how to properly convert byte array in java to char pointer in jni

Question

I am new to C/C++, so I don't know how to properly convert a byte array in Java to a char array in JNI.

For example, I have an inputString containing "Hello world!", I use this to get the byte array:

byte[] data = inputString.getBytes();

Then in the JNI layer, I use:

jbyte *array = (*env)->GetByteArrayElements(env, data, &isCopy);
argv[1] = (char *)array;

Then when I print the argv[1], it is "Hello world!ZE".

Why is there extra characters "ZE" here?

Would anyone explain this, and provide me the right solution to convert byte[] to char* pointer? I need the char* pointer because the interface I am given is char* pointer for me.

Remy Lebeau · Accepted Answer · 2022-01-05 23:46:36Z

1

Your byte[] array is not null-terminated (does not end with a 0x00 byte), but you are clearly treating it as if it were. When printing the bytes, the "extra" characters are random values coming from surrounding memory when the print accesses beyond the bounds of the array, which is undefined behavior.

Use (*env)->GetArrayLength() to find out how many bytes are actually in the array, and then don't access more bytes than that, eg:

jint len = (*env)->GetArrayLength(env, data);
jbyte *array = (*env)->GetByteArrayElements(env, data, &isCopy);
argv[1] = (char*) array;
...
printf("%.*s", len, argv[1]);
...
(*env)->ReleaseByteArrayElements(env, data, array, JNI_ABORT);

If you really need a null-terminated string (or just need to hold on to the data longer than the Java byte[] array will exist), you will have to allocate your own char[] buffer with extra room for a terminator, and then copy the byte[] array data into that buffer, eg:

jint len = (*env)->GetArrayLength(env, data);

jbyte *array = malloc(len + 1);
if (array) {
    (*env)->GetByteArrayRegion(env, data, 0, len, array);
    array[len] = 0x00;
}

argv[1] = (char*) array;
...
printf("%s", argv[1]);
...
free(argv[1]);

edited Jan 5, 2022 at 23:46

answered Jan 5, 2022 at 23:21

Remy Lebeau

609k36 gold badges516 silver badges875 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

JUAN CHEN Over a year ago

I used this function (*env)->GetArrayLength(), it showed the length is 12 which is correct. So I still don't know why there is extra "ZE"

Remy Lebeau Over a year ago

I already told you why - you are treating the char* as a null-terminated string, but the byte[] data IS NOT null-terminated!

JUAN CHEN Over a year ago

o right, I should read your comment carefully. Thank you.

Collectives™ on Stack Overflow

how to properly convert byte array in java to char pointer in jni

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related