Question

I’ve run into a small issue here. I have an unsigned char array, and I am trying to access bytes 2-3 (0xFF and 0xFF) and get their value as a short.

Code:

 unsigned char Temp[512] = {0x00,0xFF,0xFF,0x00};
 short val = (short)*((unsigned char*)Temp+1)

While I would expect val to contain 0xFFFF it actually contains 0x00FF. What am I doing wrong?

Answer 1

There’s no guarantee that you can access a short when the data is improperly aligned.

On some machines, especially RISC machines, you’d get a bus error and core dump for misaligned access. On other machines, the misaligned access would involve a trap into the kernel to fix up the error — which is only a little quicker than the core dump.

To get the result reliably, you’d be best off doing shifting and or:

val = *(Temp+1) << 8 | *(Temp+2);

or:

val = *(Temp+2) << 8 | *(Temp+1);

Note that this explicitly offers big-endian (first option) or little-endian (second) interpretation of the data.

Also note the careful use of << and |; if you use + instead of |, you have to parenthesize the shift expression or use multiplication instead of shift:

val = (*(Temp+1) << 8) + *(Temp+2);
val = *(Temp+1) * 256 + *(Temp+2);

Be logical and use either logic or arithmetic and not a mixture.