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messing around with bitmaps

This is a discussion on messing around with bitmaps within the C Programming forums, part of the General Programming Boards category; Originally Posted by nonoob Seems to be true but there's no point in switching off the pack directive. In this ...

  1. #16
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    Quote Originally Posted by nonoob View Post
    Seems to be true but there's no point in switching off the pack directive.
    In this specific case, it's probably not a big deal. I'm betting the OP is on an x86 architecture, which supports misaligned reads/writes from memory. Other (typically RISC) architectures, like ARM, are not so friendly to misaligned reads, and the resulting code may be several times slower for accessing misaligned data that results from packing structures (unless you're lucky enough to be able to organize your struct members to be naturally aligned when packed).

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    What if the address was pulled out of the structure declared with malloc() into a void pointer, decremented by two, then assigned to a pointer used for the next member in the structure with that structures data type? I guess that would be a pain though since you would need pointers for each member in the structure that doesn't naturally align, and using the wrong pointer on a member would garble the data block.

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    That would be a giant mess just begging for extra bugs. Besides, after taking the void pointer and incrementing it and assigning or casting it to, say, a uint16_t pointer to dereference, would still result in a misaligned read. It's an assembly/hardware issue you can't just sweep under the rug with nasty pointer ju-ju. There's absolutely nothing wrong with packing structures and having misaligned reads when you have to. It's just that you shouldn't go around packing structs unnecessarily.
    memcpy likes this.

  4. #19
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    If developing with the Windows SDK, you should just include Windows.h and use the pre-defined BITMAPFILEHEADER structure. The other structures are all present as well. They are declared with the correct packing.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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