End while loop when 'Enter'

Question

I am trying to make a loop that will run infinitely without user input, until 'Enter' is pressed. This is the simplified version of my program:

do {

printf("Hello\n");

}while(!getchar());

When I compile and run the program only output one 'Hello' and the program will not continue, until Enter is pressed then the program will exit. May I know which part am I wrong?

Thank you.

Answer 1

It is not so easy. Your problem is that the standard I/O functions are synchronous. Your getchar is waiting for some input (a line of input to be precise) and it blocks execution of program until Enter is pressed. To continue execution without blocking you need to use asynchronous I/O operations or select (or poll). Select allows you to detect whether the next I/O operation would block or not. Look at the documentation of select and try this:

#include<stdio.h>
#include<unistd.h>
#include <sys/select.h>

int main() {
    fd_set          s;
    struct timeval  timeout;

    timeout.tv_sec = 0;
    timeout.tv_usec = 100000;

    do {
        printf("Hello\n"); 
        fflush(stdout);
        FD_ZERO(&s);
        FD_SET(STDIN_FILENO, &s);
        select(STDIN_FILENO+1, &s, NULL, NULL, &timeout);
    } while (FD_ISSET(STDIN_FILENO, &s) == 0);

}