Manpages - gethostid.3

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

  #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)

*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.

*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.

*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.

For an explanation of the terms used in this section, see *attributes*(7).

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*().

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.)

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

*hostid*(1), *gethostbyname*(3)

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