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When a socketpair is closed, it results in an End of Transmission (EOT) signal to the other end of the socketpair. This signal should trigger a default SIGHUP (Hangup) signal to the process, which is usually used to terminate processes that have been disconnected from their controlling terminals.

However, in some cases, the default behavior of the system may be overridden by the process or the specific configuration of the system. In Ubuntu, it's possible that the SIGHUP signal is being suppressed or handled in a non-default way, which prevents it from being sent when a socketpair is closed.

It's also possible that the process handling the socketpair closure is not properly checking for the SIGHUP signal or is ignoring it. This can happen if the process has explicitly disabled the SIGHUP signal or if it's not designed to properly handle it.

Overall, the specific reason for the lack of default SIGHUP signal on Ubuntu may depend on the specific application or process involved and its configuration.