gethostid(3) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #include <unistd.h>

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

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.20:
               _DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
           Up to and including glibc 2.19:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

           Since glibc 2.21:
           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), 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

       │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

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

SEE ALSO         top

       hostid(1), gethostbyname(3)

COLOPHON         top

       This page is part of release 5.13 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

Linux                          2021-03-22                   GETHOSTID(3)

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