Returning jbyteArray from native c in Android

Question

I use native c to read data from an audio file to jbyte pointer. Now i want to send it to java as an jbyteArray.

jbyteArray Java_com_app_audio_player_readData(JNIEnv * env, jobject jobj,jstring readPath)
{

FILE *fin;
const char *inFile= (*env)->GetStringUTFChars(env,readPath,0);
fin = fopen(inFile, "r");

fseek(fin, 0, SEEK_END); // seek to end of file
 int size = ftell(fin); // get current file pointer
fseek(fin, 0, SEEK_SET);

jbyte *data=(jbyte *)malloc(size*sizeof(jbyte));
int charCnt = 0;

charCnt=fread(data, 1, size, fin);


jbyteArray result=(*env)->NewByteArray(env, size);

     //-- I want to convert data to jbyteArray and return it to java    

fclose(fin);
 return result;

}

How it is done?

Solution

use SetByteArrayRegion

charCnt=fread(data, 1, size, fin);

jbyteArray result=(*env)->NewByteArray(env, size);

(*env)->SetByteArrayRegion(env, result, 0, size, data);

one could also use GetByteArrayElements eg:

jboolean isCopy;
jbyte* rawjBytes = (*env)->GetByteArrayElements(env, result, &isCopy);

//do stuff to raw bytes
memcpy(rawjBytes, data, size*sizeof(jbyte));

(*env)->ReleaseByteArrayElements(env, result, rawjBytes, 0);

see here for more details on SetByteArrayRegion, GetByteArrayElements and ReleaseByteArrayElements.

NB: this question is probably a special case of this question

Comments

[violet313]

@Roman ~but the intention here is to use memcpy to copy data into rawjBytes?! & demonstrates an alternative to SetByteArrayRegion where lets say, large data needs to be constructed dynamically/incrementally: we can use GetByteArrayElements to gain access to the jbyteArray contents; then calling ReleaseByteArrayElements with zero-valued 3rd arg ensures data is copied back (whether-or-not the underlying memory is pinned).

[Roman Truba]

Ow... I'm sorry, I don't get it. Thank you very much anyway, it's helped me lot.

[violet313]

@Roman ~ no problemo. & of course one could do what you're suggesting: gain access to the rawjBytes and memcpy them into some other memory location. so i appreciate the potential for confusion &&so it's good you pointed it out :)

[Trent Gray-Donald]

In your SetByteArrayRegion() example, I think you're using "jbyteArray" in the args list when you mean to use "result", right? (ie: use the var name, not its type)

[violet313]

@Trent Gray-Donald +1 ~blinking! imagine failing to notice that before.lol. ~fixed.