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    <p>As ams said above, don't take a pointer to a member of a struct that's packed. This is simply playing with fire. When you say <code>__attribute__((__packed__))</code> or <code>#pragma pack(1)</code>, what you're really saying is "Hey gcc, I really know what I'm doing." When it turns out that you do not, you can't rightly blame the compiler.</p> <p>Perhaps we can blame the compiler for it's complacency though. While gcc does have a <code>-Wcast-align</code> option, it isn't enabled by default nor with <code>-Wall</code> or <code>-Wextra</code>. This is apparently due to gcc developers considering this type of code to be a brain-dead "<a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628#c5">abomination</a>" unworthy of addressing -- understandable disdain, but it doesn't help when an inexperienced programmer bumbles into it.</p> <p>Consider the following:</p> <pre><code>struct __attribute__((__packed__)) my_struct { char c; int i; }; struct my_struct a = {'a', 123}; struct my_struct *b = &amp;a; int c = a.i; int d = b-&gt;i; int *e __attribute__((aligned(1))) = &amp;a.i; int *f = &amp;a.i; </code></pre> <p>Here, the type of <code>a</code> is a packed struct (as defined above). Similarly, <code>b</code> is a pointer to a packed struct. The type of of the expression <code>a.i</code> is (basically) an int <a href="http://en.wikipedia.org/wiki/Value_%28computer_science%29">l-value</a> with 1 byte alignment. <code>c</code> and <code>d</code> are both normal <code>int</code>s. When reading <code>a.i</code>, the compiler generates code for unaligned access. When you read <code>b-&gt;i</code>, <code>b</code>'s type still knows it's packed, so no problem their either. <code>e</code> is a pointer to a one-byte-aligned int, so the compiler knows how to dereference that correctly as well. But when you make the assignment <code>f = &amp;a.i</code>, you are storing the value of an unaligned int pointer in an aligned int pointer variable -- that's where you went wrong. And I agree, gcc should have this warning enabled by <em>default</em> (not even in <code>-Wall</code> or <code>-Wextra</code>).</p>
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