#pragma pack effect

Question

I was wondering if someone could explain to me what the #pragma pack preprocessor statement does, and more importantly, why one would want to use it.

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Answer 1

Data elements (e.g. members of classes and structs) are typically aligned on WORD or DWORD boundaries for current generation processors in order to improve access times. Retrieving a DWORD at an address which isn't divisible by 4 requires at least one extra CPU cycle on a 32 bit processor. So, if you have e.g. three char members char a, b, c;, they actually tend to take 6 or 12 bytes of storage.

#pragma allows you to override this to achieve more efficient space usage, at the expense of access speed, or for consistency of stored data between different compiler targets. I had a lot of fun with this transitioning from 16 bit to 32 bit code; I expect porting to 64 bit code will cause the same kinds of headaches for some code.