xfree86 man page for gethostid

GETHOSTID(3)						     Linux Programmer's Manual						      GETHOSTID(3)

NAME
       gethostid, sethostid - get or set the unique identifier of the current host

SYNOPSIS
       #include <unistd.h>

       long gethostid(void);
       int sethostid(long hostid);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       gethostid():
	   _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 is intended
       to be unique among all UNIX systems in existence.  This normally resembles 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 attributes(7).

       +------------+---------------+---------------------------+
       |Interface   | Attribute     | Value			|
       +------------+---------------+---------------------------+
       |gethostid() | Thread safety | MT-Safe hostid env locale |
       +------------+---------------+---------------------------+
       |sethostid() | Thread safety | MT-Unsafe const:hostid	|
       +------------+---------------+---------------------------+

CONFORMING TO
       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().

NOTES
       In  the	glibc  implementation,	the hostid is stored in the file /etc/hostid.  (In glibc versions before 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.)

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

SEE ALSO
       hostid(1), gethostbyname(3)

COLOPHON
       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							      GETHOSTID(3)