.\" Copyright (C) 1995, Thomas K. Dyas .\" and Copyright (C) 2019, Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Created 1995-08-06 Thomas K. Dyas .\" Modified 2000-07-01 aeb .\" Modified 2002-07-23 aeb .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" .TH SETFSGID 2 2019-05-09 "Linux" "Linux Programmer's Manual" .SH NAME setfsgid \- set group identity used for filesystem checks .SH SYNOPSIS .B #include .PP .BI "int setfsgid(uid_t " fsgid ); .SH 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 filesystem objects, while the effective group ID is used for some other kinds of permissions checks (see .BR credentials (7)). .PP 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 .BR setfsgid () to change its filesystem group ID to the value given in .IR fsgid . .PP .BR setfsgid () will succeed only if the caller is the superuser or if .I fsgid matches either the caller's real group ID, effective group ID, saved set-group-ID, or current the filesystem user ID. .SH RETURN VALUE On both success and failure, this call returns the previous filesystem group ID of the caller. .SH VERSIONS This system call is present in Linux since version 1.2. .\" This system call is present since Linux 1.1.44 .\" and in libc since libc 4.7.6. .SH CONFORMING TO .BR setfsgid () is Linux-specific and should not be used in programs intended to be portable. .SH NOTES The filesystem group ID concept and the .BR setfsgid () system call were invented for historical reasons that are no longer applicable on modern Linux kernels. See .BR setfsuid (2) for a discussion of why the use of both .BR setfsuid (2) and .BR setfsgid () is nowadays unneeded. .PP The original Linux .BR setfsgid () system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added .BR setfsgid32 () supporting 32-bit IDs. The glibc .BR setfsgid () wrapper function transparently deals with the variation across kernel versions. .SS 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 \fIerrno\fP to .B EINVAL without attempting the system call. .SH 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 determine whether the call succeeded or failed. Instead, the caller must resort to looking at the return value from a further call such as .IR setfsgid(\-1) (which will always fail), in order to determine if a preceding call to .BR setfsgid () changed the filesystem group ID. At the very least, .B EPERM should be returned when the call fails (because the caller lacks the .B CAP_SETGID capability). .SH SEE ALSO .BR kill (2), .BR setfsuid (2), .BR capabilities (7), .BR credentials (7) .SH COLOPHON This page is part of release 5.04 of the Linux .I 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/.