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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)

Linux                                                                          2021-03-22                                                                   GETHOSTID(3)

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