kill
- C course
- Advanced
- Lesson 16/83
- 5 min
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.
Practice 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);.
Starter code
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
int caught = 0;
// Write handler function
// Register with signal
// kill to self and print
int main(void) {
return 0;
}Sample 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;
}Expected output
caught=1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does kill(getpid(), SIGUSR1) do?
- Sends to another process
- Blocks the signal
- Sends to the process group
- Sends to itself Correct answer
Explanation: kill(getpid(), ...) sends a signal to the process itself.
2 Match sending concepts
| Term | Meaning |
|---|---|
kill |
Sends signal |
getpid |
Own PID |
raise |
Signal to self |
SIGTERM |
Termination request |
3 kill call
kill(____(), SIGUSR1);kill(getpid(), SIGUSR1);Explanation: kill(getpid(), SIGUSR1) sends a signal to itself.