Timeout Function

问题:

I want to make a code in which Name of User will be asked to input, but in a time limit of 15 seconds. If user cross the limit & failed to input a name(or any string), then code will be terminated & "Time Out" Massage will be display otherwise Name should be saved & "Thanks" massage will be display. I had try like this but it's wrong & not working. Please give me a solution for this.. Thanks.

#include <stdio.h>
#include <time.h>

int timeout ( int seconds )
{
    clock_t endwait;
    endwait = clock () + seconds * CLOCKS_PER_SEC ;
    while (clock() < endwait) {}

    return  1;
}

int main ()
{
    char name[20];
    printf("Enter Username: (in 15 seconds)\n");
    printf("Time start now!!!\n");

    scanf("%s",name);
    if( timeout(5) == 1 ){
        printf("Time Out\n");
        return 0;
    }

    printf("Thnaks\n");
    return 0;
}

回答1:

Probably this dummy program might help you:

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>

#define WAIT 3

int main ()
{
    char            name[20] = {0}; // in case of single character input
    fd_set          input_set;
    struct timeval  timeout;
    int             ready_for_reading = 0;
    int             read_bytes = 0;

    /* Empty the FD Set */
    FD_ZERO(&input_set );
    /* Listen to the input descriptor */
    FD_SET(0, &input_set);

    /* Waiting for some seconds */
    timeout.tv_sec = WAIT;    // WAIT seconds
    timeout.tv_usec = 0;    // 0 milliseconds

    /* Invitation for the user to write something */
    printf("Enter Username: (in %d seconds)\n", WAIT);
    printf("Time start now!!!\n");

    /* Listening for input stream for any activity */
    ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
    /* Here, first parameter is number of FDs in the set, 
     * second is our FD set for reading,
     * third is the FD set in which any write activity needs to updated,
     * which is not required in this case. 
     * Fourth is timeout
     */

    if (ready_for_reading == -1) {
        /* Some error has occured in input */
        printf("Unable to read your input\n");
        return -1;
    } 

    if (ready_for_reading) {
        read_bytes = read(0, name, 19);
        if(name[read_bytes-1]=='\n'){
        --read_bytes;
        name[read_bytes]='\0';
        }
        if(read_bytes==0){
            printf("You just hit enter\n");
        } else {
            printf("Read, %d bytes from input : %s \n", read_bytes, name);
        }
    } else {
        printf(" %d Seconds are over - no data input \n", WAIT);
    }

    return 0;
}

Update:
This is now tested code.

Also, I have taken hints from man for select. This manual already contains a code snippet which is being used to read from the terminal and timeout in 5 seconds in case of no activity.

Just a brief explanation in case the code is not well written enough:

1. We add the input stream (fd = 1) to the FD set.
2. We initiate select call to listen to this FD set created for any activity.
3. In case any activity occurs within the timeout period, that is read through the read call.
4. In case there was no activity, timeout occurs.

Hope this helps.

回答2:

scanf() is not the best function to get an input in a limited time frame.

Instead I would build a specific input function around select() (for managing timeout) and read() (for getting input) system calls.

回答3:

One thing you have to think about, is that you have a single thread of execution in your program. As such, the timeout function will only be called when the scanf function will be terminated. This is not what you want.

One way to do this task, is to use the select function. It waits for a potentially limited amount of time (your timeout) for availability of input on some file descriptors (stdin for you).