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POHow do I read in hex values from a binary file and decipher some bytes containing bitflag values?
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  1. POHow do I read in hex values from a binary file and decipher some bytes containing bitflag values?
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    copied!<p>I'm reading a bunch of bytes from a binary file. It is a RAR file. I'm interested in the 11th and 12th bytes of the file because the header specifications state:</p> <blockquote> <p>HEAD_FLAGS Bit flags: 2 bytes</p> <pre><code> 0x0001 - Volume attribute (archive volume) 0x0002 - Archive comment present RAR 3.x uses the separate comment block and does not set this flag. 0x0004 - Archive lock attribute 0x0008 - Solid attribute (solid archive) 0x0010 - New volume naming scheme ('volname.partN.rar') 0x0020 - Authenticity information present RAR 3.x does not set this flag. 0x0040 - Recovery record present 0x0080 - Block headers are encrypted 0x0100 - First volume (set only by RAR 3.0 and later) other bits in HEAD_FLAGS are reserved for internal use </code></pre> </blockquote> <p>The file that I'm playing with has <code>00</code> and <code>0D</code> as positions 11 and 12 respectively. </p> <p>I can't make sense of these two values as they are bit flags (which I have failed to understand).</p> <p>I have these two values in <code>byte</code> array that is 12 bytes long. What I need to check in this sequence is whether the flag <code>0x0100</code> and <code>0x0001</code> is set or not.</p> <p>I'm lost with this. Thanks.</p> <hr> <p>I've inspected some of the files in a Hex editor and what i've seen is that 11th and 12th bytes need to be read together. That's why the specs list all the bit flags are 4 letter hex codes. Checking the bit flags individually yields incorrect results.</p> <hr> <p>Assimilating as much information from the answers/tips, 've solved this in the following way:</p> <pre><code>FileInputStream fisFileInputStream = new FileInputStream((new File("C:\\testarchive.r00")); byte[] bytHeader = new byte[20]; //The first 20 bytes are the RAR header. fisFileInputStream.read(bytHeader); short val=(short)( ((bytHeader[10]&amp;0xFF)&lt;&lt;8) | (bytHeader[11]&amp;0xFF) ); //Joining the two bytes into a short System.out.println("Volume Attribute (0x0001): " + ((val &amp; 0x0001) != 0)); System.out.println("First volume (0x0100): " + ((val &amp; 0x0100) != 0)); </code></pre> <p>I've tried this code with multiple RAR archives — spanned ones, non-spanned ones, the first file of a spanned archive, another file of a spanned archive.</p> <p>The code itself works fine except for a very minor quirk. I get the opposite results from my hex values i.e. </p> <p>When inspecting a file which <strong>is not the first file</strong> in a spanned archive, I get the volume atrribute <code>0x0001</code> as not set (<code>false</code>), and the "First volume" <code>0x100</code> as set (<code>true</code>). </p> <p>When inspecting a file which <strong>is the first file</strong> in a spanned archive, I get the exact opposite results.</p> <p>Now I modify my code to believe that the original specs are wrong (highly unlikely) and that <code>0x0001</code> means that is is the first file in a spanned archive and <code>0x0100</code> means that is a spanned archive, then it's all okay.</p> <p>..but I guess I'm doing something wrong here with my bit flag logic. Any ideas?</p>
 

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