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copied!<p>In C, you can manipulate bits with the bitwise operators, such as <code>&</code> and <code>|</code>, as well as the bitshift operators <code><<</code> and <code>>></code>.</p> <p>For instance, to turn off the high bit of a given byte, you can do this.</p> <pre><code>char c = 0xBF; // initial value is bit pattern 10111111 c &= 0x7F; // perform AND against the bit pattern 01111111 // final value is bit pattern 00111111 (0x3F) </code></pre> <p>Does that make sense? Obviously, you need to be able to convert from a bit pattern to hex, but that's not too hard.</p> <p>You can use similar masking to extract the bits you want, and put them in an output buffer.</p> <p><strong>Update:</strong></p> <p>You have 64 bits (8 bytes) of input, and want 56 bits (7 bytes) of output.</p> <p>Let's represent your input as the following, where each letter represents a single bit The 'x' bits are the ones you want to throw away.</p> <pre><code>xAAAAAAA xBBBBBBB xCCCCCCC xDDDDDDD xEEEEEEE xFFFFFFF xGGGGGGG xHHHHHHH </code></pre> <p>So you want your final answer to be:</p> <pre><code>AAAAAAAB BBBBBBCC CCCCCDDD DDDDEEEE EEEFFFFF FFGGGGGG GHHHHHHH </code></pre> <p>So in C, we might have code like this:</p> <pre><code>unsigned char data[8] = {/* put data here */}; // chop off the top bit of the first byte data[0] <<= 1; // the bottom bit of data[0] needs to come from the top data bit of data[1] data[0] |= (data[1] >> 6) & 0x01; // use similar transformations to fill in data[1], data[2], ... data[6] // At the end, data[7] will be useless </code></pre> <p>Of course this is not optimized at all, but hopefully you get the idea.</p>

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