在C中使用信号处理函数后，如何避免scanf？

Question

I have this code:

#include <stdio.h>
#include <signal.h>
void signal_handler(int signal) {
    printf("Caught signal in CHILD.\n");
}

int main(void) {
    int s;
    signal(SIGTSTP, signal_handler);
    while(1){
         printf("%s@%s/# ",getlogin(),get_current_dir_name());
         scanf("%d",&s);
    }
    return 0;
}

when i run the code it prints:

something: ^ZCaught signal in CHILD.

As far i understand that the scanf doesn't execute when i press the ctr-z. Although after the printf inside my function it goes straight to the scanf, waits for input and then starts the loop again.Is there any way to avoid scanf when i press ctr-z and start the while loop again? I tried something like that

void signal_handler(int signal) {
    printf("Caught signal in CHILD.\n");
    printf("%s@%s/# ",getlogin(),get_current_dir_name());
}

but it didn't work. After the second printf goes straight to the scanf, waits for input and then starts the loop again. Can i, somehow, start the loop again?

Answer 1

The signal handler is interrupting scanf during its read of STDIN. However, because of the way you set signal disposition, the read system call restarts immediately upon return of the signal handler. That's why you are "stuck" in the scanf rather than back at the top of your loop.

One important thing you can do is to use sigaction rather than signal . This will force you to specify the behavior of interrupted calls: restart them or not?

The next thing to do is to limit your signal handlers to functions that are async-signal-safe , lest you risk misery .

As an aside, another change to make is to give us all the required includes ( <unistd.h> ?) and defines ( _GNU_SOURCE ?) to make your program work.

Answer 2

As commented the worst solution should be:

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

void signal_handler(int signal) {
    printf("Caught signal in CHILD.\n");
    exit(1);

}

int main(void) {
    int s;
    signal(SIGTSTP, signal_handler);
    while(1){
         printf("test\n");
         scanf("%d",&s);
    }
    return 0;
}

Better solution

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

static volatile int keepRunning = 1;

void signal_handler(int signal) {
    printf("Caught signal in CHILD.\n");
    keepRunning = 0;

}

int main(void) {
    int s;
    signal(SIGTSTP, signal_handler);
    while(keepRunning){
         printf("test\n");
         scanf("%d",&s);
    }
    return 0;
}

EDIT after comments

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

static volatile int skipPrintf= 1;

void signal_handler(int signal) {
    printf("Caught signal in CHILD.\n");
    skipPrintf= 1;

}

int main(void) {
    int s;
    signal(SIGTSTP, signal_handler);
    while(1){
         if (skipPrintf == 0)
         {
             printf("test\n");
         }
         else
         {
             skipPrintf = 0;
         }
         scanf("%d",&s);
    }
    return 0;
}