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POCan a C++ compiler re-order elements in a struct
 

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  1. POCan a C++ compiler re-order elements in a struct
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    <p>Can a C++ compiler (specifically g++) re-order the internal elements of a struct?</p> <p>I'm seeing some strange behaviour where I have a structure that contains something like the following:</p> <pre><code>Struct SomeStruct{ ... ... long someLong; long someLongArray[25]; unsigned long someUnsignedLong; unsigned long someUnsignedLongArray[8]; unsigned long int someUnsignedLongInt; ... ... }; </code></pre> <p>When I write output this to file, the order of <em>someUnsignedLongArray</em> and <em>someLongArray</em> seem to be reversed (i.e. the elements in <em>someLongArray[]</em> appear after <em>someUnsignedLong</em> and the elements of <em>someUnsignedLongArray[]</em> appear after <em>someLong</em>). Is this possible??</p> <p>Thanks</p> <hr> <p>Update: As requested, I am writing out the structure using the following:</p> <pre><code>int fd = open(fspec,O_RDWR|O_CREAT|O_TRUNC,0666); int writeRes = write(fd,(char *)&amp;someStruct,sizeof(SomeStruct)); </code></pre> <p>For completeness, here is the full struct:</p> <pre><code>struct SomeStruct{ byte someByte; byte someByteArray[6]; char someChar; char someCharArray[5]; char someCharArrayArray[3][5]; short someShort; signed short someShortArray[2]; unsigned short someUnsignedShort; unsigned short someUnsignedShortArray[8]; int someInt; int someIntArray[3]; int someIntArrayArrayArrayArray[4][3][2][6]; int *pSomeInt; unsigned int someUnsignedInt; unsigned int someUnsignedIntArray[9]; long someLong; long someLongArray[25]; unsigned long someUnsignedLong; unsigned long someUnsignedLongArray[8]; unsigned long int someUnsignedLongInt; long long someLongLong; long long someLongLongArray[5]; bool someBool; bool someBoolArray[3]; unsigned long long someUnsignedLongLong; unsigned long long someUnsignedLongLongArray[5]; unsigned long long someUnsignedLongLongArrayArray[5][2]; unsigned long long int *pSomeUnsignedLongLongInt; }; </code></pre>
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    1. COHow do you write the struct to a file?
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    2. COAre you certain that you are seeing what you think you are seeing? ie have you written unique bit patterns into each element and can trace them to the file? Also what #pragma pack (if any) are you using around the struct?
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    3. COPerhaps he's writing the bytes of the struct, instead of the elements of the struct, to the file.
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