Cannot call a C module from Fortran

[Cottonwood]

I want to call a C module from a Fortran program. Whatever I tried -
the linker could not find the C module. I know about the leading
underscore and switched even that off. I abstracted everything
possible. When I replace the C module by a Fortran subroutine linking
works. Here the Fortran subroutine that replaced the C module for
testing:

      subroutine qqcprint
      return
      end

But when I use this C module in the same library (replacing the Fortran
testmodule) linking didn't work any longer.

void qqcprint()
{
return;
}

I also tried it with this one:

extern void qqcprint()
{
return;
}

The result was always the same. With the Fortran subroutine linking
works, with one of the C modules I get the message

qqmodlib.a(qqprint.o):qqprint.for:(.text+0x42): undefined reference to
`qqcprint'

What am I doing wrong?