#include <unistd.h>

int getgroups(int gidsetsize, gid_t grouplist[]);

/* Returns: number of supplementary group IDs if OK, −1 on error */

#include <grp.h> /* on Linux */
#include <unistd.h> /* on FreeBSD, Mac OS X, and Solaris */

int setgroups(int ngroups, const gid_t grouplist[]);

#include <grp.h> /* on Linux and Solaris */
#include <unistd.h> /* on FreeBSD and Mac OS X */

int initgroups(const char *username, gid_t basegid);

/* Both return: 0 if OK, −1 on error */