GENWiki

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.