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.