Java subprocess does not write its output until it completes

I wrote this simple test file that describes the problem I am facing:

I am creating a subprocess from Java and simultaneously starting a thread that MUST write each line as soon as it reads from the standard output of the subprocess.

Instead, I get that the output of the subprocess is completely written when it exits. Here's the result:

Mon Jul 15 19:17:13 CEST 2013: starting process
Mon Jul 15 19:17:14 CEST 2013: process started
Mon Jul 15 19:17:14 CEST 2013: waiting for process termination
Mon Jul 15 19:17:14 CEST 2013: readerThread is starting
Mon Jul 15 19:17:19 CEST 2013: process terminated correctly
Mon Jul 15 19:17:19 CEST 2013: Thread[Thread-0,5,main] got line: foo(7)
Mon Jul 15 19:17:19 CEST 2013: Thread[Thread-0,5,main] got line: foo(49)
Mon Jul 15 19:17:19 CEST 2013: Thread[Thread-0,5,main] got line: foo(73)
Mon Jul 15 19:17:19 CEST 2013: Thread[Thread-0,5,main] got line: foo(58)
Mon Jul 15 19:17:19 CEST 2013: Thread[Thread-0,5,main] got line: foo(30)
Mon Jul 15 19:17:19 CEST 2013: readerThread is terminating

      

        
        
        
      

    

with this code:

public class MiniTest {
    static void println(String x) {
        System.out.println(new Date() + ": " + x);
    }

    public static void main(String[] args) throws IOException, InterruptedException {
        ProcessBuilder pb = new ProcessBuilder("bin/dummy", "foo", "5");

        println("starting process");
        Process p = pb.start();
        println("process started");

        new ReaderThread(p).start();

        println("waiting for process termination");
        p.waitFor();
        println("process terminated correctly");
    }

    static class ReaderThread extends Thread {
        private Process p;

        public ReaderThread(Process p) {
            this.p = p;
        }

        public void run() {
            println("readerThread is starting");
            BufferedReader r = new BufferedReader(new InputStreamReader(p.getInputStream()));
            String line;
            try {
                while((line = r.readLine()) != null) {
                    println(this + " got line: " + line);
                }
            } catch(IOException e) {
                println("read error: " + e);
            }
            println("readerThread is terminating");
        }
    }
}

      

        
        
        
      

    

Note: the subprocess is very simple, it prints out a line every second, for a certain number of iterations (and when testing on the command line, it does this):

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
    char *f = argv[1];
    int n = atoi(argv[2]);
    while(n-- > 0) {
        printf("%s(%d)\n", f, rand() % 100);
        sleep(1);
    }
    return 0;
}