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    <p>Packing and byte alignment, as described in the C FAQ <a href="http://www.c-faq.com/struct/align.html" rel="noreferrer">here</a>:</p> <blockquote> <p>It's for alignment. Many processors can't access 2- and 4-byte quantities (e.g. ints and long ints) if they're crammed in every-which-way.</p> <p>Suppose you have this structure:</p> <pre><code>struct { char a[3]; short int b; long int c; char d[3]; }; </code></pre> <p>Now, you might think that it ought to be possible to pack this structure into memory like this:</p> <pre><code>+-------+-------+-------+-------+ | a | b | +-------+-------+-------+-------+ | b | c | +-------+-------+-------+-------+ | c | d | +-------+-------+-------+-------+ </code></pre> <p>But it's much, much easier on the processor if the compiler arranges it like this:</p> <pre><code>+-------+-------+-------+ | a | +-------+-------+-------+ | b | +-------+-------+-------+-------+ | c | +-------+-------+-------+-------+ | d | +-------+-------+-------+ </code></pre> <p>In the packed version, notice how it's at least a little bit hard for you and me to see how the b and c fields wrap around? In a nutshell, it's hard for the processor, too. Therefore, most compilers will pad the structure (as if with extra, invisible fields) like this:</p> <pre><code>+-------+-------+-------+-------+ | a | pad1 | +-------+-------+-------+-------+ | b | pad2 | +-------+-------+-------+-------+ | c | +-------+-------+-------+-------+ | d | pad3 | +-------+-------+-------+-------+ </code></pre> </blockquote>
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