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%VAL, %REF and %LOCGNU 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.