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
calling process is privileged (more precisely: has the CAP_SETGID capa-
bility in its user namespace), 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
EINVAL The group ID specified in gid is not valid in this user name-
space.
EPERM The calling process is not privileged (does not have the
CAP_SETGID capability in its user namespace), and gid does not
match the real group ID or saved set-group-ID of the calling
process.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
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 varia-
tion across kernel versions.
C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process share the same
credentials. The NPTL threading implementation handles the POSIX re-
quirements by providing wrapper functions for the various system calls
that change process UIDs and GIDs. These wrapper functions (including
the one for setgid()) employ a signal-based technique to ensure that
when one thread changes credentials, all of the other threads in the
process also change their credentials. For details, see nptl(7).
SEE ALSO
getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7),
user_namespaces(7)
COLOPHON
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Linux 2019-03-06 SETGID(2)