in particular for Fortran .mod files. The latter is particularly
important as different compilers/different versions have different .mod files.

Currently, Linux distributions usually either do not ship .mod files or they
ship them simply in /usr/include.


The /usr/{local/,}include/fortran/ directory should be used (and searched) for
Fortran INCLUDE / #include files.

Module files (or arch-dependent include files) should be searched for in an
compiler/architecture-dependent subdirectory, which can be queried with
-print-mod-dir-suffix:
   gfortran $(FFLAGS) -print-mod-dir-suffix
(Note: -m... flags in the $(FFLAGS) might change the result.)

Thus, the search order is:
 LOCAL_INCLUDE_DIR/fortran/$(gfortran -print-mod-dir-suffix) # .mod files
 LOCAL_INCLUDE_DIR/fortran  # include files
 LOCAL_INCLUDE_DIR          # For backward compatibility
followed by the same order for SYSTEM_INCLUDE_DIR and then STANDARD_INCLUDE_DIR

The result of -print-mod-dir-suffix should be
  gcc-<module_version>/lib{,<qual>}

The reason for "lib{,<qual>}" is to allow for architecture-defined directories;
using directly the target (e.g. x86_64-unknown-linux-gnu) instead of the lib
won't work as for x86_64 one can have -m32 and -m64 but target remains the
same, while for i686 and x86_64 -m32, the target changes but the produced
binaries are the same. Additionally, for PowerPC, the default can be 32 or 64
bit.

For the FHS standard tacking, see:
  http://bugs.linux-foundation.org/show_bug.cgi?id=757

(The proposal was discussed with Jakub, Richard and Matthias.)