setregid() - set real and effective group ID
#include <unistd.h>
int setregid(gid_t rgid, gid_t egid)
The real and effective group ID's of the current process are set to the arguments. Unprivileged users may change the real group ID to the effective group ID and vice-versa; only the superuser may make other changes.
Supplying a value of -1 for either the real or effective group ID forces the system to substitute the current ID in place of the -1 parameter.
The setregid(2) function was intended to allow swapping the real and effective group IDs in set-group-ID programs to temporarily relinquish the set-group-ID value. This function did not work correctly, and its purpose is now better served by the use of the setegid(2) function (see setuid(2)).
When setting the real and effective group IDs to the same value, the standard setgid(2) function is preferred.
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.
The setregid(2) call can return the following errors:
getgid(2)
setegid(2)
setgid(2)
setuid(2)