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300 lines
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<head>
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<title>Separate Debug Files - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="GDB-Files.html#GDB-Files" title="GDB Files">
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<link rel="prev" href="File-Caching.html#File-Caching" title="File Caching">
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<link rel="next" href="MiniDebugInfo.html#MiniDebugInfo" title="MiniDebugInfo">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<body>
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<div class="node">
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<a name="Separate-Debug-Files"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MiniDebugInfo.html#MiniDebugInfo">MiniDebugInfo</a>,
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Previous: <a rel="previous" accesskey="p" href="File-Caching.html#File-Caching">File Caching</a>,
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Up: <a rel="up" accesskey="u" href="GDB-Files.html#GDB-Files">GDB Files</a>
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<hr>
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</div>
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<h3 class="section">18.3 Debugging Information in Separate Files</h3>
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<p><a name="index-separate-debugging-information-files-1275"></a><a name="index-debugging-information-in-separate-files-1276"></a><a name="index-g_t_0040file_007b_002edebug_007d-subdirectories-1277"></a><a name="index-debugging-information-directory_002c-global-1278"></a><a name="index-global-debugging-information-directories-1279"></a><a name="index-build-ID_002c-and-separate-debugging-files-1280"></a><a name="index-g_t_0040file_007b_002ebuild_002did_007d-directory-1281"></a>
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<span class="sc">gdb</span> allows you to put a program's debugging information in a
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file separate from the executable itself, in a way that allows
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<span class="sc">gdb</span> to find and load the debugging information automatically.
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Since debugging information can be very large—sometimes larger
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than the executable code itself—some systems distribute debugging
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information for their executables in separate files, which users can
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install only when they need to debug a problem.
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<p><span class="sc">gdb</span> supports two ways of specifying the separate debug info
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file:
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<ul>
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<li>The executable contains a <dfn>debug link</dfn> that specifies the name of
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the separate debug info file. The separate debug file's name is
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usually <samp><var>executable</var><span class="file">.debug</span></samp>, where <var>executable</var> is the
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name of the corresponding executable file without leading directories
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(e.g., <samp><span class="file">ls.debug</span></samp> for <samp><span class="file">/usr/bin/ls</span></samp>). In addition, the
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debug link specifies a 32-bit <dfn>Cyclic Redundancy Check</dfn> (CRC)
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checksum for the debug file, which <span class="sc">gdb</span> uses to validate that
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the executable and the debug file came from the same build.
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<li>The executable contains a <dfn>build ID</dfn>, a unique bit string that is
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also present in the corresponding debug info file. (This is supported
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only on some operating systems, when using the ELF or PE file formats
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for binary files and the <span class="sc">gnu</span> Binutils.) For more details about
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this feature, see the description of the <samp><span class="option">--build-id</span></samp>
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command-line option in <a href="../ld/Options.html#Options">Command Line Options</a>. The debug info file's name is not specified
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explicitly by the build ID, but can be computed from the build ID, see
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below.
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</ul>
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<p>Depending on the way the debug info file is specified, <span class="sc">gdb</span>
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uses two different methods of looking for the debug file:
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<ul>
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<li>For the “debug link” method, <span class="sc">gdb</span> looks up the named file in
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the directory of the executable file, then in a subdirectory of that
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directory named <samp><span class="file">.debug</span></samp>, and finally under each one of the
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global debug directories, in a subdirectory whose name is identical to
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the leading directories of the executable's absolute file name. (On
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MS-Windows/MS-DOS, the drive letter of the executable's leading
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directories is converted to a one-letter subdirectory, i.e.
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<samp><span class="file">d:/usr/bin/</span></samp> is converted to <samp><span class="file">/d/usr/bin/</span></samp>, because Windows
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filesystems disallow colons in file names.)
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<li>For the “build ID” method, <span class="sc">gdb</span> looks in the
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<samp><span class="file">.build-id</span></samp> subdirectory of each one of the global debug directories for
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a file named <samp><var>nn</var><span class="file">/</span><var>nnnnnnnn</var><span class="file">.debug</span></samp>, where <var>nn</var> are the
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first 2 hex characters of the build ID bit string, and <var>nnnnnnnn</var>
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are the rest of the bit string. (Real build ID strings are 32 or more
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hex characters, not 10.)
