Over the last few kernel releases, the kernel has gained the concept of a
“pidfd” — a file descriptor that represents a process. What started as a
way of sending signals to processes without race conditions has evolved
into a more complete process-management interface. Now one of the last
pieces is being put into place: the ability to wait for processes using
pidfds. But, naturally, that API has had to go through some revisions
first.

Source: LWN.net – [$] Completing the pidfd API