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       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(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 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

       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).


       kill(2), setfsuid(2), capabilities(7), credentials(7)


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