Signal Handling in C: Capturing and Managing Signals

In C, a signal is a mechanism used to notify a program that a specific event has occurred. These events may be triggered by external systems, such as user input (like Ctrl+C), software faults, or timer expirations. Signals can interrupt the normal execution flow of a program and allow it to respond appropriately to different situations.

In this article, we will discuss how to capture and handle signals in C, along with some common application scenarios. We will demonstrate the basic usage through specific code examples.

What is a Signal?

A signal can be viewed as an asynchronous message that the operating system sends to a process to indicate that a certain condition has occurred. For example:

SIGINT: Sent when the user presses Ctrl+C
SIGTERM: Request to terminate the process
SIGSEGV: Invalid memory access leading to a segmentation fault

Each process can set custom handling for these signals by defining corresponding callback functions to respond to different types of signals.

How to Capture and Handle Signals

To capture and handle signals in C, you need to include the <signal.h> header file, which provides a series of data structures and functions related to signals. This section will introduce the most commonly used methods to install and use custom signal handling functions.

Step 1: Include the Necessary Libraries

First, you need to include the necessary header files:

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

Step 2: Define the Signal Handling Function

Next, you need to define a callback function to handle interrupts. This function will be called when a specific event (such as Ctrl+C) occurs. For example, we can create a simple example as follows:

void signal_handler(int signum) {
    printf("Received signal %d. Exiting gracefully...\n", signum);
    exit(0);
}

This signal_handler function will execute upon receiving the specified termination signal (SIGINT), outputting a message and then safely exiting the program.

Step 3: Install the Signal Handler

Now, you need to install the signal handler defined above to make it effective. Add the following code in the main function:

int main() {
    // Register SIGINT signal (usually Ctrl+C)
    signal(SIGINT, signal_handler);
    // Infinite loop to test the effect of CTRL+C
    while (1) {
        printf("Running... Press Ctrl+C to stop.\n");
        sleep(1); // Output once per second to make the loop less CPU intensive.
    }
    return 0;
}

The complete code is as follows:

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

void signal_handler(int signum) {
    printf("Received signal %d. Exiting gracefully...\n", signum);
    exit(0);
}

int main() {
    // Register SIGINT signal (usually Ctrl+C)
    signal(SIGINT, signal_handler);
    // Infinite loop to test the effect of CTRL+C
    while (1) {
        printf("Running... Press Ctrl+C to stop.\n");
        sleep(1); // Output once per second to make the loop less CPU intensive.
    }
    return 0;
}

Compiling and Running

Save the above code to a .c file, assuming it is named signal_example.c. Then compile and run it using the following command:

gcc -o signal_example signal_example.c
./signal_example

When you run this program, it will continuously print “Running… Press Ctrl+C to stop.” to standard output. When you press Ctrl + C, it will trigger the SIGINT signal and call the previously registered signal_handler() function, thus safely exiting the program and displaying a message.

Conclusion

Today, we explored the basic implementation in C—how to capture and respond to various signals sent by the system through custom callback functions. I hope the above content helps you gain a deeper understanding of the importance and operational steps of applying this functionality under specific conditions, laying a foundation for expanding your programming skills. In actual development, effectively utilizing this feature can enhance software robustness and improve user experience.