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setregid — set real and effective group ID
Standard C Library (libc, -lc)
#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. If the real group ID is changed, the saved group ID is changed to the new value of the effective group ID. Unprivileged users may change the real group ID to the effective group ID and vice-versa; only the super-user 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 argument. The setregid() system call 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 system call did not work correctly, and its purpose is now better served by the use of the setegid(2) system call. When setting the real and effective group IDs to the same value, the standard setgid() system call is preferred.
The setregid() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
[EPERM] The current process is not the super-user and a change other than changing the effective group-id to the real group-id was specified.
The setregid() system call appeared in 4.2BSD.