gethostid

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



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

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                    │
       └────────────┴───────────────┴───────────────────────────────────────────┘

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

SEE ALSO
       hostid(1), gethostbyname(3)

COLOPHON
       This page is part of release 5.11 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                              2021-03-22                       GETHOSTID(3)