Question

I wait to start a process, pass an integer to it, and get a value which it prints out. I tried the following (in Windows):

public class Example {

    public static Boolean call() throws IOException {
        String mFilename = "f.exe";
        int mParam = 0;
        Process p = Runtime.getRuntime().exec(mFilename);
        BufferedReader input = new BufferedReader
                (new InputStreamReader(p.getInputStream()));
        BufferedWriter output = new BufferedWriter
                (new OutputStreamWriter(p.getOutputStream()));
        output.write(mParam);
        output.flush();
        char retVal = (char) input.read();
        return !(retVal == '0');
    }

    public static void main(String[] args) throws IOException {
        System.out.println(call());
    }
}

mFilename is a path to the executable. The process returns 1 for positive input, 0 for negative, and remains in an infinite loop if mParam is 0. However, I found out that no matter what value I pass to it, retVal is always 1. What am I doing wrong?

Excuse me for deleting the question a few minutes before, I thought the problem was in retVal but apparently it was not the only problem in this code.

Child process, written in C++:

#include <iostream>
#include <Windows.h>

bool f(int x)
{
    if (x)
        return x > 0;

    while (1)
        Sleep(100);
}

int main()
{
    int x;
    std::cin>>x;
    std::cout<<f(x);
}

Update:

I found at least one of the errors. Since mParam is an integer, which consists of an optional sign and a bunch of digits, and output.write writes characters, I need to write my integer as a character array:

char[] arr = String.valueOf(mParam).toCharArray();
output.write(arr);

However, regardless of the input, after this fix program hangs at

char retVal = (char) input.read();

Answer 1

Well, the solution turned out to be quite simple: std::cin just wanted a newline to successfully read the integer. Resulting code is as follows:

public Boolean call() throws IOException, InterruptedException, ExecutionException, TimeoutException {
    Process p = Runtime.getRuntime().exec(mFilename);
    final BufferedReader input = new BufferedReader
            (new InputStreamReader(p.getInputStream()));
    final BufferedWriter output = new BufferedWriter
            (new OutputStreamWriter(p.getOutputStream()));
    output.write(Integer.toString(mParam));
    output.newLine();
    output.flush();

    ExecutorService executor = Executors.newFixedThreadPool(2);
    Callable<Character> readTask = new Callable<Character>() {
        @Override
        public Character call() throws Exception {
            return Character.valueOf((char)input.read());
        }
    };
    Future<Character> future = executor.submit(readTask);
    char readVal;
    try {
        readVal = future.get(3L, TimeUnit.SECONDS);
    } finally {
        p.destroy();
        executor.shutdownNow();
    }

    return !(readVal == '0');
}

Note that I added support for the ‘infinite loop’-case scenario. If we are unable to get the output from stdout in 3 seconds, we throw exception and terminate the child process. I don’t process the exception here because the function has to return some result, but in the case of infinite loop I don’t have any result to return.