gethostid,  sethostid - get or set the unique identifier of the current


   #include <unistd.h>

   long gethostid(void);
   int sethostid(long hostid);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       _BSD_SOURCE || _XOPEN_SOURCE >= 500
       Since glibc 2.21:
       In glibc 2.19 and 2.20:
       Up to and including glibc 2.19:
           _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)


   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.


   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.


   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.


   For  an  explanation  of  the  terms  used   in   this   section,   see

   │InterfaceAttributeValue                     │
   │gethostid() │ Thread safety │ MT-Safe hostid env locale │
   │sethostid() │ Thread safety │ MT-Unsafe const:hostid    │


   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().


   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.)


   It is impossible to ensure that the identifier is globally unique.


   hostid(1), gethostbyname(3)


   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


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