How To Allow Atomics Use In C++ Signal Handlers

C++11 allows the use of atomics in signal handlers, and with the advent of C++ 17 the signal handler feature was again improved. The std::atomic_flag is an atomic boolean type that is guaranteed to be lock-free and can be used in signal handlers. Moreover, the <csignal> header in C++ has an integer type std::sig_atomic_t that can be accessed as an atomic entity even in the presence of asynchronous interrupts made by signals. In this post, we explain how to use atomic_flag in C++.

How to allow atomics use in C++ signal handlers?

The <csignal> header in C++ (or signal.h in C) has an an integer type std::sig_atomic_t that can be accessed as an atomic entity even in the presence of asynchronous interrupts made by signals. In this post, we explain how to allow atomics use in C++ signal handlers.

Here is how we can use it:

#include <csignal>

volatile sig_atomic_t flag = 0;

Is there a full example of how to use atomics in C++ signal handlers?

Here is an example that has a signal handler and uses sig_atomic_t flag from <csignal>

#include <iostream>

#include <csignal>

volatile sig_atomic_t flag = 0;

void handler(int signum)

{

flag = 1 ;

}

bool signal_check()

{

if ( signal( SIGINT, handler) == SIG_ERR )

{

return false;

}

if (flag) std::cout << "Flag ON (signal received)\n";

else std::cout << "Flag OFF (signal not received)\n";

return true;

}

int main(void)

{

bool b;

b = signal_check();

std::cout << "Signal Check:" << b << std::endl;

raise (SIGINT);

b = signal_check();

std::cout << "Signal Check:" << b << std::endl;

system("pause");

return 0;

}

The output will be as follows:

Flag OFF (signal not received)

Signal Check:1

Flag ON (signal received)

Signal Check:1

Press any key to continue . . .

For more information on this feature, see Allow atomics use in signal handlers Proposal document.

If you want to know more about signals and error handlers, I recommend you “CERT C Coding Standard, Second Edition, The: 98 Rules for Developing Safe, Reliable, and Secure Systems”.
https://www.informit.com/store/cert-c-coding-standard-second-edition-98-rules-for-9780321984043

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How To Allow Atomics Use In C++ Signal Handlers