RT-Signals in UNIX

August 23, 2025

This article explores signals in UNIX, focusing on real-time signals (RT-signals), their differences with normal signals, and how to send and handle them.

Introduction to Signals in UNIX

Signals are a mechanism in UNIX that allows processes to communicate with each other. Normal signals are limited, so real-time signals were created in POSIX.1b for more reliable communication.

Difference Between RT-Signals and Normal Signals

In normal signals, if multiple signals are sent before being handled, the kernel merges them and delivers only once. RT-signals, however, are queued individually in the kernel's sigqueue structure, allowing each signal to be handled separately. This is useful when each signal carries important data.

struct task_struct 
{
    …
    struct sigpending;
    …
};

struct sigpending
{
    sigset_t signal;
    struct list_head list;
    spinlock_t lock;
};

struct sigqueue 
{
    struct k_siginfo info;
    struct list_head list;
    struct task_struct * task;
};

Sending RT-Signals in Shell

Example:

$ kill -SIGRTMIN+3 {pid}

RT-Signal Range

You can use signals from SIGRTMIN to SIGRTMAX. Access them with SIGRTMIN+n or SIGRTMAX-n.

Sending RT-Signals with kill System Call

#include <signal.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    argc--; argv++;
    int pid = atoi(argv[argc]);
    kill(pid, SIGRTMIN + 3);
    return 0;
}

Handling RT-Signals

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

void handler(int signum)
{
    printf("signal received\n");
    exit(0);
}

int main()
{
    signal(SIGRTMIN + 1, handler);
    printf("pid : %d\n", getpid());
    while(1)
        pause();
}

You can send the signal with:

$ kill -SIGRTMIN+1 {pid}

Summary

Real-time signals allow sending multiple signals without losing any, making them useful when signal delivery and data are critical.

Check out a small kill command using the kill system call: GitHub - shellStone/kill

RT-Signals in UNIX: A Deep Dive