gethostid(3) - Linux manual page (original) (raw)

gethostid(3) Library Functions Manual gethostid(3)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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 top

   **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)](../man3/gethostbyname.3.html), 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](../man3/errno.3.html)_ 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)](../man7/attributes.7.html).
   ┌─────────────┬───────────────┬──────────────────────────────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├─────────────┼───────────────┼──────────────────────────────────┤
   │ **gethostid**() │ Thread safety │ MT-Safe hostid env locale        │
   ├─────────────┼───────────────┼──────────────────────────────────┤
   │ **sethostid**() │ Thread safety │ MT-Unsafe const:hostid           │
   └─────────────┴───────────────┴──────────────────────────────────┘

VERSIONS top

   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)](../man2/gethostname.2.html), passes that hostname to [gethostbyname_r(3)](../man3/gethostbyname%5Fr.3.html) 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 top

   **gethostid**()
          POSIX.1-2008.

   **sethostid**()
          None.

HISTORY top

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

BUGS top

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

SEE ALSO top

   [hostid(1)](../man1/hostid.1.html), [gethostbyname(3)](../man3/gethostbyname.3.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 gethostid(3)

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