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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 (has the  CAP_SETGID  capability  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 4.15 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)