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

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

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

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.

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.

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

       Standard C library (libc, -lc)

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

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.

hostid(1), gethostbyname(3)

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

gethostid()
              POSIX.1-2008.

       sethostid()
              None.

#include<unistd.h>longgethostid(void);intsethostid(longhostid);

   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)

       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.)