What are signals?

Learn the signal concept and basic signals.

What are signals?

A signal is a software interrupt sent by the operating system to a process. It reports that an event occurred; the process can catch it and respond with a function.

Each signal has a name and a number. Common ones: SIGINT (interrupt via Ctrl+C), SIGTERM (termination request) and SIGKILL (forced kill). SIGKILL and SIGSTOP cannot be caught. Signals are defined in <signal.h>.

Signal sources

Signals can come from various sources: user (Ctrl+C → SIGINT), kernel (divide by zero → SIGFPE), another process (via kill) or the process itself (via raise).

A default action applies: terminate, core dump, ignore or stop. You can install your own function instead of the default; this is called a signal handler. You register it with signal or sigaction.

Exercise Print signal names

Print three signal names, one per line, with printf: printf("signal: SIGINT\n");, printf("signal: SIGTERM\n"); and printf("signal: SIGKILL\n");.

Show example solution

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

int main(void) {
    printf("signal: SIGINT\n");
    printf("signal: SIGTERM\n");
    printf("signal: SIGKILL\n");
    return 0;
}

Test yourself

Which signal is sent with Ctrl+C?
- SIGINT
- SIGTERM
- SIGKILL
- SIGHUP
Show answer

SIGINT — SIGINT is the user interrupt via Ctrl+C.
Signal header file
```
#include <____.h>
```
Show answer
```
#include <signal.h>
```
Match signals to meanings

SIGINT

Ctrl+C interrupt
SIGTERM

Termination request
SIGKILL

Forced kill
SIGHUP

Hangup detected

Next lesson: Signal handler

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