.\" 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>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.TH GETHOSTID 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
gethostid, sethostid \- get or set the unique identifier of the current host
.SH SYNOPSIS
.B #include <unistd.h>
.PP
.B long gethostid(void);
.br
.BI "int sethostid(long " hostid );
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.ad l
.br
.BR gethostid ():
.RS 4
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.RE
.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
.ad b
.SH DESCRIPTION
.BR gethostid ()
and
.BR sethostid ()
respectively get or set a unique 32-bit identifier for the current machine.
The 32-bit identifier is 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).
.TS
allbox;
lb lb lbw25
l l l.
Interface	Attribute	Value
T{
.BR gethostid ()
T}	Thread safety	MT-Safe hostid env locale
T{
.BR sethostid ()
T}	Thread safety	MT-Unsafe const:hostid
.TE
.sp 1
.SH CONFORMING TO
4.2BSD; these functions were dropped in 4.4BSD.
SVr4 includes
.BR gethostid ()
but not
.BR sethostid ().
.PP
POSIX.1-2001 and POSIX.1-2008 specify
.BR gethostid ()
but not
.BR sethostid ().
.SH NOTES
In the glibc implementation, the
.I hostid
is stored in the file
.IR /etc/hostid .
(In glibc versions before 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 BUGS
It is impossible to ensure that the identifier is globally unique.
.SH SEE ALSO
.BR hostid (1),
.BR gethostbyname (3)