Android jni datatype losing precision (ByteBuffer, int)

Question

What am I doing: I add four integers in C. on the way, I lose information.

See code below:

//c-file
jbyte *inputByteArray = (*env)->GetDirectBufferAddress (env, obj);

// checked every value, also sizeof(val1)= 4 etc...
int val1 = (int) *(inputByteArray + 1); //120 
int val2 = (int) *(inputByteArray + 2); //120
int val3 = (int) *(inputByteArray + 3); //180
int val4 = (int) *(inputByteArray + 4); //180
int result = val1 + val2 + val3 + val4;

return result; 
//return type is int

//output: 88, should be 600
// 88 binary: 0000 0000 0101 1000
//600 binary: 0000 0010 0101 1000

The special thing about this is the following, which might be causing the problem:

The 4 values for the input are from a handed-over Buffer from Java, which is a direct ByteBuffer. It is directly allocated in Java, in order NOT be moved by the garbage collector. On the c-side I hand the buffer over via pointer from "GetDirectBufferAddress" (see code), and the single values do match the values in the array.

Does anyone know about this strange behaviour?

When I am using IntBuffer to hand over the numbers, it works by the way.
Im working here on performance, so I want small buffers and my data values are small enough to use ByteBuffer. (this is only a fragment of a larger calculation on the c-code side)

Since this is on Android, I did not manage to debug into the c-code...

Edit: I am using eclipse/SDK/NDK in current versions on Android 3.2.1 testing device

Don't know jni well but why +1..4 instead of +0..3. Arrays indexes are zero based or something different in jni or you pass more values? Please show/write complete test code (native + java). — Deucalion, Apr 17 '12 at 21:36
this is the c-file. these are not array indexes. they are relative adresses to the starting pointer, which points to the (in java allocated) memory block. the values at each position (in c) are fine, i checked. — mojjj, Apr 18 '12 at 08:10
byte range in java is -128...127 (same as (signed) char in c). It cannot contain 180. — Deucalion, Apr 18 '12 at 09:23

score 0 · Accepted Answer · answered Apr 19 '12 at 05:38

As @Deucalion says your array looks dodgy. Unless you are trying to add array1, array[2], array[3] and array[4]. Without using array[0].
Anyway assuming that what you have done is what you intend, your value is exactly what you will get.
Byte range is -128 t0 +127. And so 180 is actually stored as -76. And voila !!

Android jni datatype losing precision (ByteBuffer, int)

1 Answers1