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POPacking data of different sizes into a list of unsigned ints
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copied!<p>I have a set of data that represents a hardware structure that I need to manipulate in python. The real structure is 4 KB in size...I'll just whip up a quick example:</p> <pre><code>Byte(s) Value 0-1 0x0102 2-3 0x0304 4-23 "AStrWith20Characters" 24-63 "WoahThisStringHas40CharactersItIsHuge!!!" 64-71 "Only8Chr" 72-74 0x050607 74 0x08 75-127 0x00 (padding) </code></pre> <p>The idea is I pack this structure up into a list of 32 bit Ints, pass the list off to another function, and <em>that</em> function then writes the whole shebang out to memory. The Memory Writing function accepts 64 Bytes at a time, so I'd have to make two calls.</p> <p>So, for the example above, I'd need to make these calls (I'll use big endian for readability):</p> <pre><code>WrToMemory([0x01020304, 0x41537472, 0x57697468, 0x3230436, 0x61726163, 0x74657273, 0x576F6168, 0x5468697, 0x53747269, 0x6E674861, 0x73343043, 0x6861726, 0x63746572, 0x73497449, 0x73487567, 0x65212121]) WrToMemory([0x4F6E6C79, 0x38436872, 0x05060708, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x00000000]) </code></pre> <p>The problem is that I don't know how to pack this data in any kind of efficient way. I have been struggling with using <code>struct</code> and <code>array</code>, but either I am missing something conceptually or I am simply using the wrong tool for the job.</p> <p>Ultimately, I know I can write a big hairy function that checks data types, does a bunch of shifting and masking, converts ASCII to hex, and the constructs data packets I need. But that seems...inelegant. </p> <p>I figure there's got to be a way to tell python "This is a number that is two bytes long. Pack it up. Another two byte number, append it to the last one. Now this is a string, with 20 bytes. Append it. Go on until we have 64 bytes. Then convert the list of 64 bytes we just made into a lsit of 16 unsigned integers.</p> <p>It seems like the <code>pack</code> and <code>unpack</code> functions are what I should be using, but like I said above...I must be missing something. How does <code>pack</code> handle the 3 byte number, for example? If you use a format string like <code>HB</code>, then <code>pack</code> expects two arguments:</p> <pre><code>>>> pack('HB', 0x050607) Traceback (most recent call last): File "<stdin>", line 1, in <module> struct.error: pack requires exactly 2 arguments </code></pre> <p>And I can't use <code>unpack</code>, because that expects a string, not a number.</p> <p>Hope I've made my problem clear. Any help is much appreciated!</p>

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