How can I call a subroutine of C from the program of Fortran ?

In the case of SX cross compiler, you can compile in the following procedure.

% sxc++ -S a.c
% sxf90 b.f90 a.s

In the case of intel compiler, you can compile in the following procedure.

% icc -c a.c
% ifort b.f90 a.o