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RedHat 9 (Linux i386) - man page for setfsgid (redhat section 2)

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

       setfsgid - set group identity used for file system checks

       #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, whenever the effective group ID is changed, fsgid will also
       be changed to the new value of the effective group ID.

       Explicit calls to setfsuid 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 with-
       out 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.

       setfsgid is Linux specific and should not be used in programs intended to be portable.  It
       is present since Linux 1.1.44 and in libc since libc 4.7.6.

       No  error messages of any kind are returned to the caller. At the very least, EPERM should
       be returned when the call fails.

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

       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 signal  permission  handling  is  slightly

       kill(2), setfsuid(2)

Linux 1.3.15				    2002-07-23				      SETFSGID(2)

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