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 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 correspond‐
              ing 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 ver‐
       sions 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 gethostby‐
       name_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 3.74 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                                       2010-09-20                               GETHOSTID(3)