What is the difference between sigaction and signal in C?

sigaction and signal are both functions used for handling signals in UNIX/Linux systems, but they have key differences in functionality and reliability:

1. Reliability and Behavior Control:

   • sigaction provides more control over signal handling, such as setting whether signals are automatically blocked during processing and the ability to restore to default handling. This makes sigaction more reliable than signal, especially in multi-threaded environments.
   • signal may behave inconsistently across different systems due to varying implementations, leading to differences in signal handling behavior.

2. Portability:

   • sigaction is part of the POSIX standard, offering better cross-platform support.
   • signal, while widely available, may exhibit inconsistent behavior across different systems.

3. Functionality:

   • sigaction allows detailed definition of signal handling behavior, such as specifying whether to block other signals during processing. Additionally, the sigaction structure provides a way to specify extra information for the signal handler function (e.g., sa_flags and sa_mask).
   • signal only permits specifying a single handler function and does not support complex configurations.

4. Example:

   • Imagine a program that needs to capture the SIGINT signal (typically generated when the user presses Ctrl+C). Using sigaction, you can precisely control the program's behavior upon receiving this signal, for example, blocking other signals during handler execution to prevent interruption while processing the signal.

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

void handler(int signum) {
    printf("Caught signal %d\n", signum);
}

int main() {
    struct sigaction sa;
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;

    if (sigaction(SIGINT, &sa, NULL) == -1) {
        perror("sigaction");
        return 1;
    }

    while (1) {
        sleep(1);  // Simulating a long-running program
    }
    return 0;
}

In this example, even during SIGINT processing, the program is not interrupted by other registered signals, ensuring handling integrity and program stability.

In summary, while signal is sufficient for simple applications, sigaction is the better choice when precise and reliable signal handling is required.