1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
3 .\" %%%LICENSE_START(VERBATIM)
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25 .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
26 .\" Modified 2000-07-01 aeb
27 .\" Modified 2002-07-23 aeb
28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added notes on capability requirements
31 .TH SETFSGID 2 2013-08-08 "Linux" "Linux Programmer's Manual"
33 setfsgid \- set group identity used for filesystem checks
35 .B #include <sys/fsuid.h>
37 .BI "int setfsgid(uid_t " fsgid );
41 changes the value of the caller's filesystem group ID\(emthe
42 group ID that the Linux kernel uses to check for all accesses
44 Normally, the value of
45 the filesystem group ID
46 will shadow the value of the effective group ID.
48 effective group ID is changed,
49 the filesystem group ID
50 will also be changed to the new value of the effective group ID.
56 are usually used only by programs such as the Linux NFS server that
57 need to change what user and group ID is used for file access without a
58 corresponding change in the real and effective user and group IDs.
59 A change in the normal user IDs for a program such as the NFS server
60 is a security hole that can expose it to unwanted signals.
64 will succeed only if the caller is the superuser or if
66 matches either the caller's real group ID, effective group ID,
67 saved set-group-ID, or current the filesystem user ID.
69 On both success and failure,
70 this call returns the previous filesystem group ID of the caller.
72 This system call is present in Linux since version 1.2.
73 .\" This system call is present since Linux 1.1.44
74 .\" and in libc since libc 4.7.6.
77 is Linux-specific and should not be used in programs intended
80 When glibc determines that the argument is not a valid group ID,
81 it will return \-1 and set \fIerrno\fP to
86 Note that at the time this system call was introduced, a process
87 could send a signal to a process with the same effective user ID.
88 Today signal permission handling is slightly different.
91 for a discussion of why the use of both
99 system call supported only 16-bit group IDs.
100 Subsequently, Linux 2.4 added
102 supporting 32-bit IDs.
105 wrapper function transparently deals with the variation across kernel versions.
107 No error indications of any kind are returned to the caller,
108 and the fact that both successful and unsuccessful calls return
109 the same value makes it impossible to directly determine
110 whether the call succeeded or failed.
111 Instead, the caller must resort to looking at the return value
112 from a further call such as
114 (which will always fail), in order to determine if a preceding call to
116 changed the filesystem group ID.
120 should be returned when the call fails (because the caller lacks the
126 .BR capabilities (7),