Ok, I'm dumb, byte alignment for the third time

This is a discussion on Ok, I'm dumb, byte alignment for the third time within the C++ Programming forums, part of the General Programming Boards category; salem let me explain, I went to delete the FIRST thread I made on byte alignment after I realized I ...

  1. #1
    Shadow12345
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    Ok, I'm dumb, byte alignment for the third time

    salem let me explain, I went to delete the FIRST thread I made on byte alignment after I realized I posted two threads on the same topic (I forgot I made the first). Then, I couldn't find the most recent one I made so I assume you deleted it, now none exist but I still need to understand what this means.

    Anyway I know that PACK_STRUCT expands to nothing, but I don't know what the rest of those '#' directives mean, and I would like someone to tell me. I don't know what the pack/push __attribute__((packed)) lines mean. When I just include the structures without the defines the application doesn't work properly.


    Code:
    #ifdef _MSC_VER
    #	pragma pack( push, packing )
    #	pragma pack( 1 )
    #	define PACK_STRUCT
    #elif defined( __GNUC__ )
    #	define PACK_STRUCT	__attribute__((packed))
    #else
    #	error you must byte-align these structures with the appropriate compiler directives
    #endif
    
    typedef unsigned char byte;
    typedef unsigned short word;
    
    // File header
    struct MS3DHeader
    {
    	char m_ID[10];
    	int m_version;
    } PACK_STRUCT;
    ....//MORE STRUCTS THAT DON'T NEED TO TAKE UP SPACE
    
    // Default alignment
    #ifdef _MSC_VER
    #	pragma pack( pop, packing )
    #endif
    
    #undef PACK_STRUCT
    yeah well salem don't beat me up plz

  2. #2
    Skunkmeister Stoned_Coder's Avatar
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    Its all explained in the msvc helpfiles and also at msdn i guess.
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  3. #3
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    This all has to do with the way different compiler makers chose to deal with the alignment issue. Microsoft chose to use #pragmas, and to use them you have to specify the point where you should start a particular alignment, and where you should end it (the code you give is single byte alignment). That is the #pragma push & pop code, and it is defined to only be compiled for Microsoft.
    The gcc compiler uses a different approach: the __attribute__((packed)) stuff. Under that approach, you tag a particular structure to be packed (rather than a block of code). I think I read once in the gcc documenation a statement to the effect that pragmas are never a good idea, and that is why they didnt use them. Right or wrong, that was their approach to solving the alignment issue.

    The code block you showed was code that was conditionally defined to work on either gcc (__GNUC__) or Microsoft VC (_MSC_VER).

    Hopefully that clears up something.
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