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 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.

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 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-09-20							      GETHOSTID(3)