C signal handling

In the C Standard Library, a signal processing defines how a program handles various signals while it executes. A signal can report some exceptional behavior within the program (such as division by zero), or a signal can report some asynchronous event outside the program (such as someone striking an interactive attention key on a keyboard).

Standard signals

The C standard defines only 6 signals. They are all defined in signal.h header (csignal header in C++):

• SIGABRT - abnormal termination.
• SIGFPE - floating point exception.
• SIGILL - invalid instruction.
• SIGINT - interactive attention request sent to the program.
• SIGSEGV - invalid memory access.
• SIGTERM - termination request sent to the program.

Additional signals may be specified in the signal.h header by the implementation. For example, Unix and Unix-like operating systems (such as Linux) defines more than 15 additional signals.

Handling

A signal can be generated by calling raise or kill system calls. raise sends a signal to the current process, kill sends a signal to a specific process.

A signal handler can be specified for all but two signals (SIGKILL and SIGSTOP cannot be caught, blocked or ignored). A signal handler is a function that the target environment calls when the corresponding signal occurs. The target environment suspends execution of the program until the signal handler returns or calls longjmp. For maximum portability, an asynchronous signal handler should only:

• make calls (that succeed) to the function signal
• assign values to objects of type volatile sig_atomic_t
• return control to its caller

If the signal reports an error within the program (and the signal is not asynchronous), the signal handler can terminate by calling abort(), exit(), or longjmp().

Functions

Function	Description
raise	artificially raises a signal
signal	sets the action taken when the program receives a specific signal

Example usage

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

static void catch_function(int signal) {
    puts("Interactive attention signal caught.");
}

int main(void) {
    if (signal(SIGINT, catch_function) == SIG_ERR) {
        fputs("An error occurred while setting a signal handler.\n", stderr);
        return EXIT_FAILURE;
    }
    puts("Raising the interactive attention signal.");
    if (raise(SIGINT) != 0) {
        fputs("Error raising the signal.\n", stderr);
        return EXIT_FAILURE;
    }
    puts("Exiting.");
    return 0;
}

External links

• XGC's manual for signal.h