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    <p>The struct module mimics C structures. It takes more CPU cycles for a processor to read a 16-bit word on an odd address or a 32-bit dword on an address not divisible by 4, so structures add "pad bytes" to make structure members fall on natural boundaries. Consider:</p> <pre><code>struct { 11 char a; 012345678901 short b; ------------ char c; axbbcxxxdddd int d; }; </code></pre> <p>This structure will occupy 12 bytes of memory (x being pad bytes).</p> <p>Python works similarly (see the <a href="http://docs.python.org/library/struct.html" rel="noreferrer">struct</a> documentation):</p> <pre><code>&gt;&gt;&gt; import struct &gt;&gt;&gt; struct.pack('BHBL',1,2,3,4) '\x01\x00\x02\x00\x03\x00\x00\x00\x04\x00\x00\x00' &gt;&gt;&gt; struct.calcsize('BHBL') 12 </code></pre> <p>Compilers usually have a way of eliminating padding. In Python, any of =&lt;>! will eliminate padding:</p> <pre><code>&gt;&gt;&gt; struct.calcsize('=BHBL') 8 &gt;&gt;&gt; struct.pack('=BHBL',1,2,3,4) '\x01\x02\x00\x03\x04\x00\x00\x00' </code></pre> <p>Beware of letting struct handle padding. In C, these structures:</p> <pre><code>struct A { struct B { short a; int a; char b; char b; }; }; </code></pre> <p>are typically 4 and 8 bytes, respectively. The padding occurs at the end of the structure in case the structures are used in an array. This keeps the 'a' members aligned on correct boundaries for structures later in the array. Python's struct module does not pad at the end:</p> <pre><code>&gt;&gt;&gt; struct.pack('LB',1,2) '\x01\x00\x00\x00\x02' &gt;&gt;&gt; struct.pack('LBLB',1,2,3,4) '\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00\x04' </code></pre>
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    1. COWhat I'm wondering is why Python didn't pack the data in such a format in the first place. "01 01 00" it packed byte 0x01, short 0x01, but it's trying to unpack it like "01 00 01 00". Anyway, I solved my problem, I'm always adding '<' before all my format codes, to make them unpadded little endian. Thanks for the explaination. :)
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    2. COhad a similar problem, the '=' nor '@' did not solve... using code I did on mac on windows
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