System Calls on Nalar

Avoid PID Reuse Races on Linux with pidfds

Fri, 04 Sep 2026 00:00:00 +0700

A process ID looks like an identity, but it is really a reusable number.

That distinction matters in long-running supervisors, job managers, test harnesses, and other programs that observe a process and then act on it later. Between those two operations, the original process can exit and Linux can eventually reuse the same PID for an unrelated process.

Traditional PID-based code can therefore have a time-of-check/time-of-use race:

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check PID 4242 -> original process exits -> PID 4242 is reused -> signal PID 4242

Linux PID file descriptors, usually called pidfds, provide another model. Instead of repeatedly identifying a task by a reusable integer, a program obtains a file descriptor that refers to a particular process and can use that descriptor with pidfd-aware APIs.

Handle Linux Signals in Event Loops with signalfd

Fri, 04 Sep 2026 00:00:00 +0700

Unix signals are asynchronous by design: a signal can interrupt a program between ordinary instructions and transfer control to a signal handler. That model is useful, but it creates an awkward boundary for event-driven programs.

A network server may already spend most of its time inside poll(), epoll_wait(), or another readiness API. Its sockets, pipes, and timers appear as file-descriptor events, while SIGTERM and SIGHUP arrive through a separate execution path with much stricter rules about what code may safely run.

Integrate Timers into Linux Event Loops with timerfd

Fri, 04 Sep 2026 00:00:00 +0700

Event loops work best when unrelated kinds of work have one common waiting mechanism. Sockets become readable. Pipes become writable. A child-process descriptor or signal descriptor can become ready. Timers are often the awkward exception.

A program can call sleep() or nanosleep(), but that blocks the thread instead of letting it wait for I/O. It can pass a timeout to poll() or epoll_wait(), but one timeout becomes difficult to manage when the program has several independent deadlines. Traditional POSIX timers can deliver signals, which introduces a second asynchronous control path.