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 ||
           _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 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
       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  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.  (In glibc versions before 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)

COLOPHON
       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-08-08                      GETHOSTID(3)
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