gethostid, sethostid - get or set the unique identifier of the current host
Standard C library (
libc,
-lc)
#include <unistd.h>
long gethostid(void);
int sethostid(long hostid);
gethostid():
Since glibc 2.20:
_DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
Up to and including glibc 2.19:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
sethostid():
Since glibc 2.21:
_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)
gethostid() and
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
gethostbyname(3), and thus usually never needs to be set.
The
sethostid() call is restricted to the superuser.
gethostid() returns the 32-bit identifier for the current host as set by
sethostid().
On success,
sethostid() returns 0; on error, -1 is returned, and
errno is set to indicate the error.
sethostid() can fail with the following errors:
- EACCES
- The caller did not have permission to write to the file
used to store the host ID.
- EPERM
- 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
attributes(7).
Interface |
Attribute |
Value |
gethostid () |
Thread safety |
MT-Safe hostid env locale |
sethostid () |
Thread safety |
MT-Unsafe const:hostid |
4.2BSD; these functions were dropped in 4.4BSD. SVr4 includes
gethostid()
but not
sethostid().
POSIX.1-2001 and POSIX.1-2008 specify
gethostid() but not
sethostid().
In the glibc implementation, the
hostid is stored in the file
/etc/hostid. (Before glibc 2.2, the file
/var/adm/hostid was
used.)
In the glibc implementation, if
gethostid() cannot open the file
containing the host ID, then it obtains the hostname using
gethostname(2), passes that hostname to
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.)
It is impossible to ensure that the identifier is globally unique.
hostid(1),
gethostbyname(3)