Passing a string to C-code in Android NDK

Question

The program should take an Image from the SD card and adjust its brightness. And the image is taken from the SD card via the NDK C-code. It is to be noted that the string depicting the path to the image is passed to the NDK via JNI.

Java code:

private void adjustBrightness() {
   imagePath     = (Environment.getExternalStorageDirectory().getPath()+"earthglobe.jpeg").toCharArray();
   brightness(imagePath, brightness);
}

public native void brightness(char[] imagePath, float brightness);

NDK code:

JNIEXPORT void JNICALL Java_com_example_ImageActivity_brightness(JNIEnv * env,char[] bitmappath, jfloat brightnessValue)
{
   string bmpath    =   bitmappath+'\0';
   jobject  obj = fopen( bitmappath , "rb" );
}

Solution

You cannot pass char[] this way.

In Java use:

public static native void brightness(String imagePath, float brightness);

In native use:

std::string ConvertJString(JNIEnv* env, jstring str)
{
   if ( !str ) std::string();

   const jsize len = env->GetStringUTFLength(str);
   const char* strChars = env->GetStringUTFChars(str, (jboolean *)0);

   std::string Result(strChars, len);

   env->ReleaseStringUTFChars(str, strChars);

   return Result;
}

JNIEXPORT void JNICALL Java_com_example_ImageActivity_brightness(JNIEnv * env, jobject obj, jstring bitmappath, jfloat brightnessValue)
{
   std::string bmpath = ConvertJString( env, bitmappath );
   FILE* f = fopen( bmpath.c_str(), "rb" );

   // do something useful here

   fclose( f );
}

Comments

[Sreekanth Karumanaghat]

Thanks,I want the bitmap as a Jobject and not a file,because there is already a function to retrieve the pixels from the Jobject... Is there any way I can do this?

[Sergey K.]

Use java.nio.ByteBuffer to pass pixels around.

[Sreekanth Karumanaghat]

I think you got me wrong....either I want to retrieve the bmp as JObject (in NDK)or I need to get a method to get pixels from the file in the NDK.

[Sergey K.]

You cannot get the pixels via jObject in NDK. You will have to pass a ByteBuffer.

[Sreekanth Karumanaghat]

Sorry I cannot pass ByteBuffer...because that will cause a memory problem...I already have a library that returns the pixels when the JObject corresponding to bmp is passed to it...Is there anyway I can convert the File to JObject inside the NDK?

[Sergey K.]

You cannot convert a FILE to jObject inside NDK.

[Sreekanth Karumanaghat]

let us continue this discussion in chat

[Rizwan Sohaib]

hello Sergey K. I am try to get the string parameters the way you suggesting, but I get UnsatisfiedLikerError. If i keep the parameters of the function to void, then the functions are complied successfully. Can you please help me on this?

[Sergey K.]

@RizwanSohaib: Hey, why not to ask a question and put your code there? Don't be shy!

[Rizwan Sohaib]

I am not shy, this is the same question as I have but this solution is not working for me..

[Sergey K.]

@RizwanSohaib: then your code is different. Could you post it as a question?

[Rizwan Sohaib]

Ok, I shall post it in some time and will give you the link to review

[Sergey K.]

@wEight std::string