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

       setgid - set group identity

       #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
       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.

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       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

       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 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).

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

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Linux                              2021-03-22                          SETGID(2)