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    <p>The first cast is legal; the second cast may not be legal.</p> <p>The first cast tells the compiler to use the knowledge of the type of the variable to make a conversion to the desired type; the compiler does it, provided that a proper conversion is defined in the language standard.</p> <p>The second cast tells the compiler to forget its knowledge of the variable's type, and re-interpret its internal representation as that of a different type<sup> *</sup>. This has limited applicability: as long as the binary representation matches that of the type pointed by the target pointer, this conversion will work. However, this is not equivalent to the first cast, because in this situation value conversion never takes place.</p> <p>Switching the type of the variable being cast to something with a different representation, say, a <code>float</code>, illustrates this point well: the first conversion produces a correct result, while the second conversion produces garbage:</p> <pre><code>float test = 123456.0f; printf("VALUES:\n"); printf("%f\n", test + 1); printf("%lu\n", (unsigned long)test + 1); printf("%lu\n", *((unsigned long *)&amp;test) + 1); // Undefined behavior </code></pre> <p>This prints</p> <pre><code>123457.000000 123457 1206984705 </code></pre> <p><a href="http://ideone.com/nBgjpL" rel="nofollow noreferrer">(demo)</a></p> <p><hr/> <sup> *</sup> This is valid only when one of the types is a character type and the pointer alignment is valid, type conversion is trivial (i.e. when there is no conversion), when you change qualifiers or signedness, or when you cast to/from a <code>struct</code>/<code>union</code> with the first member being a valid conversion source/target. Otherwise, this leads to undefined behavior. See C 2011 (N1570), 6.5 7, for complete description. Thanks, <a href="https://stackoverflow.com/users/298225/eric-postpischil">Eric Postpischil</a>, for pointing out the situations when the second conversion is defined.</p>
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    1. POWhat's the difference between "(type)variable" and "*((type *)&variable)", if any?
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    1. USdasblinkenlight
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    1. POWhat's the difference between "(type)variable" and "*((type *)&variable)", if any?
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    1. CO@EricPostpischil I edited the answer to include your comment - thanks for a great comment!
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    2. COActually, I was too restrictive: In addition to conversions where a character type is one of the pointed-to types, there are also conversions that can change qualifiers or signedness (and, trivially, conversions that do not change type). These are in C 2011 (N1570) 6.5 7, along with one more that allows certain aggregates or union types. But all that might be too much for a simple question.
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      1. USEric Postpischil
    3. COIn most cases, `*(type*)&variable` isn't actually *illegal* (i.e., a constraint violation requiring a compile-time diagnostic). It's often going to cause undefined behavior, though.
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      1. USKeith Thompson
 

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