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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
   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.

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 4.09 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/.



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