Communicate with C program using stdin/out from Java

Question

I want my Java program to communicate with a C program. This is just a simple example but I can\'t get it working. The Java program is supposed to run the C program and write to

Answer 1

The problem is that you forgot to add the new line character \n to the string that you are sending from Java thus fscanf keeps waiting indefinitely for the process to "press enter". I've made some modifications to the code and the changes are documented in comments.

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class Main {
    public static void main(String[] args) {
        try {
            // 1. run C program
            Process proc = Runtime.getRuntime().exec("./hello");
            InputStream in = proc.getInputStream();
            OutputStream out = proc.getOutputStream();
            // 2. write 'hello' to 'hello' program
            // <change>
            // don't forget to add the new line here or in the 
            // writeToProc method
            writeToProc(out, "hello\n");
            // 3. read response
            readFromProc(in);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    // write message to process
    public static void writeToProc(OutputStream out, String msg) throws IOException {
        // <change>
        // Using UTF-8 encoding since all chars in C are byte sized
        byte[] buff = msg.getBytes("UTF-8");
        out.write(buff);
        out.flush();
        System.out.println("done writing: " + new String(buff));
    }

    // read stdin of process
    public static void readFromProc(InputStream in) throws IOException {
        byte[] buff = new byte[256];
        int read = in.read();
        int index = 0;
        while(read != -1) {
            buff[index] = (byte) read;
            read = in.read();
            ++index;
        }
        String str = new String(buff, 0, index);
        System.out.println("proc says: " + str);
    }
}