int setnetgrent(const char *netgroup);
int getnetgrent(char **host, char **user, char **domain);
int getnetgrent_r(char **host, char **user, char **domain, char *buf, int buflen);
int innetgr(const char *netgroup, const char *host, const char *user, const char *domain);
The setnetgrent() call defines the netgroup that will be searched by subsequent getnetgrent() calls. The getnetgrent() function retrieves the next netgroup entry, and returns pointers in host, user, domain. A NULL pointer means that the corresponding entry matches any string. The pointers are valid only as long as there is no call to other netgroup related functions. To avoid this problem you can use the GNU function getnetgrent_r() that stores the strings in the supplied buffer. To free all allocated buffers use endnetgrent().
In most cases you only want to check if the triplet (hostname,username,domainname) is a member of a netgroup. The function innetgr() can be used for this without calling the above three functions. Again, a NULL pointer is a wildcard and matches any string. The function is thread-safe.