1. Signal Fundamentals

Signals in Linux are software interrupts delivered to a process by the operating system or other processes. They serve as notifications that a particular event has occurred, requiring attention or action from the receiving process.

When a process receives a signal, it can:

• Execute the default action associated with that signal
• Handle the signal with a custom function (signal handler)
• Ignore the signal (if permitted)

2. Purpose of Signals

Signals serve several important purposes in Linux systems:

• Inter-Process Communication (IPC): Processes can send signals to each other to comumnicate events. For example, a parent process might send a signal to a child proces to instruct it to terminate.
• Exception Handling: When a process performs an illegal operation (such as accessing invalid memory or dividing by zero), the kernel sends an appropriate signal to notify the process of the error.
• System Management: System administrators use signals to control process behavior, such as terminating unresponsive processes.

3. Types of Signals

Linux defines two categories of signals:

• Standard Signals: These are numbered from 1 to 31 and represent the traditional Unix signals. They are supported by all Unix-like systems.
• Real-time Signals: Numbered from 32 to 64, these are Linux-specific extensions that provide additional functionality for real-time applications.

To view all available signals on your system, execute:

kill -l

4. Common Signals and Their Meanings

Below are some of the most frequently encountered signals in Linux programming:

Signal	Number	Description	Default Action
SIGHUP	1	Hangup detected on controlling terminal	Terminate
SIGINT	2	Interrupt from keyboard (Ctrl+C)	Terminate
SIGQUIT	3	Quit from keyboard (Ctrl+\)	Terminate (core dump)
SIGILL	4	Illegal instruction	Terminate (core dump)
SIGABRT	6	Abort signal from abort()	Terminate (core dump)
SIGFPE	8	Floating-point exception	Terminate (core dump)
SIGKILL	9	Kill signal (cannot be caught or ignored)	Terminate
SIGSEGV	11	Invalid memory reference	Terminate (core dump)
SIGPIPE	13	Broken pipe: write to pipe with no readers	Terminate
SIGALRM	14	Timer signal from alarm()	Terminate
SIGTERM	15	Termination signal	Terminate
SIGCHLD	17	Child process stopped or terminated	Ignore
SIGCONT	18	Continue if stoped	Continue
SIGSTOP	19	Stop process (cannot be caught or ignored)	Stop
SIGTSTP	20	Stop typed at terminal (Ctrl+Z)	Stop

Key actions include:

• Term: Terminate the process
• Core: Terminate the process and generate a core dump for debugging
• Ignore: Ignore the signal
• Cont: Continue the process if it was stopped
• Stop: Stop the process (pause execution without terminating)

5. Signal Handling in Programming

In C programming, the signal() function is used to define how a process should handle a specific signal:

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

void handle_sigint(int sig) {
    printf("Caught signal %d (SIGINT)\n", sig);
    // Perform cleanup or other actions here
}

int main() {
    // Register the signal handler
    signal(SIGINT, handle_sigint);
    
    printf("Waiting for SIGINT (Ctrl+C)...\n");
    while(1) {
        sleep(1); // Infinite loop waiting for signal
    }
    
    return 0;
}

For more robust signal handling, especially in complex applications, consider using the sigaction() system call, which provides more control and reliability than the simpler signal() function.