kill

Learn to send signals with kill.

The kill function

kill(pid, sig) sends a signal to a specific process. A positive pid targets that process, 0 targets the process group, -1 targets all processes the caller owns.

It returns 0 on success, -1 on error. To send a signal to itself, a process uses kill(getpid(), sig); same as raise(sig). The kill command-line tool uses the same system call.

Signal sending patterns

A parent can send a termination request to a child with kill(child_pid, SIGTERM). Two processes can use SIGUSR1 and SIGUSR2 for custom signaling.

Before sending, make sure the receiver has a handler installed; otherwise the default action (usually termination) applies. Calling kill with signal number 0 checks if the process exists; it sends no signal but tests permissions.

Exercise Send to self with kill

Define int caught = 0; and void handler(int sig) with caught = 1;. Register with signal(SIGUSR1, handler);. Send to self with kill(getpid(), SIGUSR1);. Print with printf("caught=%d\n", caught);.

Show example solution

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

int caught = 0;

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

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

Test yourself

What does kill(getpid(), SIGUSR1) do?
- Sends to another process
- Blocks the signal
- Sends to the process group
- Sends to itself
Show answer

Sends to itself — kill(getpid(), ...) sends a signal to the process itself.
Match sending concepts

kill

Sends signal
getpid

Own PID
raise

Signal to self
SIGTERM

Termination request

kill call

kill(____(), SIGUSR1);

Show answer

kill(getpid(), SIGUSR1);

Next lesson: Signal safety

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