Скачать или смотреть Resolving the Issue of SIGUSR1 Not Reaching the Second Child Process

Discover how to solve the issue of a child process not receiving the `SIGUSR1` signal after being recreated in a forked environment. This guide dives into coding solutions and best practices for process management in C.
---
This video is based on the question https://stackoverflow.com/q/68154741/ asked by the user 'giggiox' ( https://stackoverflow.com/u/8591417/ ) and on the answer https://stackoverflow.com/a/68155720/ provided by the user 'Craig Estey' ( https://stackoverflow.com/u/5382650/ ) 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: process killed more than one time does't receive signal SIGUSR1

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

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Introduction

When working with C programming and processes, especially with signals such as SIGUSR1, developers often encounter unique challenges. One particularly intriguing issue arises when a parent process attempts to manage a child process created via a fork, only to find that subsequent instances of the child no longer receive the expected signals. This article will dissect a specific case where a parent process creates a child process every two seconds, but the subsequent child never receives the intended SIGUSR1 signal.

The Problem

Our tests feature two scripts: father.c (the parent program) and child.c (the child program). The parent is expected to continuously send a signal to the child process, which then signals back to the parent when it has received the signal. However, the second child appears to be unresponsive, failing to receive SIGUSR1 as its predecessor did. How can this issue be resolved?

Understanding the Implementation

Let’s break down the working of father.c and child.c and the roles each plays:

The Parent Process - father.c

Functionality:

Creates a child process.

Sends SIGUSR1 signals to the child every two seconds.

On receiving SIGUSR1, the parent signals the child to terminate and forks a new child.

Core Functionality

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

This function handles signaling, but there lies the key issue: the parent may be sending signals while the child is still establishing its signal handling.

The Child Process - child.c

Functionality:

Sets up a signal handler for SIGUSR1.

After receiving the signal, it should notify the parent with another signal.

Core Functionality

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

However, if the child is killed before it can properly handle the signal, the expected behavior is disrupted.

Solution: Utilize Different Signals

Thanks to community insights, we realize that the problem can also be tackled by using different signals for the parent and child processes. This involves modifying key parts of both father.c and child.c:

Code Modifications

In father.c:

Change from handling SIGUSR1 to SIGUSR2:

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

Update the signal setting to SIGUSR2.

In child.c:

Modify the signal sent back to the parent to SIGUSR2:

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

Why Does This Solution Work?

Using different signal numbers allows for the establishment of distinct communication pathways between the parent and child, preventing any interference that might arise when they are configured to respond to the same signal type.

Additional Best Practices

Signal Management: Reinstate signal handlers regularly to prevent the handler from misfiring or becoming blocked.

Waiting for Process: Utilize waitpid() after killing the child to ensure the parent waits for termination confirmation.

Debugging: Keep track of signal receipt and handling in your code with debug logs for follow-up.

Conclusion

The ability to manage signals and processes effectively is crucial for robust programming in C. By recognizing the challenge posed by simultaneous signal handling and by employing unique signals for distinct processes, we can steer clear of common pitfalls. Feel free to customize your signal management strategies as per your application requirements. Happy coding!