Signal handler

Learn to catch signals with the signal function.

The signal function

signal(sig, handler) registers a handler for a signal. handler is a pointer to a function that takes the signal number and returns void.

For example, signal(SIGUSR1, handler) calls handler when SIGUSR1 arrives. Inside the handler you should only do safe operations; the signal can arrive at any time. signal is simple but its behavior varies across systems; production code prefers sigaction.

Sending signals with raise

raise(sig) sends the sig signal to the calling process. It has the same effect as kill(getpid(), sig).

The handler interrupts the process flow when the signal arrives and is called. When the handler returns, the process resumes where it was. raise is synchronous: the call waits until the handler returns. So the line after raise(SIGUSR1) runs after the handler finishes. Ideal for testing.

Exercise Catch a signal

Define global int caught = 0;. Write void handler(int sig) with caught = 1;. Register with signal(SIGUSR1, handler);. Send with raise(SIGUSR1); and print with printf("caught=%d\n", caught);.

Show example solution

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

int caught = 0;

void handler(int sig) {
    caught = 1;
}

int main(void) {
    signal(SIGUSR1, handler);
    raise(SIGUSR1);
    printf("caught=%d\n", caught);
    return 0;
}

Test yourself

Match handler concepts

signal

Registers handler
raise

Signal to self
handler

Handler function
SIGUSR1

User signal
What does raise(SIGUSR1) do?
- Sends to another process
- Sends to the caller
- Ignores the signal
- Blocks the signal
Show answer

Sends to the caller — raise sends the signal to the calling process.

Handler registration

signal(SIGUSR1, ____);

Show answer

signal(SIGUSR1, handler);

Next lesson: sigaction

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