Question

Just a quick and specific question, this has stumped me for half an hour almost.

char * bytes = {0x01, 0xD8};
int value = 0;

value = bytes[0];  // result is 1 (0x0001)
value <<= 8;       // result is 256 (0x0100)
value |= bytes[1]; // result is -40? (0xFFD8) How is this even happening?

The last operation is the one of interest to me, how is it turning a signed integer of 256 into -40?

edit: changed a large portion of the example code for brevity

Answer 1

In your case the type char is equivalent to signed char, which means that when you save the value 0xD8 in a char, it will come out as a negative number.

The usual arithmetic conversions that happen during the |= operation are value-preserving, so the negative number is preserved.

To solve the problem, you can either make all your data types unsigned when you have binary arithmetics. Or you can write value |= ((unsigned char) buffer[0]) or value |= buffer[0] & 0xFF.