Thank you primo.
My distro (FC2) has 3 versions of md5.c, all different. Only the smaller one goes through gcc -S without error but when displaying the C source, it looks as though it might be just the cypher itself (no input or output passed). It also has 2 versions of sha1.c, both give errors.
Also, the assembly code shown is AS and I use NASM, so it looks like I am probably better off linking a compiled C version of it.
Would you have any suggestion on how to pick a suitable source of the program (md5 or other)?