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  in its user namespace), and gid does not
              match the real group ID or saved  set-group-ID  of  the  calling

       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                             2019-03-06                         SETGID(2)
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