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POsizeof() part of a C struct - sort of
 

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    <p>I want to grab part of the data inside some C structs to partially serialize/deserialize them, writing the bytes from memory to disk, and viceversa. </p> <p>The structs are not known in advance, they are dynamically built with my own C code generator (as well as the code that will serialize it). The serializable fields will be placed at the beginning of the struct.</p> <p>Let's say a have a struct with 4 fields, and the first two are to be serialized:</p> <pre><code>typedef struct { int8_t x1; int32_t x2; /* 1 + 4 = 5 bytes (if packed) */ int8_t y1; int32_t y2; /* 1 + 4 +1 + 4 = 10 bytes (if packed) */ } st; </code></pre> <p>I plan to grab the pointer to the struct variable and write/read the <code>n</code> bytes that cover those two first fields (<code>x1, x2</code>). I don't think I need to worry about alignment/packing because I don't intend the serialization to survive different compilations (only a unique executable is expected to read/write the data). And, as I'm targeting a wide scope of compilers-architectures, I don't want to place assumptions on alignment-packing or compiler specific tricks.</p> <p>Then, I need to count bytes. And I can't just do <code>sizeof(st.x1)+sizeof(st.x2)</code> because of alingment-padding. So, I'm planning to substract pointers, from the start of the struct to the first "non persistent" field:</p> <pre><code>st myst; int partsize = (char*)&amp;myst.y1 - (char*)(&amp;myst); printf("partial size=%d (total size=%d)\n",partsize,sizeof(myst)); </code></pre> <p>This seems to work. And it can be placed in a macro.</p> <p>(For the record: I tried also to write another macro that does not requrire an instance of the struct, something like <a href="https://stackoverflow.com/questions/3553296/c-sizeof-single-struct-member">this</a>, but it doesnt seem possible here - but this does not matter me much).</p> <p>My question: Is this correct and safe? Can you see any potential pitfall, or some better approach?</p> <p>Among other things: Does C standard (and de-facto compilers) assume that the structs fields lay in memory in the same order as they are defined in source? This probably is a stupid question, but I'd want to be sure...</p> <p>UPDATE: Some conclusions from the answers and my own findings:</p> <ol> <li><p>There seems to be no problem with my approach. In particular, C dictates that struct fields will never change order.</p></li> <li><p>One could also (as suggested by an aswer) count from the last persistent field and add its size : <code>(char*)&amp;myst.x2 + sizeof(&amp;myst.x2) - (char*)(&amp;myst)</code> . That would be equivalent, except that it would include not the padding bytes (if present) for the last field. A very small advantage - and a very small disadvantage, in being less simple.</p></li> <li><p>But the accepted answer, with <code>offsetof</code>, seems to be preferable than my proposal. It's clear-expressive and pure compile-time, it does not require an instance of the struct. It further seems to be standard, available in any compiler. If one does not need a compile-time construct, and has an instance of the struct available (as is my scenario) both solutions are esentially equivalent.</p></li> </ol>
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    1. CORelated question: http://stackoverflow.com/questions/2748995/c-struct-memory-layout
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    2. COI removed the C++ tag, as there is no C++ anywhere near this question.
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