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

Answer 1

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 !!