Understanding sigwait and signalfd: Are They Safe Together in Multithreading?

Explore if using `sigwait` and `signalfd` concurrently in your multi-threaded C+ + program can lead to race conditions, and learn about the underlying mechanisms that ensure signal safety.
---
This video is based on the question https://stackoverflow.com/q/70387878/ asked by the user 'MrAzzaman' ( https://stackoverflow.com/u/2837253/ ) and on the answer https://stackoverflow.com/a/70400473/ provided by the user 'MrAzzaman' ( https://stackoverflow.com/u/2837253/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Does using sigwait and signalfd concurrently in a multithreaded program result in a race condition?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding sigwait and signalfd: Are They Safe Together in Multithreading?

In the world of multithreaded programming, particularly in C+ + , managing signals can indeed pose a challenge, especially when dealing with potential race conditions. A common scenario involves the coexistence of sigwait and signalfd. This post explores the question: Does using sigwait and signalfd concurrently result in a race condition?

The Problem at Hand

In a sample multithreaded application, you might have a setup where one thread is waiting on a network socket using poll, while another thread uses sigwait to handle signals. The concern arises when signals are involved—specifically whether sigwait, which consumes signals, could cause race conditions with signalfd, which signals notifications.

Consider a situation where:

A dedicated thread awaits signals via sigwait.

Another thread listens for network connections using a signalfd, combined with poll.

The confusion lies in the mechanics of how these two function together. There is a fear that if sigwait consumes a signal before signalfd can be notified of it, the poll call in the primary thread could end up hanging indefinitely. But is this actually the case?

The Solution: Insights from the Linux Source Code

After investigating the intricacies of Linux’s kernel code responsible for signal handling, it's clear that there is no race condition in this interaction. Here’s an overview of the findings:

Order of Operations:

When a signal is raised, the signalfd functionality is invoked before the signal itself is sent. This guarantees that any threads waiting on signals via signalfd are notified of the signal prior to its consumption by any sigwait calls.

Signal Notification Mechanism:

The sequence in the kernel’s handling can be summarized as follows:

[[See Video to Reveal this Text or Code Snippet]]

This means that when a signal is raised, the handler for signalfd is explicitly notified first, ensuring it can respond before sigwait takes action to consume the signal.

Conclusion: What This Means for Your Code

Given the underlying mechanisms in the Linux kernel, developers can feel confident in the use of sigwait and signalfd concurrently without the need for additional locking or synchronization techniques to avoid race conditions. The operation of these functions has been designed to work cohesively, effectively preventing the block or hang scenarios when correctly implemented.

In summary, you can utilize both sigwait and signalfd in your multithreaded applications with peace of mind, knowing that Linux's signal handling mechanisms safeguard against potential race conditions. Happy coding!

Видео Understanding sigwait and signalfd: Are They Safe Together in Multithreading? канала vlogize