What happens when piping the `yes` to another command?

Question

If the yes command echos "y" continuously until killed, then it will never get done right? If it will never get done then how does it pipe it's output to the next command??

Answer 1

The yes program will write to the pipe, concurrently with the reader. If the pipe is empty, the reader will block in the kernel in a call to read() waiting for more input. If the pipe is full, the write will block in the kernel in a call to write() waiting for the reader to free up some space in the pipe.

The kernel delivers a SIGPIPE signal to a process if it ties to write to a pipe that has no reader. When the reading process closes the read-end of the pipe (either explicilty or as a result of its termination), the next time the writing process tries to write to the pipe, it will receive the SIGPIPE signal.

To illustrate this point, consider this simplified version of the yes program that prints a continuous stream of y. This program differs from yes in that it generates a message when it receives the SIGPIPE signal:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

static void handler(int sig) {
    #define msg "received SIGPIPE signal, terminating\n"
    write(2, msg, sizeof msg);
    exit(1);
    #undef msg
}

int main(void) {
    // Instruct the kernel to call the handler() function when
    // this process receives the SIGPIPE signal.  The default behavior
    // when this signal is received is for the process to terminate.
    signal(SIGPIPE, handler);

    for (;;) {
        if (write(1, "y\n", 2) < 0) {
            fprintf(stderr, "%s\n", strerror(errno));
            return 2;
        }
    }
    return 0;
}

I can compile and run the program and see that it behaves like yes:

$ gcc ex.c -o myes

$ ./myes
y
y
y
...

If I pipe the output to another process, when that process terminates (and the read-end of the pipe is closed), the myes program receives the SIGPIPE signal (as indicated by the associated message).

$ ./myes | head -n5
y
y
y
y
y
received SIGPIPE signal, terminating