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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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setfsgid(2) System Calls Manual setfsgid(2)
setfsgid - set group identity used for filesystem checks
Standard C library (libc, -lc)
#include <sys/fsuid.h>
[[deprecated]] int setfsgid(gid_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 caller.
Linux.
Linux 1.2.
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.
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were invented for historical reasons that are no longer applicable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.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 setfsgid(2)
Pages that refer to this page: setfsuid(2), setresuid(2), syscalls(2), credentials(7), path_resolution(7), user_namespaces(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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