The functions and data structures for accessing the system group database are declared in the header file grp.h.
The group
structure is used to hold information about an entry in
the system group database. It has at least the following members:
char *gr_name
The name of the group.
gid_t gr_gid
The group ID of the group.
char **gr_mem
A vector of pointers to the names of users in the group. Each user name is a null-terminated string, and the vector itself is terminated by a null pointer.