man2/setgid.2

   1 .\" Copyright (C), 1994, Graeme W. Wilford. (Wilf.)
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  26 .\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
  27 .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
  28 .\" Modified 2002-03-09 by aeb
  29 .\"
  30 .TH SETGID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  31 .SH NAME
  32 setgid \- set group identity
  33 .SH SYNOPSIS
  34 .B #include <sys/types.h>
  35 .br
  36 .B #include <unistd.h>
  37 .PP
  38 .BI "int setgid(gid_t " gid );
  39 .SH DESCRIPTION
  40 .BR setgid ()
  41 sets the effective group ID of the calling process.
  42 If the calling process is privileged (more precisely: has the
  43 .B CAP_SETGID
  44 capability in its user namespace),
  45 the real GID and saved set-group-ID are also set.
  46 .PP
  47 Under Linux,
  48 .BR setgid ()
  49 is implemented like the POSIX version with the
  50 .B _POSIX_SAVED_IDS
  51 feature.
  52 This allows a set-group-ID program that is not set-user-ID-root
  53 to drop all of its group
  54 privileges, do some un-privileged work, and then reengage the original
  55 effective group ID in a secure manner.
  56 .SH RETURN VALUE
  57 On success, zero is returned.
  58 On error, \-1 is returned, and
  59 .I errno
  60 is set appropriately.
  61 .SH ERRORS
  62 .TP
  63 .B EINVAL
  64 The group ID specified in
  65 .I gid
  66 is not valid in this user namespace.
  67 .TP
  68 .B EPERM
  69 The calling process is not privileged (does not have the
  70 \fBCAP_SETGID\fP capability in its user namespace), and
  71 .I gid
  72 does not match the real group ID or saved set-group-ID of
  73 the calling process.
  74 .SH CONFORMING TO
  75 POSIX.1-2001, POSIX.1-2008, SVr4.
  76 .SH NOTES
  77 The original Linux
  78 .BR setgid ()
  79 system call supported only 16-bit group IDs.
  80 Subsequently, Linux 2.4 added
  81 .BR setgid32 ()
  82 supporting 32-bit IDs.
  83 The glibc
  84 .BR setgid ()
  85 wrapper function transparently deals with the variation across kernel versions.
  86 .\"
  87 .SS C library/kernel differences
  88 At the kernel level, user IDs and group IDs are a per-thread attribute.
  89 However, POSIX requires that all threads in a process
  90 share the same credentials.
  91 The NPTL threading implementation handles the POSIX requirements by
  92 providing wrapper functions for
  93 the various system calls that change process UIDs and GIDs.
  94 These wrapper functions (including the one for
  95 .BR setgid ())
  96 employ a signal-based technique to ensure
  97 that when one thread changes credentials,
  98 all of the other threads in the process also change their credentials.
  99 For details, see
 100 .BR nptl (7).
 101 .SH SEE ALSO
 102 .BR getgid (2),
 103 .BR setegid (2),
 104 .BR setregid (2),
 105 .BR capabilities (7),
 106 .BR credentials (7),
 107 .BR user_namespaces (7)