The files fsusage.c and fsusage.h have been stolen from the GNU Coreutils package (version 5.0) and are the actual implementation of the fsusage function. The only changes that have been made to these files is that some code for SVR2 has been ripped out since it depended on some more code (SVR3 was released in 1986 and introduced shared libraries), and the functions have been prefixed with glibtop_private to avoid name clashes with user code.