gethostid(3) Library Functions Manual gethostid(3)
NAME
gethostid, sethostid - get or set the unique identifier of the current
host
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
long gethostid(void);
int sethostid(long hostid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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)
DESCRIPTION
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 resem-
bles the Internet address for the local machine, as returned by geth-
ostbyname(3), and thus usually never needs to be set.
The sethostid() call is restricted to the superuser.
RETURN VALUE
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.
ERRORS
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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────┬───────────────┬─────────────────────────────────────────┐
│Interface │ Attribute │ Value │
├────────────┼───────────────┼─────────────────────────────────────────┤
│gethostid() │ Thread safety │ MT-Safe hostid env locale │
├────────────┼───────────────┼─────────────────────────────────────────┤
│sethostid() │ Thread safety │ MT-Unsafe const:hostid │
└────────────┴───────────────┴─────────────────────────────────────────┘
STANDARDS
4.2BSD; these functions were dropped in 4.4BSD. SVr4 includes geth-
ostid() but not sethostid().
POSIX.1-2001 and POSIX.1-2008 specify gethostid() but not sethostid().
NOTES
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 con-
taining 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.)
BUGS
It is impossible to ensure that the identifier is globally unique.
SEE ALSO
hostid(1), gethostbyname(3)
Linux man-pages 6.03 2022-12-15 gethostid(3)
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