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CentOS 7.0 - man page for gethostid (centos section 3)

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 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED sethostid(): _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 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.
CONFORMING TO
4.2BSD; these functions were dropped in 4.4BSD. SVr4 includes gethostid() but not sethostid(). POSIX.1-2001 specifies 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 3.53 of the Linux man-pages project. A description of the project, and informa- tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-09-20 GETHOSTID(3)


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