TOMS515 is a FORTRAN77 library which lists the subsets of size K selected from a set of size N, by Bill Buckles, Matthew Lybanon.
TOMS515 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version and a Python version.
SUBSET, a FORTRAN77 library which enumerates combinations, partitions, subsets, index sets, and other combinatorial objects.
Original FORTRAN77 version by Bill Buckles, Matthew Lybanon. This FORTRAN77 version by John Burkardt.
You can go up one level to the FORTRAN77 source codes.