#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

A compiler may place structure members on particular byte boundaries for reasons of performance on a particular architecture. This may leave unused padding between members. Structure packing forces members to be contiguous.

This may be important for example if you require a structure to conform to a particular file or communications format where the data you need the data to be at specific positions within a sequence. However such usage does not deal with endian-ness issues, so although used, it may not be portable.

It may also to exactly overlay the internal register structure of some I/O device such as a UART or USB controller for example, in order that register access be through a structure rather than direct addresses.