NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

GETHOSTID(3)              Linux Programmer's Manual             GETHOSTID(3)

NAME         top

       gethostid,  sethostid  - get or set the unique identifier of the cur‐
       rent host

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       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         top

       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         top

       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         top

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

SEE ALSO         top

       hostid(1), gethostbyname(3)

COLOPHON         top

       This page is part of release 4.07 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/.

Linux                            2016-03-15                     GETHOSTID(3)