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 names-
              pace.

       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
       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
       requirements 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
       This page is part of release 4.16 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.



Linux                             2017-09-15                         SETGID(2)