| .\" 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) |
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| .\" 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. |
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| .\" Since the Linux kernel and libraries are constantly changing, this |
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| .\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com> |
| .TH GETHOSTID 3 2016-03-15 "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> |
| .sp |
| .B long gethostid(void); |
| .br |
| .BI "int sethostid(long " hostid ); |
| .sp |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .sp |
| .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. |
| |
| 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 (). |
| |
| 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 |
| |
| .SH CONFORMING TO |
| 4.2BSD; these functions were dropped in 4.4BSD. |
| SVr4 includes |
| .BR gethostid () |
| but not |
| .BR sethostid (). |
| |
| 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 |
| |
| 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) |