6.1.20 Argument list functions %VAL, %REF and %LOC ¶

GNU Fortran supports argument list functions %VAL, %REF and %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 reference and %LOC passes its memory location. Since gfortran already passes scalar arguments by reference, %REF is in effect a do-nothing. %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 https://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top.

Also, c_by_val.f and its partner c_by_val.c of the GNU Fortran testsuite are worth a look.