#include <unistd.h> /* glibc uses <sys/fsuid.h> */

       int setfsgid(uid_t fsgid);

       The system call setfsgid() sets the group ID that the Linux kernel uses
       to check for all accesses to the file system.  Normally, the  value  of
       fsgid  will shadow the value of the effective group ID.  In fact, when-
       ever the effective group ID is changed, fsgid will also be  changed  to
       the new value of the effective group ID.

       Explicit  calls  to setfsuid(2) and setfsgid() are usually only used by
       programs such as the Linux NFS server that need to change what user and
       group  ID is used for file access without a corresponding change in the
       real and effective user and group IDs.  A change in the normal user IDs
       for a program such as the NFS server is a security hole that can expose
       it to unwanted signals.  (But see below.)

       setfsgid() will only succeed if the caller is the superuser or if fsgid
       matches  either the real group ID, effective group ID, saved set-group-
       ID, or the current value of fsgid.

       On success, the previous value of fsgid is  returned.   On  error,  the
       current value of fsgid is returned.

       This system call is present in Linux since version 1.2.

       setfsgid()  is  Linux-specific  and  should  not  be  used  in programs
       intended to be portable.

       When glibc determines that the argument is not a  valid  group  ID,  it
       will  return  -1  and set errno to EINVAL without attempting the system

       Note that at the time this system call was introduced, a process  could
       send a signal to a process with the same effective user ID.  Today sig-
       nal permission handling is slightly different.

       The original Linux setfsgid() system call supported only  16-bit  group
       IDs.  Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs.
       The glibc setfsgid() wrapper  function  transparently  deals  with  the
       variation across kernel versions.

       No  error messages of any kind are returned to the caller.  At the very
       least, EPERM should be returned when the call fails (because the caller
       lacks the CAP_SETGID capability).
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