The following discussion assumes that you are running g77 in f2c
compatibility mode, i.e. not using -fno-f2c.
It provides some
advice about quick and simple techniques for linking Fortran and C (or
C++), the most common requirement.
For the full story consult the
description of code generation.
See Debugging and Interfacing.
When linking Fortran and C, it's usually best to use g77 to do
the linking so that the correct libraries are included (including the
maths one).
If you're linking with C++ you will want to add
-lstdc++, -lg++ or whatever.
If you need to use another
driver program (or ld directly),
you can find out what linkage
options g77 passes by running g77 -v.