Converting an unsigned char array to jstring

Question

I'm having issues trying to convert an unsigned char array to jstring.

The context is I'm using a shared c library from Java. So I'm implementing a JNI c++ file. The function I use from the library returs a unsigned char* num_carte_transcode

And the function I implemented in the JNI C++ file returns a jstring.

So I need to convert the unsigned char array to jstring.

I tried this simple cast return (env)->NewStringUTF((char*) unsigned_char_array);

But though the array should only contain 20 bytes, I get randomly 22 or 23 bytes in Java... (Though the 20 bytes are correct)

EDIT1: Here some more details with example

JNIEXPORT jstring JNICALL Java_com_example_demo_DemoClass_functionToCall
  (JNIEnv *env, jobject, ) {
  // Here I define an unsigned char array as requested by the library function
  unsigned char unsigned_char_array[20];
  // The function feeds the array at execution
  function_to_call(unsigned_char_array);

  // I print out the result for debug purpose
  printf("result : %.*s (%ld chars)\n", (int) sizeof unsigned_char_array, unsigned_char_array, (int) sizeof unsigned_char_array);

  // I get the result I want, which is like: 92311221679609987114 (20 numbers)

  // Now, I need to convert the unsigned char array to jstring to return to Java
  return (env)->NewStringUTF((char*) unsigned_char_array);
  // Though... On debugging on Java, I get 21 bytes 92311221679609987114 (check the image below)
}

And sometimes I get 22 bytes, sometimes 23 ... though the expected result is always 20 bytes.

Answer 1

The string passed to NewStringUTF must be null-terminated. The string you're passing, however, is not null-terminated, and it looks like there's some garbage at the end of the string before the first null.

I suggest creating a larger array, and adding a null terminator at the end:

JNIEXPORT jstring JNICALL Java_com_example_demo_DemoClass_functionToCall
  (JNIEnv *env, jobject recv) {
  unsigned char unsigned_char_array[20 + 1]; // + 1 for null terminator
  function_to_call(unsigned_char_array);    
  unsigned_char_array[20] = '\0'; // add null terminator

  printf("result : %s (%ld chars)\n", unsigned_char_array, (int) sizeof unsigned_char_array);

  return (env)->NewStringUTF((char*) unsigned_char_array); // should work now
}