gethostid

gethostid is a Unix-like system library function that returns the host identifier for the current machine. It provides a 32-bit value, typically returned as an unsigned long, that historically served as a machine-wide identifier for older network services and programs.

The host ID is commonly stored on the system as a 4-byte value, often in the file /etc/hostid, and read at boot or when the system initializes networking. On some configurations, if the file is absent or inaccessible, the host ID may be derived from hardware characteristics or other system data, but the exact behavior depends on the operating system and its startup scripts. The function gethostid reads this stored identifier and returns it to the caller.

In practice, the host ID has been used by legacy software such as the Network Information Service (NIS) to identify hosts within a network and by the hostid utility, which prints the numeric identifier. Because modern networking relies more on DNS names and dynamic identifiers, the use of gethostid and the host ID value is largely historical or for compatibility with older software.

Portability and implementation details vary by platform. The function is part of traditional Unix APIs and is available on many BSD and Linux systems, but it is not central to contemporary network configuration. Developers requiring a unique machine identifier today typically rely on hostnames, DNS, UUIDs, or other mechanisms rather than the historic host ID.