Question

I have an exercise where I am required to print a file slowly (1 second intervals) until the file ends, unless the user types a character.

So far, the program outputs the file in one second intervals which is great, but when I type a character, nothing happens. My guess is that I am using select wrong somehow.

This is the final program I ended up submitting.

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

int main(void)
{
    FILE* infile;
    char str[100];
    fd_set readset;
    struct timeval tv;

    // open a file
    if((infile = fopen("infile", "r")) == NULL)
    {
        (void)printf("Couldn't open the file\n");
        exit(1);
    }
    // file was opened successfully
    else
    {       
        // while we are not at the end of a file
        while(fgets(str, 100, infile) != NULL)
        {
            FD_ZERO(&readset);
            FD_SET(fileno(stdin), &readset);
            // set the time value to 1 second
            tv.tv_sec = 1;
            tv.tv_usec = 0;
            select(fileno(infile)+1, &readset, NULL, NULL, &tv);
            // the user typed a character so exit
            if(FD_ISSET(fileno(stdin), &readset))
            {
                fclose(infile);
                exit(0);
            }
            // the user didn't type a character so print the next line
            else
            {
                fgets(str, 100, stdin);
                puts(str);
            }
        }

        // clean up
        fclose(infile);
    }

    // report success
    return 0;
}

Thanks for the help!

Answer 1

This is a working version, using tcgetattr/tcsetattr:

#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

int main(void) {
    FILE* infile;
    char str[100];
    fd_set readset;
    struct timeval tv;
    struct termios ttystate, ttysave;

    // open a file
    if((infile = fopen("infile", "r")) == NULL)
    {
        (void)printf("Couldn't open the file\n");
        exit(1);
    }
    // file was opened successfully

    //get the terminal state
    tcgetattr(STDIN_FILENO, &ttystate);
    ttysave = ttystate;
    //turn off canonical mode and echo
    ttystate.c_lflag &= ~(ICANON | ECHO);
    //minimum of number input read.
    ttystate.c_cc[VMIN] = 1;

    //set the terminal attributes.
    tcsetattr(STDIN_FILENO, TCSANOW, &ttystate);

    // while we are not at the end of a file
    while(fgets (str, 100, infile))
    {
        // set the time value to 1 second
        tv.tv_sec = 1;
        tv.tv_usec = 0;

        FD_ZERO(&readset);
        FD_SET(fileno(stdin), &readset);

        select(fileno(stdin)+1, &readset, NULL, NULL, &tv);
        // the user typed a character so exit
        if(FD_ISSET(fileno(stdin), &readset))
        {
            fgetc (stdin); // discard character
            break;
        }
        // the user didn't type a character so print the next line
        else
        {
            puts(str);
            // not needed: sleep(1);
        }
    }

    // clean up
    fclose(infile);

    ttystate.c_lflag |= ICANON | ECHO;
    //set the terminal attributes.
    tcsetattr(STDIN_FILENO, TCSANOW, &ttysave);
    // report success
    return 0;
}

The sleep(1); isn’t needed anymore.