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 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.

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 attributes(7).
Interface | Attribute | Value
 | Thread safety | MT-Safe hostid env locale
 | Thread safety | MT-Unsafe const:hostid

VERSIONS
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.)

STANDARDS

gethostid()
    POSIX.1-2008.

sethostid()
    None.

HISTORY
4.2BSD; dropped in 4.4BSD. SVr4 and POSIX.1-2001 include gethostid() but not sethostid().

BUGS
It is impossible to ensure that the identifier is globally unique.

SEE ALSO
hostid(1), gethostbyname(3)