GNU Fortran supports argument list functions
%LOC statements, for backward compatibility with g77.
It is recommended that these should be used only for code that is
accessing facilities outside of GNU Fortran, such as operating system
or windowing facilities. It is best to constrain such uses to isolated
portions of a program–portions that deal specifically and exclusively
with low-level, system-dependent facilities. Such portions might well
provide a portable interface for use by the program as a whole, but are
themselves not portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
%VAL passes a scalar argument by value,
%REF passes it by
%LOC passes its memory location. Since gfortran
already passes scalar arguments by reference,
%REF is in effect
%LOC has the same effect as a fortran pointer.
An example of passing an argument by value to a C subroutine foo.:
C C prototype void foo_ (float x); C external foo real*4 x x = 3.14159 call foo (%VAL (x)) end
For details refer to the g77 manual http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top.
Also, the gfortran testsuite c_by_val.f and its partner c_by_val.c are worth a look.