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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 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 4.15 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)