gethostid(3) — Linux manual page

gethostid(3)            Library Functions Manual            gethostid(3)

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)

COLOPHON

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

Linux man-pages 6.9.1          2024-05-02                   gethostid(3)

Pages that refer to this page: hostid(1), machine-id(5), attributes(7)

---