Section: Linux Programmer's Manual (3)
gethostid, sethostid - get or set the unique identifier of the current host
int sethostid(long hostid);
Feature Test Macro Requirements for glibc (see
_BSD_SOURCE || _XOPEN_SOURCE >= 500
Since glibc 2.21:
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)
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
This normally resembles the Internet address for the local
machine, as returned by
and thus usually never needs to be set.
call is restricted to the superuser.
returns the 32-bit identifier for the current host as set by
returns 0; on error, -1 is returned, and
is set to indicate the error.
can fail with the following errors:
The caller did not have permission to write to the file used
to store the host ID.
The calling process's effective user or group ID is not the same
as its corresponding real ID.
For an explanation of the terms used in this section, see
||Thread safety||MT-Safe hostid env locale|
||Thread safety||MT-Unsafe const:hostid|
4.2BSD; these functions were dropped in 4.4BSD.
POSIX.1-2001 and POSIX.1-2008 specify
In the glibc implementation, the
is stored in the file
(In glibc versions before 2.2, the file
In the glibc implementation, if
cannot open the file containing the host ID,
then it obtains the hostname using
passes that hostname to
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.)
It is impossible to ensure that the identifier is globally unique.
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