man3/gethostid.3

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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   3 .\" Updated with additions from Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
   4 .\" Portions Copyright 1993 Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
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  28 .\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  29 .TH GETHOSTID 3 2015-08-08 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 gethostid, sethostid \- get or set the unique identifier of the current host
  32 .SH SYNOPSIS
  33 .B #include <unistd.h>
  34 .sp
  35 .B long gethostid(void);
  36 .br
  37 .BI "int sethostid(long " hostid );
  38 .sp
  39 .in -4n
  40 Feature Test Macro Requirements for glibc (see
  41 .BR feature_test_macros (7)):
  42 .in
  43 .sp
  44 .ad l
  45 .br
  46 .BR gethostid ():
  47 .RS 4
  48 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
  49 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  50 .RE
  51 .BR sethostid ():
  52 .nf
  53     Since glibc 2.21:
  54 .\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8
  55         _DEFAULT_SOURCE
  56     In glibc 2.19 and 2.20:
  57         _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  58     Up to and including glibc 2.19:
  59         _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  60 .fi
  61 .ad b
  62 .SH DESCRIPTION
  63 .BR gethostid ()
  64 and
  65 .BR sethostid ()
  66 respectively get or set a unique 32-bit identifier for the current machine.
  67 The 32-bit identifier is intended to be unique among all UNIX systems in
  68 existence.
  69 This normally resembles the Internet address for the local
  70 machine, as returned by
  71 .BR gethostbyname (3),
  72 and thus usually never needs to be set.
  73
  74 The
  75 .BR sethostid ()
  76 call is restricted to the superuser.
  77 .SH RETURN VALUE
  78 .BR gethostid ()
  79 returns the 32-bit identifier for the current host as set by
  80 .BR sethostid ().
  81
  82 On success,
  83 .BR sethostid ()
  84 returns 0; on error, \-1 is returned, and
  85 .I errno
  86 is set to indicate the error.
  87 .SH ERRORS
  88 .BR sethostid ()
  89 can fail with the following errors:
  90 .TP
  91 .B EACCES
  92 The caller did not have permission to write to the file used
  93 to store the host ID.
  94 .TP
  95 .B EPERM
  96 The calling process's effective user or group ID is not the same
  97 as its corresponding real ID.
  98 .SH ATTRIBUTES
  99 For an explanation of the terms used in this section, see
 100 .BR attributes (7).
 101 .TS
 102 allbox;
 103 lb lb lbw25
 104 l l l.
 105 Interface       Attribute       Value
 106 T{
 107 .BR gethostid ()
 108 T}      Thread safety   MT-Safe hostid env locale
 109 T{
 110 .BR sethostid ()
 111 T}      Thread safety   MT-Unsafe const:hostid
 112 .TE
 113
 114 .SH CONFORMING TO
 115 4.2BSD; these functions were dropped in 4.4BSD.
 116 SVr4 includes
 117 .BR gethostid ()
 118 but not
 119 .BR sethostid ().
 120
 121 POSIX.1-2001 and POSIX.1-2008 specify
 122 .BR gethostid ()
 123 but not
 124 .BR sethostid ().
 125 .SH NOTES
 126 In the glibc implementation, the
 127 .I hostid
 128 is stored in the file
 129 .IR /etc/hostid .
 130 (In glibc versions before 2.2, the file
 131 .I /var/adm/hostid
 132 was used.)
 133 .\" libc5 used /etc/hostid; libc4 didn't have these functions
 134
 135 In the glibc implementation, if
 136 .BR gethostid ()
 137 cannot open the file containing the host ID,
 138 then it obtains the hostname using
 139 .BR gethostname (2),
 140 passes that hostname to
 141 .BR gethostbyname_r (3)
 142 in order to obtain the host's IPv4 address,
 143 and returns a value obtained by bit-twiddling the IPv4 address.
 144 (This value may not be unique.)
 145 .SH BUGS
 146 It is impossible to ensure that the identifier is globally unique.
 147 .SH SEE ALSO
 148 .BR hostid (1),
 149 .BR gethostbyname (3)