The functions and data structures for accessing the system group database are declared in the header file grp.h.
group structure is used to hold information about an entry in
the system group database. It has at least the following members:
The name of the group.
The group ID of the group.
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.