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.

UTF-8 should be passed. As one java char or int code point can be more than 1 byte. Is the unsigned char* indeed 20 bytes long, and is the java char[] 20 long, that is 40 bytes, or what is exactly happening? There should probably be no problem with 7 bits ASCII. — Joop Eggen
– Joop Eggen, Commented Jun 27, 2022 at 14:37
Hello @JoopEggen. thanks for your quick reply. I edited the intial question to add more details :) — Hyukchan Kwon
– Hyukchan Kwon, Commented Jun 27, 2022 at 14:47
Please use text instead of images. See meta.stackoverflow.com/questions/285551/… for more details on the reason. Kindly share the input and the result as an array in text. — aled
– aled, Commented Jun 27, 2022 at 14:54

Jorn Vernee · Accepted Answer · 2022-06-27 14:57:03Z

2

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
}

answered Jun 27, 2022 at 14:57

Jorn Vernee

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2 Comments

Hyukchan Kwon Over a year ago

Alright, that was it ! Thanks very much for your help :)

Remy Lebeau Over a year ago

Something to note: NewStringUTF() expects modified UTF-8, so if function_to_call() outputs standard UTF-8 then NewStringUTF() won't work correctly for Unicode codepoint U+0000 or codepoints > U+FFFF. JNI doesn't have any functions that handle standard UTF-8, but you can use JNI to call a String constructor that accepts standard UTF-8 (this would also allow you to specify the actual byte count so you don't have to worry about the null terminator).

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