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POC: Data structures alignment
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copied!<p>I'm working with structures and have several questions about them. As I understand structure variables will be placed at memory sequentially. Length of blocks(words) depends on machine architecture (32 bit - 4 byte, 64 bit - 8 bytes).</p> <p>Lets say we have 2 data structures:</p> <pre><code>struct ST1 { char c1; short s; char c2; double d; int i; }; </code></pre> <p>In memory it will be:</p> <pre><code>32 bit - 20 bytes 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | ------------------------------------------------------------------------------------------ c1| PB| s | s | c1| PB| PB| PB| d | d | d | d | d | d | d | d | i | i | i | i | 64 bit - 24 bytes | 20 | 21 | 22 | 23 | previous sequence + --------------------- | PB | PB | PB | PB | </code></pre> <p>But we can rearrange it, to make this data fit into machine word. Like this:</p> <pre><code>struct ST2 { double d; int i; short s; char c1; char c2; }; </code></pre> <p>In this case for both 32 and 64 bit it will be represented at the same way (16 bytes):</p> <pre><code> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | ---------------------------------------------------------------------- d | d | d | d | d | d | d | d | i | i | i | i | s | s | ch1| ch2| </code></pre> <p>I have a couple of questions:</p> <ul> <li>It's like wild guess but main rule for <code>struct</code> is to define variables with bigger size at the beginning?</li> <li>As I understand it's not working with stand-alone variables. Like <code>char str[] = "Hello";</code>?</li> <li>Padding byte, what code it has? Is it somewhere at ASCII table? Sorry, couldn't find it.</li> <li>2 structures with all members represented at memory by different addresses and they can be placed not sequentially at memory?</li> <li>Such structure: <code>struct ST3 { char c1; char c2; char c3;} st3;</code> Has <code>size = 3</code>, I understand that if we will add a member with other type into it, it will be aligned. But why it's not aligned before it?</li> </ul>

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