GNU cflow is a flow graph generator that is part of the GNU Project. It reads a collection of C source files and generate a C flow graph of external references. It uses only sources and doesn't need to run the program.
It was initially an implementation of the cflow UNIX utility.
cflow is a unix utility generating a C-language flowgraph.
Besides GNU, there are other implementations of cflow, like the one for Tru64 Unix.