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2011-10-19T14:52:00.507
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In most architecture, the underlying access must match the alignment of the accessed data. This mean, that if you have a 32 bit value, you can access it efficiently if it is stored at an address that is dividable by four. If you use <code>#pragma pack</code>, the location of the variable can be anything, and the compiler must access the element piece by piece and combine them together. Concretely, below is the generated code to read a normal <code>int</code> on a V850E (a popular microcontoller in the embedded world): <pre><code>LD.W a[zero],r5 </code></pre> Correspondingly, the following is the code to access an <code>int</code> in a packed structure: <pre><code>LD.BU g+3[zero],r1 SHL 8,r1 LD.BU g+2[zero],r6 OR r1,r6 SHL 8,r6 LD.BU g+1[zero],r7 OR r6,r7 SHL 8,r7 LD.BU g[zero],r1 OR r7,r1 </code></pre> Another reason not to use packed structs is that it's not possible to dereference a pointer to a member of a packed struct, unless the architecture support unaligned pointer accesses. The reason for this is that type of the point will be a plain <code>int</code> pointer, and the compiler has no knowledge that it must access whatever it points to piece-by-piece. I would strongly recommend that you don't use '#pragma pack' at all, unless it's absolutely necessary. If you have control over the struct definition, there are techniques to make sure the struct layout is padding-free. If not, a better approach would be to copy any unaligned data to a new, aligned, struct and use it in your application.
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1. POWhen should we not use #pragma pack?
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1. USLindydancer
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