signal.h - Signals

Category: 📂 std/ (C Standard Library)
Header: <signal.h>
Scope: C89, C99, C11

The signal.h header declares a type and two functions and defines several macros, for handling various signals during program execution.

Facilities Overview

Facility Category	Key Symbols	Description
Signal Handling	signal, raise	Registering handlers and sending signals.
Standard Signals	SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, SIGTERM	Predefined signal types.
Signal Actions	SIG_DFL, SIG_IGN	Default and ignore actions for signals.

Types & Variables

sig_atomic_t

typedef int sig_atomic_t;

An integer type that can be accessed as an atomic entity, even in the presence of asynchronous signals. Commonly int or volatile int.

Signal Actions

SIG_DFL

#define SIG_DFL /* implementation-defined */

Requests the default signal handling action.

SIG_IGN

#define SIG_IGN /* implementation-defined */

Requests that the signal be ignored.

Functions

Signal Handling Functions

signal

void (*signal(int sig, void (*func)(int)))(int)

Sets a handler for signal sig. The func can be a pointer to a function, or one of the macros SIG_DFL (default action) or SIG_IGN (ignore signal).

Returns: The previous value of the signal handler, or SIG_ERR on error (sets errno).

Example

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

void handler(int sig) {
    printf("Caught signal %d\n", sig);
    exit(0);
}

int main(void) {
    // Always check if signal registration failed
    if (signal(SIGINT, handler) == SIG_ERR) {
        perror("Failed to register SIGINT handler");
        return 1;
    }
    
    if (signal(SIGTERM, SIG_IGN) == SIG_ERR) {
        perror("Failed to ignore SIGTERM");
        return 1;
    }
    
    printf("Waiting for signals...\n");
    while(1); 
    return 0;
}

raise

int raise(int sig)

Sends the signal sig to the executing program.

Returns: 0 if successful, non-zero if unsuccessful.

Example

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

void handler(int sig) {
    printf("Signal %d raised\n", sig);
}

int main(void) {
    if (signal(SIGUSR1, handler) == SIG_ERR) {
        perror("signal");
        return 1;
    }

    if (raise(SIGUSR1) != 0) {
        fprintf(stderr, "Failed to raise signal\n");
        return 1;
    }
    
    return 0;
}

Standard Signals

SIGABRT

#define SIGABRT /* implementation-defined */

Abnormal termination signal, such as initiated by abort().

SIGFPE

#define SIGFPE /* implementation-defined */

Erroneous arithmetic operation (e.g., division by zero).

SIGILL

#define SIGILL /* implementation-defined */

Invalid executable code.

SIGINT

#define SIGINT /* implementation-defined */

Interactive attention signal (usually Ctrl+C).

SIGSEGV

#define SIGSEGV /* implementation-defined */

Invalid access to storage (segmentation violation).

SIGTERM

#define SIGTERM /* implementation-defined */

Termination request sent to the program.

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