How to mask a signal when I handle a variable in signal _handler and in main function

Question

I am trying to copy the contents from one file to another, with SIGINT the program gets interrupted to print the number of bytes copied. I tried to use the sigprocmask while the flag initialization, flag check, and flag clear to avoid a race condition. But I don't know how to check whether this sigprocmask works or not. I have been trying to find out quite a long time for this.

void signal_handler(int num)
{
    flag = 1;
}
int main()
{
    signal(SIGINT, signal_handler);
    ret = sigaddset(&set, SIGINT);  
     /* Code for 
      * copying the bytes from one file to another
      */
    sigprocmask(SIG_BLOCK, &set, NULL);         
    if (flag == 1)
        printf("The number of bytes copied are :%llu\n", bytes); 
    flag = 0;
    sigprocmask(SIG_UNBLOCK, &set, NULL);
    }
}

Answer 1

It should work as you expect. The only thing you need to ensure is that flag is of type volatile sig_atomic_t to avoid data race on flag.

Here's an example. The loop prints the value of bytes continuously (it'd wrap around at some point when bytes reaches UINT64_MAX). You can repeatedly send SIGINT to test.

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

volatile sig_atomic_t flag = 0;

void signal_handler(int num)
{
    flag = 1;
}

int main(void)
{
    uint64_t bytes = 0;
    sigset_t set;

    signal(SIGINT, signal_handler);
    int ret = sigaddset(&set, SIGINT);

    while(1) {
        bytes++;
        sigprocmask(SIG_BLOCK, &set, NULL);
        if (flag == 1)
            printf("The number of bytes copied are :%" PRIu64 "\n", bytes);
        flag = 0;
        sigprocmask(SIG_UNBLOCK, &set, NULL);
    }
}