如何在几秒钟后终止 scanf？

Question

I'm using signals in C language. The program consists in wait a few seconds for a user keyboard input if the time ends, the program terminates. However, I always have to enter text although time was over if not the program never ends. Is there any way to avoid scanf?

Here is my code

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <stdlib.h>
#define NSECS 10
#define TRUE 1
#define FALSE 0
# define BELLS "\007\007\007"

int alarm_flag = FALSE;

void bye(){
    printf("Good bye...");
}

void setflag(){
    alarm_flag = TRUE;
}

int main() {
char name[20];
    do {
        

        printf("File name: \n"); 
        scanf("%s", name);

        signal(SIGALRM, setflag);
        alarm(NSECS);
        
        if (alarm_flag == TRUE){
            printf(BELLS);
            atexit(adios); 
            return 0;
        } // if the user enters a file name, the program continues 

           

Answer 1

You are almost there—set the alarm first, and call scanf afterwards. The signal will interrupt the call to read() inside scanf() , which causes scanf() to return immediately.

volatile int alarm_flag = 0;
void setflag(int sig) {
    alarm_flag = 1;
}

...

struct sigaction act = {};
act.sa_handler = set_flag;
sigaction(SIGALRM, &act, NULL);

...

    alarm(NSECS);
    scanf("%s", name);
    if (alarm_flag) {
        ...

A couple notes:

• alarm_flag should be volatile .

• setflag should take one int parameter.

• Declare your functions as func(void) not func() . Declaring a function as func() is an old-fashioned style from before 1990 or so and there is no benefit to using it today. (Note that C++ is different.)

More notes:

• You should not use == TRUE or == FALSE . In this particular case it may work fine, but there are cases where it doesn't. So I would almost never use == TRUE .

• As an exercise this code with alarm is reasonable, but if you want to do this kind of thing in a production application you would probably use something like libuv instead of alarm() . It's not that there's something wrong with this approach, just that using non-blocking IO and libuv would probably be easier to deal with.