#include <unistd.h>int setregid(gid_t rgid, gid_t egid);
The setregid() function is used to set the real and effective group IDs of the calling process. If rgid is -1, the real group ID is not changed; if egid is -1, the effective group ID is not changed. The real and effective group IDs may be set to different values in the same call.
If the effective user ID of the calling process is super-user, the real group ID and the effective group ID can be set to any legal value.
If the effective user ID of the calling process is not super-user, either the real group ID can be set to the saved set-group-ID from execve(2), or the effective group ID can either be set to the saved set-group-ID or the real group ID.
In either case, if the real group ID is being changed (that is, if rgid is not -1), or the effective group ID is being changed to a value not equal to the real group ID, the saved set-group-ID is set equal to the new effective group ID.
Upon successful completion, 0 is returned. Otherwise, -1 is returned, errno is set to indicate the error, and neither of the group IDs will be changed.
The setregid() function will fail if:
The value of rgid or egid is less than 0 or greater than UID_MAX (defined in <limits.h>).
The calling process's effective UID is not the super-user and a change other than changing the real group ID to the saved set-group-ID or changing the effective group ID to the real group ID or the saved group ID, was specified.
If a set-group-ID process sets its effective group ID to its real group ID, it can still set its effective group ID back to the saved set-group-ID.