gethostid, sethostid - get or set the unique identifier of the current host
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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:
The caller did not have permission to write to the file used to store the host ID.
The calling process's effective user or group ID is not the same as its corresponding real ID.
In the glibc implementation, the
hostid is stored in the
/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.
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