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</ul>
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<p>So, for example, suppose you ask <span class="sc">gdb</span> to debug
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<samp><span class="file">/usr/bin/ls</span></samp>, which has a debug link that specifies the
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file <samp><span class="file">ls.debug</span></samp>, and a build ID whose value in hex is
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<code>abcdef1234</code>. If the list of the global debug directories includes
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<samp><span class="file">/usr/lib/debug</span></samp>, then <span class="sc">gdb</span> will look for the following
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debug information files, in the indicated order:
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<ul>
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<li><samp><span class="file">/usr/lib/debug/.build-id/ab/cdef1234.debug</span></samp>
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<li><samp><span class="file">/usr/bin/ls.debug</span></samp>
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<li><samp><span class="file">/usr/bin/.debug/ls.debug</span></samp>
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<li><samp><span class="file">/usr/lib/debug/usr/bin/ls.debug</span></samp>.
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</ul>
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<p><a name="debug_002dfile_002ddirectory"></a>Global debugging info directories default to what is set by <span class="sc">gdb</span>
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configure option <samp><span class="option">--with-separate-debug-dir</span></samp>. During <span class="sc">gdb</span> run
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you can also set the global debugging info directories, and view the list
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<span class="sc">gdb</span> is currently using.
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<a name="index-set-debug_002dfile_002ddirectory-1282"></a>
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<dl><dt><code>set debug-file-directory </code><var>directories</var><dd>Set the directories which <span class="sc">gdb</span> searches for separate debugging
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information files to <var>directory</var>. Multiple path components can be set
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concatenating them by a path separator.
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<p><a name="index-show-debug_002dfile_002ddirectory-1283"></a><br><dt><code>show debug-file-directory</code><dd>Show the directories <span class="sc">gdb</span> searches for separate debugging
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information files.
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</dl>
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<p><a name="index-g_t_0040code_007b_002egnu_005fdebuglink_007d-sections-1284"></a><a name="index-debug-link-sections-1285"></a>A debug link is a special section of the executable file named
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<code>.gnu_debuglink</code>. The section must contain:
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<ul>
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<li>A filename, with any leading directory components removed, followed by
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a zero byte,
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<li>zero to three bytes of padding, as needed to reach the next four-byte
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boundary within the section, and
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<li>a four-byte CRC checksum, stored in the same endianness used for the
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executable file itself. The checksum is computed on the debugging
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information file's full contents by the function given below, passing
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zero as the <var>crc</var> argument.
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</ul>
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<p>Any executable file format can carry a debug link, as long as it can
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contain a section named <code>.gnu_debuglink</code> with the contents
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described above.
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<p><a name="index-g_t_0040code_007b_002enote_002egnu_002ebuild_002did_007d-sections-1286"></a><a name="index-build-ID-sections-1287"></a>The build ID is a special section in the executable file (and in other
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ELF binary files that <span class="sc">gdb</span> may consider). This section is
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often named <code>.note.gnu.build-id</code>, but that name is not mandatory.
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It contains unique identification for the built files—the ID remains
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the same across multiple builds of the same build tree. The default
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algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the
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content for the build ID string. The same section with an identical
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value is present in the original built binary with symbols, in its
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stripped variant, and in the separate debugging information file.
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<p>The debugging information file itself should be an ordinary
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executable, containing a full set of linker symbols, sections, and
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debugging information. The sections of the debugging information file
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should have the same names, addresses, and sizes as the original file,
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but they need not contain any data—much like a <code>.bss</code> section
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in an ordinary executable.
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<p>The <span class="sc">gnu</span> binary utilities (Binutils) package includes the
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‘<samp><span class="samp">objcopy</span></samp>’ utility that can produce
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the separated executable / debugging information file pairs using the
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following commands:
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<pre class="smallexample"> <kbd>objcopy --only-keep-debug foo foo.debug</kbd>
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<kbd>strip -g foo</kbd>
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</pre>
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<p class="noindent">These commands remove the debugging
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information from the executable file <samp><span class="file">foo</span></samp> and place it in the file
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<samp><span class="file">foo.debug</span></samp>. You can use the first, second or both methods to link the
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two files:
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<ul>
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<li>The debug link method needs the following additional command to also leave
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behind a debug link in <samp><span class="file">foo</span></samp>:
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<pre class="smallexample"> <kbd>objcopy --add-gnu-debuglink=foo.debug foo</kbd>
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</pre>
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<p>Ulrich Drepper's <samp><span class="file">elfutils</span></samp> package, starting with version 0.53, contains
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a version of the <code>strip</code> command such that the command <kbd>strip foo -f
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foo.debug</kbd> has the same functionality as the two <code>objcopy</code> commands and
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the <code>ln -s</code> command above, together.
