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    <p>No, this won't work.</p> <p>First of all, all the definitions of the structure must be the same, otherwise you get the infamous Undefined Behaviour.</p> <p>Second, look at the memory layout. What should be the offset to <code>bar</code> from the beginning of the structure? The first and the third definitions don't agree on this (the fields are most likely to be located consecutively).</p> <p>Maybe you can try a union?</p> <pre><code>struct theStruct { int fd; unsigned int flags; struct config config; int foo; // in both systems union { int bar; // only in System A int foobar; // only in system B }; }; </code></pre> <p>If you choose this, you should use only <code>bar</code> on System A and only <code>foobar</code> on System B.</p> <hr> <p>If the two systems are incompatible, and the actual type needed for <code>bar</code> is not available on System B (and vice versa), you can do with the following code:</p> <pre><code>struct theStruct { int fd; unsigned int flags; struct config config; int foo; // in both systems #ifdef SYSTEM_A int bar; // only in System A #else #ifdef SYSTEM_B int foobar; // only in system B #else #pragma error(either SYSTEM_A or SYSTEM_B must be enabled) #endif #endif }; </code></pre> <p>This way you will be always working with either with code compiled for System A or for System B, so you'll need to have different executables (which seems to be unavoidable anyway if you are compiling for the systems so much different).</p> <p>You'll need to wrap parts of your code accessing the fields into <code>#ifdef</code>s:</p> <pre><code>#ifdef SYSTEM_A s.bar = 5; #endif </code></pre> <p>-- otherwise you'll get compile errors on System B.</p>
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    1. COThis particular undefined behavior is so well defined that almost every modern operating system depends critically on it.
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    2. CO@Wug: writing a portable code is tricky even without UB, so I won't rely on portability of an UB anyway.
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    3. COThank you very much. I already thought that this will be a problem but I'm relative new to C and still learning. The union is completely new to me, have to read the docs about that and keep searching for a way around the whole situation :)
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