Input within a time limit in Standard C

Question

I\'m currently doing my assignment and it\'s compulsory to use C-Free 5.0. Just need your help to solve this piece of puzzle. I want to implement a time limit for the user t

Answer 1

Since you have fcntl.h try setting stdin to non-blocking. It's not pretty (active waiting), but if you do not have select then this is the easyest way to go:

#include <stdio.h> 
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>

int main()
{
    // get stdin flags
    int flags = fcntl(0, F_GETFL, 0);
    if (flags == -1) {
        // fcntl unsupported
        perror("fcntl");
        return -1;
    }

    // set stdin to non-blocking
    flags |= O_NONBLOCK;
    if(fcntl(0, F_SETFL, flags) == -1) {
        // fcntl unsupported
        perror("fcntl");
        return -1;
    }

    char st[1024] = {0}; // initialize the first character in the buffer, this is generally good practice
    printf ("Please enter a line of text : ");
    time_t end = time(0) + 5; //5 seconds time limit.

    // while
    while(time(0) < end  // not timed out
        && scanf("%s", st) < 1 // not read a word
        && errno == EAGAIN); // no error, but would block

    if (st[0]) // if the buffer contains something
        printf ("Thank you, you entered >%s<\n", st);

    return 0;
}

A remark to your code: if (st != NULL) will always be satisfied since st is a stack pointer.

Answer 2

Here is an example program that shows how you can use O_NONBLOCK flag on a stdin file descriptor.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define INPUT_LEN 10

int main()
{
    printf ("Please enter a line of text : ");
    fflush(stdout);
    time_t end = time(0) + 5; //5 seconds time limit.

    int flags = fcntl(STDIN_FILENO, F_GETFL, 0);
    fcntl(STDIN_FILENO, F_SETFL, flags | O_NONBLOCK);

    char answer[INPUT_LEN];
    int pos = 0;
    while(time(0) < end)
    {
        int c = getchar();

        /* 10 is new line */
        if (c != EOF && c != 10 && pos < INPUT_LEN - 1)
            answer[pos++] = c;

        /* if new line entered we are ready */
        if (c == 10)
            break;
    }

    answer[pos] = '\0';

    if(pos > 0)
        printf("%s\n", answer);
    else
        puts("\nSorry, I got tired waiting for your input. Good bye!");
}