Question

Is there a historical reason or something ? I’ve seen quite a few times something like char foo[256]; or #define BUF_SIZE 1024. Even I do mostly only use 2ⁿ sized buffers, mostly because I think it looks more elegant and that way I don’t have to think of a specific number. But I’m not quite sure if that’s the reason most people use them, more information would be appreciated.

Answer 1

There may be a number of reasons, although many people will as you say just do it out of habit.

One place where it is very useful is in the efficient implementation of circular buffers, especially on architectures where the % operator is expensive (those without a hardware divide – primarily 8 bit micro-controllers). By using a 2^n buffer in this case, the modulo, is simply a case of bit-masking the upper bits, or in the case of say a 256 byte buffer, simply using an 8-bit index and letting it wraparound.

In other cases alignment with page boundaries, caches etc. may provide opportunities for optimisation on some architectures – but that would be very architecture specific. But it may just be that such buffers provide the compiler with optimisation possibilities, so all other things being equal, why not?