Understanding signals in Linux §

What is a process ID? §

A process ID a.k.a. PID is literally what the name says, it is a number to uniquely identify a running process. You can print your program’s PID in C using the getpid() function included on the header file unistd.h.

int main(void)
{
	while (1)
	{
		printf("PID: %d\n", getpid());
		sleep(1);
	}
}

output: “ PID: 12345 ”

note: “12345” is a PID for an arbitrary process.

What is a signal? §

A signal is a one-way message to inform that something important happened sent by a process to a process, the kernel to the process, or a process to itself. Some examples of signals are SIGINT and SIGSTOP mapped to “ctrl-C” and “ctrl-Z” respectively on Unix-like Operating Systems.

Sending signals: §

You can send a signal with the command kill through the command line specifying as the first parameter the signal you want to send, and as the second parameter the PID of the process you want to send it to.

kill -INT 12345

or in C (don’t forget to include the header file signal.h):

kill(12345, SIGINT);

---

Handling signals: §

You can use the signal() function in C to handle a specific signal defined as the first parameter in the signal() function call, and pass the address of a function you would like to run when the specified signal is received.

signal(SIGINT, &sigint_handler);

Now, I will define the sigint_handler() function as:

void sigint_handler(int signal_number)
{
	printf("sigint's signal number is %d\n", signal_number);
}

The function above will be run when SIGINT (when the user presses ctrl-C or uses the kill program/function to send a signal) is sent. It will simply print the signal number for SIGINT based

on the table shown on the manual page for signal. To see it, just run:

man signal

By the way, there are some pre-existing functions that you can pass to signal() such as SIG_IGN (to ignore a signal) and SIG_DFL (for default handling of a certain signal).

Usage:

signal(SIGINT, SIG_IGN);