pthread callbacks interupt user input

Question

I have written my own stop_watch module. That will create a thread and go to sleep for a period of seconds. Once the seconds have expired it will call a callback function in the main.c and inform the user the time has expired.

This is so that the user will only have 3 seconds to enter a digit and they will have to enter 5 digits. If the time expires the program has to stop.

2 problems. 1) if they enter the digit in the required time. How can I cancel the thread. I was thinking of using thread_kill or thread_cancel? 2) How can I terminate the in the do_while loop? As the scanf will block while waiting for the user to enter.

Many thanks for any suggestions,

My code below:

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

#include "stop_watch.h"

struct data_struct *g_data_struct;

void timeout_cb(int id)
{
    printf("Digit timeout\n");
    free(g_data_struct);
}

int main()
{
    pthread_t thread_id;
    unsigned int digit = 0;

    g_data_struct = (struct data_struct*) calloc(1, sizeof(*g_data_struct));

    if(!g_data_struct)
    {
        printf("=== failed to allocate memory ===\n");
        return 0;
    }

    /* start timer for 3 seconds */
    g_data_struct->seconds = 3;
    g_data_struct->func_ptr = timeout_cb;
    thread_id = start_stopwatch(g_data_struct);

    do
    {
        printf("Enter digit: ");
        scanf("%d", &digit);
    }while(1);

    pthread_join(thread_id, NULL);
    printf("=== End of Program - all threads in ===\n");

    free(g_data_struct);

    return 0;
}



#include <stdio.h>
#include <pthread.h>

#include "stop_watch.h"

pthread_t thread_id;
static id = 10;

/* start sleeping and call the callback when seconds have expired */
static void* g_start_timer(void *args)
{
    void (*function_pointer)(int id);
    int seconds = ((struct data_struct*) args)->seconds;
    function_pointer = ((struct data_struct*) args)->func_ptr;

    sleep(seconds);

    (void) (*function_pointer)(id);

    pthread_exit(NULL);

    return 0;
}

/* Will sleep in its own thread for a period of seconds */
int start_stopwatch(struct data_struct *g_data_struct)
{
    int rc = 0;

    int seconds = g_data_struct->seconds;
    printf("=== start_stopwatch(): %d\n", seconds);

    rc =  pthread_create(&thread_id, NULL, g_start_timer, (void *) g_data_struct);

    if(rc)
    {    
        printf("=== Failed to create thread\n");
        return 1;
    }

    return thread_id;
}

This question is about C99 gcc, by the way.

Answer 1

I wouldn't recommend using a separate thread for this in the first place. But since it looks like an exercise to learn how to use threads, I'll go with it.

You can set a global boolean flag that indicates whether a digit was entered. In g_start_timer, just before calling function_pointer, check whether the flag has been set. If it was, don't call the function.

If you want the thread to die earlier, before its sleep period has expired, you can use a mutex with a try lock.

EDIT: Since you say it's not an exercise, I'll mention how I would do it. Either use a function that can time out while waiting for input on a stdin, such as select() or poll(). Or, use alarm() to interrupt. Here's an example of using alarm:

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

void alarm_signal(int signum)
{
}

int main()
{
    char c;
    unsigned int v = 0;
    struct sigaction act;
    puts("Enter digit: ");

    /* Register dummy handler for alarm */
    act.sa_handler = &alarm_signal;
    act.sa_flags = 0;
    sigemptyset(&act.sa_mask);
    sigaction(SIGALRM, &act, NULL);

    /* Set timer for 3 seconds */
    alarm(3);

    /* Read the digits */
    errno = 0;
    while(0 != read(STDIN_FILENO, &c, 1))
    {
        if(c < '0' || c > '9')
            break;
        v = v*10 + c - '0';
    }
    if(errno == EINTR)
        puts("Timed out");
    else
        printf("Value is %d\n", v);
    return 0;
}

Answer 2

Of course you can use thread_cancel to cancel a thread. As for the scanf thing, you might think of using pipes to simulate an input from the terminnal.