SETFSGID(2)               (2019-05-09)                SETFSGID(2)

     NAME
          setfsgid - set group identity used for filesystem checks

     SYNOPSIS
          #include <sys/fsuid.h>

          int setfsgid(uid_t fsgid);

     DESCRIPTION
          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 filesys-
          tem 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.

     RETURN VALUE
          On both success and failure, this call returns the previous
          filesystem group ID of the caller.

     VERSIONS
          This system call is present in Linux since version 1.2.

     CONFORMING TO
          setfsgid() is Linux-specific and should not be used in pro-
          grams intended to be portable.

     NOTES
          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          call were invented for historical reasons that are no longer
          applicable on modern Linux kernels.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()

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     SETFSGID(2)               (2019-05-09)                SETFSGID(2)

          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.

     BUGS
          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 deter-
          mine 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).

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

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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