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<li>Build ID gets embedded into the main executable using <code>ld --build-id</code> or
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the <span class="sc">gcc</span> counterpart <code>gcc -Wl,--build-id</code>. Build ID support plus
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compatibility fixes for debug files separation are present in <span class="sc">gnu</span> binary
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utilities (Binutils) package since version 2.18.
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</ul>
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<p class="noindent"><a name="index-CRC-algorithm-definition-1288"></a>The CRC used in <code>.gnu_debuglink</code> is the CRC-32 defined in
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IEEE 802.3 using the polynomial:
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<!-- TexInfo requires naked braces for multi-digit exponents for Tex -->
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<!-- output, but this causes HTML output to barf. HTML has to be set using -->
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<!-- raw commands. So we end up having to specify this equation in 2 -->
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<!-- different ways! -->
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<pre class="display"> <em>x</em><sup>32</sup> + <em>x</em><sup>26</sup> + <em>x</em><sup>23</sup> + <em>x</em><sup>22</sup> + <em>x</em><sup>16</sup> + <em>x</em><sup>12</sup> + <em>x</em><sup>11</sup>
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+ <em>x</em><sup>10</sup> + <em>x</em><sup>8</sup> + <em>x</em><sup>7</sup> + <em>x</em><sup>5</sup> + <em>x</em><sup>4</sup> + <em>x</em><sup>2</sup> + <em>x</em> + 1
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</pre>
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<p>The function is computed byte at a time, taking the least
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significant bit of each byte first. The initial pattern
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<code>0xffffffff</code> is used, to ensure leading zeros affect the CRC and
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the final result is inverted to ensure trailing zeros also affect the
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CRC.
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<p><em>Note:</em> This is the same CRC polynomial as used in handling the
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<dfn>Remote Serial Protocol</dfn> <code>qCRC</code> packet (see <a href="qCRC-packet.html#qCRC-packet">qCRC packet</a>).
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However in the case of the Remote Serial Protocol, the CRC is computed
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<em>most</em> significant bit first, and the result is not inverted, so
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trailing zeros have no effect on the CRC value.
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<p>To complete the description, we show below the code of the function
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which produces the CRC used in <code>.gnu_debuglink</code>. Inverting the
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initially supplied <code>crc</code> argument means that an initial call to
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this function passing in zero will start computing the CRC using
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<code>0xffffffff</code>.
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<p><a name="index-gnu_005fdebuglink_005fcrc32-1289"></a>
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<pre class="smallexample"> unsigned long
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gnu_debuglink_crc32 (unsigned long crc,
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unsigned char *buf, size_t len)
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{
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static const unsigned long crc32_table[256] =
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{
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0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
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0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
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0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
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0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
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0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
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0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
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0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
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0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
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0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
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0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
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0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
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0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
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0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
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0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
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0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
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0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
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0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
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0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
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0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
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0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
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0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
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0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
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|
|
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
|
||
|
|
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
|
||
|
|
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
|
||
|
|
0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
|
||
|
|
0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
|
||
|
|
0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
|
||
|
|
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
|
||
|
|
0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
|
||
|
|
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
|
||
|
|
0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
|
||
|
|
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
|
||
|
|
0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
|
||
|
|
0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
|
||
|
|
0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
|
||
|
|
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
|
||
|
|
0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
|
||
|
|
0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
|
||
|
|
0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
|
||
|
|
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
|
||
|
|
0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
|
||
|
|
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
|
||
|
|
0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
|
||
|
|
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
|
||
|
|
0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
|
||
|
|
0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
|
||
|
|
0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
|
||
|
|
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
|
||
|
|
0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
|
||
|
|
0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
|
||
|
|
0x2d02ef8d
|
||
|
|
};
|
||
|
|
unsigned char *end;
|
||
|
|
|
||
|
|
crc = ~crc & 0xffffffff;
|
||
|
|
for (end = buf + len; buf < end; ++buf)
|
||
|
|
crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
|
||
|
|
return ~crc & 0xffffffff;
|
||
|
|
}
|
||
|
|
</pre>
|
||
|
|
<p class="noindent">This computation does not apply to the “build ID” method.
|
||
|
|
|
||
|
|
</body></html>
|
||
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