NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

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

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

       gethostid():
           Since glibc 2.20:
               _DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
           Up to and including glibc 2.19:
               _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 was 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                                     │
       └────────────┴───────────────┴────────────────────────────────────────────────────────────┘

VERSIONS

       In the glibc implementation, the hostid is stored in the file /etc/hostid.  (Before  glibc
       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.)

STANDARDS

       gethostid()
              POSIX.1-2008.

       sethostid()
              None.

HISTORY

       4.2BSD; dropped in 4.4BSD.  SVr4 and POSIX.1-2001 include gethostid() but not sethostid().

BUGS

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

SEE ALSO

       hostid(1), gethostbyname(3)