Description

   The getgrent function returns the next group in the sequential
   search, returning a pointer to a structure containing the broken-
   out fields of an entry in the group database.

   When first called, getgrent returns a pointer to a group
   structure containing the first entry in the group database.
   Thereafter, it returns a pointer to the next group structure
   in the group database, so successive calls can be used to search
   the entire database.

   If an end-of-file or an error is encountered on reading, getgrent
   returns a NULL pointer and sets errno.