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POHow can I access members of a struct when it's not aligned properly?
 

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  1. POHow can I access members of a struct when it's not aligned properly?
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    <p>I'm afraid that I'm not very good at low level C stuff, I'm more used to using objects in Obj-c, so please excuse me if this is an obvious question, or if I've completely misunderstood something...</p> <p>I am attempting to write an application in Cocoa/Obj-C which communicates with an external bit of hardware (a cash till.) I have the format of the data the device sends and receives - and have successfully got some chunks of data from the device.</p> <p>For example: the till exchanges PLU (price data) in chunks of data in the following format: (from the documentation)</p> <p>Name Bytes Type</p> <pre><code>Name Bytes Type PLU code h 4 long PLU code L 4 long desc 19 char Group 3 char Status 5 char PLU link code h 4 long PLU link code l 4 long M&amp;M Link 1 char Min. Stock. 2 int Price 1 4 long Price 2 4 long Total 54 Bytes </code></pre> <p>So I have a struct in the following form in which to hold the data from the till:</p> <pre><code>typedef struct MFPLUStructure { UInt32 pluCodeH; UInt32 pluCodeL; unsigned char description[19]; unsigned char group[3]; unsigned char status[5]; UInt32 linkCodeH; UInt32 linkCodeL; unsigned char mixMatchLink; UInt16 minStock; UInt32 price[2]; } MFPLUStructure; </code></pre> <p>I have some known sample data from the till (below) which I have checked by hand and is valid </p> <p>00 00 00 00 4E 61 BC 00 54 65 73 74 20 50 4C 55 00 00 00 00 00 00 00 00 00 00 00 09 08 07 17 13 7C 14 04 00 00 00 00 09 03 00 00 05 BC 01 7B 00 00 00 00 00 00 00</p> <p>i.e. </p> <ul> <li><p>bytes 46 to 50 are <code>&lt;7B 00 00 00&gt;</code> == 123 as I would expect as the price is set to '123' on the till.</p></li> <li><p>byte 43 is <code>&lt;05&gt;</code> == 5 as I would expect as the 'mix and match link' is set to 5 on the till. </p></li> <li><p>bytes 39 to 43 are <code>&lt;09 03 00 00&gt;</code> == 777 as I would expect as the 'link code' is set to '777' on the till.</p></li> <li><p>Bytes 27,28,29 are <code>&lt;09 08 07&gt;</code> which are the three groups (7,8 &amp; 9) that I would expect.</p></li> </ul> <p>The problem comes when I try to get some of the data out of the structure programmatically: The early members work correctly right up to, and including the five 'status' bytes. However, members after that don't come out properly. (see debugger screenshot below.) </p> <p>Image 1 - <a href="http://i.stack.imgur.com/nOdER.png">http://i.stack.imgur.com/nOdER.png</a></p> <p>I assume that the reason for this is because the five status bytes push later members out of alignment - i.e. they are over machine word boundaries. Is this right?</p> <p>Image 2 - i.imgur.com/ZhbXU.png</p> <p>Am I right in making that assumption?<br> And if so, how can I get the members in and out correctly?</p> <p>Thanks for any help.</p>
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    1. COsumming all the number in column 'Bytes' gives 70, not 54; however, if you replace the 8s for the 8-byte `long` types with 4s, you get the 54 you mentioned...
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      1. POHow can I access members of a struct when it's not aligned properly?
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    2. COPLU code h should be 8 bytes long but in your structure you are using UInt32 wich is only 4 bytes long, are you sure that is correct? Shouldn't it be UInt64? (the same case happens in other PLUs and Prices)
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      1. POHow can I access members of a struct when it's not aligned properly?
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    3. COOh yes, sorry - that was a copying error by me when formatting the table for this site. Fixed now.
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