SETGID(2)                  Linux Programmer's Manual                 SETGID(2)

       setgid - set group identity

       #include <sys/types.h>
       #include <unistd.h>

       int setgid(gid_t gid);

       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.

       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

       EINVAL The  group  ID  specified in gid is not valid in this user name-

       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.

       POSIX.1-2001, POSIX.1-2008, SVr4.

       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).

       getgid(2), setegid(2),  setregid(2),  capabilities(7),  credentials(7),

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Linux                             2017-09-15                         SETGID(2)

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