1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
|
'\" t
.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\"
.\" Updated with additions from Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
.\" Portions Copyright 1993 Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.TH gethostid 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
gethostid, sethostid \- get or set the unique identifier of the current host
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.B long gethostid(void);
.BI "int sethostid(long " hostid );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR gethostid ():
.nf
Since glibc 2.20:
_DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
.\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
Up to and including glibc 2.19:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
.\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
.fi
.PP
.BR sethostid ():
.nf
Since glibc 2.21:
.\" commit 266865c0e7b79d4196e2cc393693463f03c90bd8
_DEFAULT_SOURCE
In glibc 2.19 and 2.20:
_DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
Up to and including glibc 2.19:
_BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
.fi
.SH DESCRIPTION
.BR gethostid ()
and
.BR sethostid ()
respectively get or set a unique 32-bit identifier for the current machine.
The 32-bit identifier was intended to be unique among all UNIX systems in
existence.
This normally resembles the Internet address for the local
machine, as returned by
.BR gethostbyname (3),
and thus usually never needs to be set.
.PP
The
.BR sethostid ()
call is restricted to the superuser.
.SH RETURN VALUE
.BR gethostid ()
returns the 32-bit identifier for the current host as set by
.BR sethostid ().
.PP
On success,
.BR sethostid ()
returns 0; on error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.BR sethostid ()
can fail with the following errors:
.TP
.B EACCES
The caller did not have permission to write to the file used
to store the host ID.
.TP
.B EPERM
The calling process's effective user or group ID is not the same
as its corresponding real ID.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lb lb lbx
l l l.
Interface Attribute Value
T{
.BR gethostid ()
T} Thread safety T{
MT-Safe hostid env locale
T}
T{
.BR sethostid ()
T} Thread safety T{
MT-Unsafe const:hostid
T}
.TE
.hy
.ad
.sp 1
.SH VERSIONS
In the glibc implementation, the
.I hostid
is stored in the file
.IR /etc/hostid .
(Before glibc 2.2, the file
.I /var/adm/hostid
was used.)
.\" libc5 used /etc/hostid; libc4 didn't have these functions
.PP
In the glibc implementation, if
.BR gethostid ()
cannot open the file containing the host ID,
then it obtains the hostname using
.BR gethostname (2),
passes that hostname to
.BR gethostbyname_r (3)
in order to obtain the host's IPv4 address,
and returns a value obtained by bit-twiddling the IPv4 address.
(This value may not be unique.)
.SH STANDARDS
.TP
.BR gethostid ()
POSIX.1-2008.
.TP
.BR sethostid ()
None.
.SH HISTORY
4.2BSD; dropped in 4.4BSD.
SVr4 and POSIX.1-2001 include
.BR gethostid ()
but not
.BR sethostid ().
.SH BUGS
It is impossible to ensure that the identifier is globally unique.
.SH SEE ALSO
.BR hostid (1),
.BR gethostbyname (3)
|
