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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 privileged (has the
       CAP_SETGID capability), 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 namespace.

       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
       variation 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
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