How to “try” to read input in C

Question

Im writing a program which allows to chat between two processes in Linux. To transfer messages I use IPC queues.

I have a problem with main loop: I need to check if there's any new message in the queue and if there is - print it. Then I need to check if there is any input, and if there is - scanf it (this is the problem). Any ideas?

Answer 1

Use non blocking operations. If a read() is performed on a file descriptor opened with O_NONBLOCK flag, and there's no data available at that moment, read() will return inmediately with errno = -EWOULDBLOCK .

Another option is to use select() to poll more than one descriptor.

Answer 2

to add more value to my post I'm pasting an example I found, which solves my problem

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

int main(void)
{
    fd_set rfds;
    struct timeval tv;
    int retval;

    /* Watch stdin (fd 0) to see when it has input. */
    FD_ZERO(&rfds);
    FD_SET(0, &rfds);

    /* Wait up to five seconds. */
    tv.tv_sec = 5;
    tv.tv_usec = 0;

    retval = select(1, &rfds, NULL, NULL, &tv);
    /* Don't rely on the value of tv now! */

    if (retval == −1)
        perror("select()");
    else if (retval)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
    else
        printf("No data within five seconds.\n");

    exit(EXIT_SUCCESS);
}