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 2015-03-29 "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 .sp
  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 caller is privileged (has the
  43 CAP_SETGID
  44 capability), the real GID and saved set-group-ID are also set.
  45
  46 Under Linux,
  47 .BR setgid ()
  48 is implemented like the POSIX version with the
  49 .B _POSIX_SAVED_IDS
  50 feature.
  51 This allows a set-group-ID program that is not set-user-ID-root
  52 to drop all of its group
  53 privileges, do some un-privileged work, and then reengage the original
  54 effective group ID in a secure manner.
  55 .SH RETURN VALUE
  56 On success, zero is returned.
  57 On error, \-1 is returned, and
  58 .I errno
  59 is set appropriately.
  60 .SH ERRORS
  61 .TP
  62 .B EINVAL
  63 The group ID specified in
  64 .I gid
  65 is not valid in this user namespace.
  66 .TP
  67 .B EPERM
  68 The calling process is not privileged (does not have the
  69 \fBCAP_SETGID\fP capability), and
  70 .I gid
  71 does not match the real group ID or saved set-group-ID of
  72 the calling process.
  73 .SH CONFORMING TO
  74 SVr4, POSIX.1-2001.
  75 .SH NOTES
  76 The original Linux
  77 .BR setgid ()
  78 system call supported only 16-bit group IDs.
  79 Subsequently, Linux 2.4 added
  80 .BR setgid32 ()
  81 supporting 32-bit IDs.
  82 The glibc
  83 .BR setgid ()
  84 wrapper function transparently deals with the variation across kernel versions.
  85 .\"
  86 .SS C library/kernel differences
  87 At the kernel level, user IDs and group IDs are a per-thread attribute.
  88 However, POSIX requires that all threads in a process
  89 share the same credentials.
  90 The NPTL threading implementation handles the POSIX requirements by
  91 providing wrapper functions for
  92 the various system calls that change process UIDs and GIDs.
  93 These wrapper functions (including the one for
  94 .BR setgid ())
  95 employ a signal-based technique to ensure
  96 that when one thread changes credentials,
  97 all of the other threads in the process also change their credentials.
  98 For details, see
  99 .BR nptl (7).
 100 .SH SEE ALSO
 101 .BR getgid (2),
 102 .BR setegid (2),
 103 .BR setregid (2),
 104 .BR capabilities (7),
 105 .BR credentials (7),
 106 .BR user_namespaces (7)