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POMis-aligned pointers on x86
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  1. POMis-aligned pointers on x86
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    copied!<p>Can someone provide an example were casting a pointer from one type to another fails due to mis-alignment?</p> <p>In the comments to <a href="https://stackoverflow.com/questions/544928/reading-integer-size-bytes-from-a-char-array/544964#544964">this answer</a>, bothie states that doing something like</p> <pre><code>char * foo = ...; int bar = *(int *)foo; </code></pre> <p>might lead to errors even on x86 if alignment-checking is enabled.</p> <p>I tried to produce an error condition after setting the alignment-check flag via <code>set $ps |= (1&lt;&lt;18)</code> in GDB, but nothing happened.</p> <p>What does a working (ie non-working ;)) example look like?</p> <hr> <p>None of the code snippets from the answers fail on my system - I'll try it with a different compiler version and on a different pc later.</p> <p>Btw, my own test code looked like this (now also using asm to set <code>AC</code> flag and unaligned read and write):</p> <pre><code>#include &lt;assert.h&gt; int main(void) { #ifndef NOASM __asm__( "pushf\n" "orl $(1&lt;&lt;18),(%esp)\n" "popf\n" ); #endif volatile unsigned char foo[] = { 1, 2, 3, 4, 5, 6 }; volatile unsigned int bar = 0; bar = *(int *)(foo + 1); assert(bar == 0x05040302); bar = *(int *)(foo + 2); assert(bar == 0x06050403); *(int *)(foo + 1) = 0xf1f2f3f4; assert(foo[1] == 0xf4 &amp;&amp; foo[2] == 0xf3 &amp;&amp; foo[3] == 0xf2 &amp;&amp; foo[4] == 0xf1); return 0; } </code></pre> <p>The assertion passes without problems, even though the generated code definitely contains the unaligned access <code>mov -0x17(%ebp), %edx</code> and <code>movl $0xf1f2f3f4,-0x17(%ebp)</code>.</p> <hr> <p>So will setting <code>AC</code> trigger a <code>SIGBUS</code> or not? I couldn't get it to work on my Intel dual core laptop under Windows XP with none of the GCC versions I tested (MinGW-3.4.5, MinGW-4.3.0, Cygwin-3.4.4), whereas codelogic and Jonathan Leffler mentioned failures on x86...</p>
 

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