Android - Using pipe between native and java apps

Question

I'm developing on SGS2 api v.16

I have two applications: native and Java

From the native app I open a Unix pipe with mkfifo() function and write some string to it.

And in the java app I'm trying to read the string, but somehow the app blocks and I don't know why, no indication in the logcat for why it blocked.

The 2 apps have android:sharedUserId="my.Id" in the manifest, and the share worked for sure.

In the logcat I can see the "opening input stream" log, but no log after that.. Is there a recommended way to read from a pipe file in Java? NOTE: if I open a regular file with open(...) instead of using mkfifo(...) I succeed to read from the file, the problem is only when I open a pipe file with mkfifo()

Here is the function runs from the native app:

jint Java_com_example_ndkfoo_NdkFooActivity_writeToPipe(JNIEnv* env, jobject javaThis) {
   const char* PATH = "/data/data/com.example.ndkfoo/v_pipe8";
   char* line = "Hello Pipe!";
   int pipe;
   errno = 0;

   // open a named pipe
   mode_t mode = S_IRWXU | S_IRWXG | S_IRWXO;
   pipe = mkfifo(PATH, mode);
   if(errno != 0) {
        __android_log_write(ANDROID_LOG_ERROR, "NDK_FOO_TAG", strerror(errno));
        return errno;
    }
    __android_log_write(ANDROID_LOG_DEBUG, "NDK_FOO_TAG", "file opened successfully");

   // actually write out the data and close the pipe
   int err = write(pipe, line, strlen(line));

    // close the pipe
   close(pipe);
   return err;

}

And here is the java that tries to read from the pipe

package com.example.secondparty;

import java.io.FileDescriptor;
import java.io.FileInputStream;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;

public class MainActivity extends Activity {
     private static final String TAG = MainActivity.class.getSimpleName();
     //public final static String PATH2 = "/sdcard/fifo9001";
     public final static String PATH = "/data/data/com.example.ndkfoo/v_pipe8";

     @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        readFromPipe();

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    private void readFromPipe() {
        Log.i(TAG, "Attempt started");
        FileInputStream fis;
        String message = "";
        byte buffer[] = new byte[256];

        try {
            Log.i(TAG, "openning input stream");
            fis = new FileInputStream(PATH);
            Log.i(TAG, "input stream opened");
            FileDescriptor fd =  fis.getFD();
            int len = 0;
            do {
                len = fis.read(buffer);
                if(len > 0) {
                    String newmessage = new String(buffer,0, len, "US-ASCII");
                    message += newmessage;
                }
            } while (len == buffer.length);
            Log.i(TAG, "message: "+message);
            fis.close();
        } catch (Exception e) {
            e.printStackTrace();
        }


    }
}

Answer 1

Use BufferedReader instead `FileInputStream

Answer 2

here a very helpful consideration over the case, with some examples and explanations. http://www.jonathanbeard.io/tutorials/CtoJava