SETGID(2)                 (2019-03-06)                  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 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
               namespace.

          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 variation across kernel ver-
          sions.

        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

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     SETGID(2)                 (2019-03-06)                  SETGID(2)

          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 5.10 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/.

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