focal (3) sethostid.3.gz

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():
           _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 is 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

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