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CentOS 7.0 - man page for setgid (centos section 2)

SETGID(2)						     Linux Programmer's Manual							 SETGID(2)

NAME
setgid - set group identity
SYNOPSIS
#include <sys/types.h> #include <unistd.h> int setgid(gid_t gid);
DESCRIPTION
setgid() sets the effective group ID of the calling process. If the caller is the superuser, the real GID and saved set-group-ID are also set. Under Linux, setgid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a set-group-ID program that is not set-user-ID-root to drop all of its group privileges, do some un-privileged work, and then reengage the original effective group ID in a secure manner.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EPERM The calling process is not privileged (does not have the CAP_SETGID capability), and gid does not match the real group ID or saved set-group-ID of the calling process.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The original Linux setgid() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added setgid32() supporting 32-bit IDs. The glibc setgid() wrapper function transparently deals with the variation across kernel versions.
SEE ALSO
getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-11-22 SETGID(2)