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

NAME
       setgid - set group identity

SYNOPSIS
       #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 to indicate the error.

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 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)

Linux                                                                          2021-03-22                                                                      SETGID(2)