CODEWITHSUNDEEP
android-ndkjava-native-interface
Aditya Sagam
Aditya Sagam

Reputation: 121

JNI: Reading a text file in C code and returning to sdk

Im trying to create an android app to read text from a text file using NDK .My C code reads a string into a variable and returns the string variable to java code .But when I run the Android app the string displayed in the TextView appears as symbols like @ followed by some rectangles.What is the format of returning strings from JNI? Check the below code.Can I read a system file without any other permission?What will a printf() statement do?

Java Code:

package com.example.openfile;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener{
public native String ndkopenfile();
static{
    System.loadLibrary("mylib");
}
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //TextView textView1 = new TextView(this);
    //textView1.setText(ndkopenfile());
    Button button1 = (Button)findViewById(R.id.button1);
    button1.setOnClickListener(this);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub
    String str1;
    Log.d("Click","The button has been clicked to open the file");

    str1=ndkopenfile();

    //setTitle(str1);
    TextView tv = (TextView)findViewById(R.id.textView1);
    tv.setText(str1);
}

}

C code:

JNIEXPORT jstring JNICALL Java_com_example_openfile_MainActivity_ndkopenfile
(JNIEnv *env, jobject this)
{

    jstring str1[20];
    FILE* fp = fopen("/sdcard/x.txt","w+");
    if(fp!=NULL)
    {
        fgets(str1,20,fp);
        fflush(fp);
        fclose(fp);
        return(*env)->NewStringUTF(env,str1);
    }
    else
    {
        fclose(fp);
        return(*env)->NewStringUTF(env,"Error opening file!");
    }
 }

Upvotes: 1

Views: 10383

Answers (2)

Zax
Zax

Reputation: 3010

Try this:

JNIEXPORT jstring JNICALL Java_com_example_openfile_MainActivity_ndkopenfile
(JNIEnv *env, jobject this)
{
        char myStr[20];
        FILE* fp = fopen("/sdcard/x.txt","w+");
        if(fp!=NULL)
        {
            fgets(myStr,20,fp);
            fflush(fp);
            fclose(fp);
            return(*env)->NewStringUTF(env,myStr);
        }
        else
        {
            fclose(fp);
            return(*env)->NewStringUTF(env,"Error opening file!");
        }
}

This technique works perfectly !!! Tested.

Upvotes: 5

Tim
Tim

Reputation: 22

I think your definition jstring str1[20]; need to modify to char str1[20]. Because jstring type is defined as a jobject, it's not basic type of char.

Upvotes: 1

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