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    <p>This is probably a good example of why (in my opinion) it's a bad idea to <code>typedef</code> array types.</p> <p>Unlike in other contexts, in function declarations a parameter of array type is always adjusted to an equivalent pointer type. When an array is passed to the function it always decays into a pointer to the first element.</p> <p>These two snippets are equivalent:</p> <pre><code>typedef unsigned char Str[257]; Str src = "blah"; Str dst; memcpy( &amp;dst, &amp;src, sizeof(Str) ); // unconventional </code></pre> <hr> <pre><code>unsigned char src[257] = "blah"; unsigned char dst[257]; memcpy(&amp;dst, &amp;src, sizeof(unsigned char[257])); // unconventional </code></pre> <p>In this latter case <code>&amp;dst</code> and <code>&amp;src</code> are both of type <code>unsigned char (*)[257]</code> but the value of these pointers are the same as the value of pointers to the first element of each array, which is what <code>dst</code> and <code>src</code> would decay into if passed directly into <code>memcpy</code> like this.</p> <pre><code>memcpy(dst, src, sizeof(unsigned char[257])); // more usual </code></pre> <p><code>memcpy</code> takes <code>void*</code> arguments so the types of the original pointers don't matter, only their values.</p> <p>Because of the rule for parameter declarations (an array type of any or unspecified size is adjusted to the equivalent pointer type), these declarations for <code>fn</code> are all equivalent:</p> <pre><code>typedef unsigned char Str[257]; void fn( Str dst, Str src ); </code></pre> <hr> <pre><code>void fn( unsigned char dst[257], unsigned char src[257] ); </code></pre> <hr> <pre><code>void fn( unsigned char dst[], unsigned char src[] ); </code></pre> <hr> <pre><code>void fn( unsigned char* dst, unsigned char* src ); </code></pre> <p>Looking at this code, it is more obvious that the values being passed into <code>memcpy</code> in this case are pointers to the passed pointers, and not pointers to the actual <code>unsigned char</code> arrays.</p> <pre><code>// Incorrect void fn( unsigned char* dst, unsigned char* src ) { memcpy(&amp;dst, &amp;src, sizeof(unsigned char[257])); } </code></pre> <p>With a typedef, the error is not so obvious, but still present.</p> <pre><code>// Still incorrect typedef unsigned char Str[257]; void fn( Str dst, Str src ) { memcpy(&amp;dst, &amp;src, sizeof(Str)); } </code></pre>
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    1. USCB Bailey
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    1. POWhy does memcpy fail copying to a local array member of a simple object?
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    1. COGack! I hate C nuances. It's been a few too many years since I had to wrestle with raw pointer/array idioms. When I'm doing this kind of port normally I use C++ and Str255 would be a class with operators to catch silliness. Unfortunately, this time, I have to use plain C as there's a heap of backend code that won't compile in C++, even after converting the K&R declarations to ANSI. For other project reasons, I can't vary the code too far from the original.
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      1. USAndy Dent
    2. COAndy Dent, if you wrote good c, you would be confident of avoiding such mistacks; while if you wrote good c++, you would use "struct Str" instead of a simple typedef, and write StrCopy(struct Str *p, struct Str *from), say OOP & encapsulation sense when writing C.
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    3. CO"if you wrote good c" - I'm porting it, I didn't write it! I can't use structs for things which assume the original Apple definitions which are typedefs. As I said, normally when porting code like this I use objects and operator overloading to get robust types.
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