title: Sending input to GDB scanf using external program
body: <p>How do send an input to GDB whenever scanf is triggered using external program??</p>
  
  <p>c file:</p>
  
  <pre><code>#include&lt;stdio.h&gt;
  
  void main()
  {
      int x;
      int y;
      printf("input x: ");
      scanf("%d",&amp;x);
      printf("input y: ");
      scanf("%d",&amp;y);
  
  }
  </code></pre>
  
  <p>Java external program:</p>
  
  <pre><code>public class Debugger extends Thread{
  
          public void run(){
          Process p = null;
          try {
          p = Runtime.getRuntime().exec("gdb a.out --interpreter=console");
         new Thread(new SyncPipe(p.getErrorStream(), System.err)).start();
         new Thread(new SyncPipe(p.getInputStream(), System.out)).start();
          PrintWriter stdin = new PrintWriter(p.getOutputStream());
  
          stdin.flush();
          stdin.println("break main");
          stdin.flush();
          stdin.println("run");
          stdin.flush();
          stdin.println("s");
          stdin.flush();
          stdin.println("45");
          stdin.flush();
  
         // stdin.close();
  
  
          } catch (Exception e) {
              e.printStackTrace();
          }
          }
      }
  
  class SyncPipe implements Runnable
  {
  
      public SyncPipe(InputStream istrm, OutputStream ostrm) {
        istrm_ = istrm;
        ostrm_ = ostrm;
  
    }
  
    public void run() {
  
        try
        {   
            int length;
             byte[] buffer = new byte[1024];
  
            for ( length = 0; (length = istrm_.read(buffer)) != -1; ){
  
                ostrm_.write(buffer, 0, length);
            } 
  
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
  
  
    }
  
  
    private final OutputStream ostrm_;
    private final InputStream istrm_;
  }
  </code></pre>
  
  <p>GDB output from external program:</p>
  
  <pre><code>GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
  Copyright (C) 2011 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later &lt;http://gnu.org/licenses/gpl.html&gt;
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
  and "show warranty" for details.
  This GDB was configured as "i686-linux-gnu".
  For bug reporting instructions, please see:
  &lt;http://bugs.launchpad.net/gdb-linaro/&gt;...
  Reading symbols from /home/charmae/workspace/AVT/a.out...done.
  (gdb) Breakpoint 1 at 0x804843d: file file4.c, line 7.
  (gdb) Starting program: /home/charmae/workspace/AVT/a.out 
  
  Breakpoint 1, main () at file4.c:7
  7       printf("input of x: ");
  (gdb) 8     scanf("%d",&amp;x);
  (gdb) Undefined command: "45".  Try "help".
  (gdb) 
  </code></pre>