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       setfsgid - set group identity used for filesystem checks


       #include <sys/fsuid.h>

       int setfsgid(uid_t fsgid);


       On Linux, a process has both a filesystem group ID and an effective group ID.  The (Linux-
       specific) filesystem group ID is used for permissions checking when  accessing  filesystem
       objects,  while  the effective group ID is used for some other kinds of permissions checks
       (see credentials(7)).

       Normally, the value of the process's filesystem group ID is the same as the value  of  its
       effective  group  ID.   This  is  so,  because  whenever a process's effective group ID is
       changed, the kernel also changes the filesystem group ID to be the same as the  new  value
       of  the  effective  group ID.  A process can cause the value of its filesystem group ID to
       diverge from its effective group ID by using setfsgid() to change its filesystem group  ID
       to the value given in fsgid.

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


       On  both  success  and  failure, this call returns the previous filesystem group ID of the


       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.


       The filesystem group  ID  concept  and  the  setfsgid()  system  call  were  invented  for
       historical reasons that are no longer applicable on modern Linux kernels.  See setfsuid(2)
       for a discussion of why the use of both setfsuid(2) and setfsgid() is nowadays unneeded.

       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.

   C library/kernel differences
       In glibc 2.15 and earlier, when the wrapper for  this  system  call  determines  that  the
       argument  can't  be passed to the kernel without integer truncation (because the kernel is
       old and does not support 32-bit group IDs), it will return -1  and  set  errno  to  EINVAL
       without attempting the system call.


       No  error  indications  of  any  kind  are  returned to the caller, and the fact that both
       successful and unsuccessful calls return the same value makes it  impossible  to  directly
       determine  whether  the  call  succeeded  or  failed.   Instead, the caller must resort to
       looking at the return value from a further call such as setfsgid(-1)  (which  will  always
       fail),  in  order  to  determine  if a preceding call to setfsgid() changed the filesystem
       group ID.  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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