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POAliasing array with pointer-to-struct without violating the standard
 

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    <p>Reading <a href="http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html" rel="nofollow">this</a> I understood that you can alias structures (without violating the standard, that is) if they have compatible members, i.e given the following struct:</p> <pre><code>typedef struct { uint32_t a; uint32_t b; } Frizzly; </code></pre> <p>The following would break aliasing rules:</p> <pre><code>uint32_t foo(uint16_t *i) { Frizzly *f = (Frizzly *)i; return f-&gt;a; } </code></pre> <p>But the following would not:</p> <pre><code>uint32_t foo(uint32_t *i) { Frizzly *f = (Frizzly *)i; return f-&gt;b; } </code></pre> <p>because the "aggregrate type" in question contains types compatible with the pointer that that we're casting into it, i.e. a pointer to type <code>uint32_t</code> can be casted into a struct that contains members (or a member) of type <code>uint32_t</code> without breaking aliasing rules.</p> <p>First, did I understand this correctly?</p> <p>Secondly, does the ordering and types of the (other) variables within the struct matter? Say, if <code>Frizzly</code> was defined as follows:</p> <pre><code>typedef struct { uint16_t b[2]; uint32_t a; } </code></pre> <p>After the cast in the second example, <code>b</code> is now backed by memory of incompatible (<code>uint32_t</code>) type. Is the cast still valid (or rather, accessing the values through the casted pointer)? Will changes to either element of <code>a</code> alter the value of first element of <code>i</code> (and the other way around) as though strict aliasing were disabled?</p> <p>Also, if the above is valid, what if I had a struct like so:</p> <pre><code>typedef struct { void *m; uint16_t hooah[4]; } Bar; </code></pre> <p>The following cast would, if I'm correct, break aliasing rules:</p> <pre><code>void test(char *boo, size_t dee) { Bar *bar = (Bar *)(boo + dee); do_other_stuff(bar); } </code></pre> <p>Could I make the cast valid simply by adding a single <code>unsigned char</code> member into the struct? In other words, casting pointers of incompatible types generally breaks aliasing rules, but since a cast from a pointer to a struct containing a member of type <code>X</code> into a pointer to <code>X</code> is an exception, can any cast from pointer-to-X to aggregrate-Y made valid simply by adding a (possibly dummy) member of type X into Y? </p> <p>(I didn't actually test the above code snippets in a compiler.)</p> <p><strong>EDIT:</strong></p> <p>I know my wording and examples might be rather poor, so I'll try to rephrase the question: if I understood correctly, it is legal for a pointer-to-struct to alias an array of elements of type 'X' as long as the struct contains members of type 'X'. Now, when dereferencing a member of the struct, does the member have to be of the type 'X', or are exceptions to the strict aliasing rules made for <em>all</em> members of the struct regardless of their types as long as there is one member of the appropriate type?</p>
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    1. COI always see people make unions of the two and use that. That way you always get the strictest version for both.
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    2. COgood question , +1
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    3. COIf it compiles, it doesn't violate standards. No?
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