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| Next: <a href="MiniDebugInfo.html#MiniDebugInfo" accesskey="n" rel="next">MiniDebugInfo</a>, Previous: <a href="Files.html#Files" accesskey="p" rel="prev">Files</a>, Up: <a href="GDB-Files.html#GDB-Files" accesskey="u" rel="up">GDB Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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| <hr> |
| <a name="Debugging-Information-in-Separate-Files"></a> |
| <h3 class="section">18.2 Debugging Information in Separate Files</h3> |
| <a name="index-separate-debugging-information-files"></a> |
| <a name="index-debugging-information-in-separate-files"></a> |
| <a name="index-_002edebug-subdirectories"></a> |
| <a name="index-debugging-information-directory_002c-global"></a> |
| <a name="index-global-debugging-information-directories"></a> |
| <a name="index-build-ID_002c-and-separate-debugging-files"></a> |
| <a name="index-_002ebuild_002did-directory"></a> |
| |
| <p><small>GDB</small> allows you to put a program’s debugging information in a |
| file separate from the executable itself, in a way that allows |
| <small>GDB</small> to find and load the debugging information automatically. |
| Since debugging information can be very large—sometimes larger |
| than the executable code itself—some systems distribute debugging |
| information for their executables in separate files, which users can |
| install only when they need to debug a problem. |
| </p> |
| <p><small>GDB</small> supports two ways of specifying the separate debug info |
| file: |
| </p> |
| <ul> |
| <li> The executable contains a <em>debug link</em> that specifies the name of |
| the separate debug info file. The separate debug file’s name is |
| usually <samp><var>executable</var>.debug</samp>, where <var>executable</var> is the |
| name of the corresponding executable file without leading directories |
| (e.g., <samp>ls.debug</samp> for <samp>/usr/bin/ls</samp>). In addition, the |
| debug link specifies a 32-bit <em>Cyclic Redundancy Check</em> (CRC) |
| checksum for the debug file, which <small>GDB</small> uses to validate that |
| the executable and the debug file came from the same build. |
| |
| </li><li> The executable contains a <em>build ID</em>, a unique bit string that is |
| also present in the corresponding debug info file. (This is supported |
| only on some operating systems, when using the ELF or PE file formats |
| for binary files and the <small>GNU</small> Binutils.) For more details about |
| this feature, see the description of the <samp>--build-id</samp> |
| command-line option in <a href="http://sourceware.org/binutils/docs/ld/Options.html#Options">Command Line Options</a> in <cite>The GNU Linker</cite>. The debug info file’s name is not specified |
| explicitly by the build ID, but can be computed from the build ID, see |
| below. |
| </li></ul> |
| |
| <p>Depending on the way the debug info file is specified, <small>GDB</small> |
| uses two different methods of looking for the debug file: |
| </p> |
| <ul> |
| <li> For the “debug link” method, <small>GDB</small> looks up the named file in |
| the directory of the executable file, then in a subdirectory of that |
| directory named <samp>.debug</samp>, and finally under each one of the global debug |
| directories, in a subdirectory whose name is identical to the leading |
| directories of the executable’s absolute file name. |
| |
| </li><li> For the “build ID” method, <small>GDB</small> looks in the |
| <samp>.build-id</samp> subdirectory of each one of the global debug directories for |
| a file named <samp><var>nn</var>/<var>nnnnnnnn</var>.debug</samp>, where <var>nn</var> are the |
| first 2 hex characters of the build ID bit string, and <var>nnnnnnnn</var> |
| are the rest of the bit string. (Real build ID strings are 32 or more |
| hex characters, not 10.) |
| </li></ul> |
| |
| <p>So, for example, suppose you ask <small>GDB</small> to debug |
| <samp>/usr/bin/ls</samp>, which has a debug link that specifies the |
| file <samp>ls.debug</samp>, and a build ID whose value in hex is |
| <code>abcdef1234</code>. If the list of the global debug directories includes |
| <samp>/usr/lib/debug</samp>, then <small>GDB</small> will look for the following |
| debug information files, in the indicated order: |
| </p> |
| <ul class="no-bullet"> |
| <li>- <samp>/usr/lib/debug/.build-id/ab/cdef1234.debug</samp> |
| </li><li>- <samp>/usr/bin/ls.debug</samp> |
| </li><li>- <samp>/usr/bin/.debug/ls.debug</samp> |
| </li><li>- <samp>/usr/lib/debug/usr/bin/ls.debug</samp>. |
| </li></ul> |
| |
| <a name="debug_002dfile_002ddirectory"></a><p>Global debugging info directories default to what is set by <small>GDB</small> |
| configure option <samp>--with-separate-debug-dir</samp>. During <small>GDB</small> run |
| you can also set the global debugging info directories, and view the list |
| <small>GDB</small> is currently using. |
| </p> |
| <dl compact="compact"> |
| <dd> |
| <a name="index-set-debug_002dfile_002ddirectory"></a> |
| </dd> |
| <dt><code>set debug-file-directory <var>directories</var></code></dt> |
| <dd><p>Set the directories which <small>GDB</small> searches for separate debugging |
| information files to <var>directory</var>. Multiple path components can be set |
| concatenating them by a path separator. |
| </p> |
| <a name="index-show-debug_002dfile_002ddirectory"></a> |
| </dd> |
| <dt><code>show debug-file-directory</code></dt> |
| <dd><p>Show the directories <small>GDB</small> searches for separate debugging |
| information files. |
| </p> |
| </dd> |
| </dl> |
| |
| <a name="index-_002egnu_005fdebuglink-sections"></a> |
| <a name="index-debug-link-sections"></a> |
| <p>A debug link is a special section of the executable file named |
| <code>.gnu_debuglink</code>. The section must contain: |
| </p> |
| <ul> |
| <li> A filename, with any leading directory components removed, followed by |
| a zero byte, |
| </li><li> zero to three bytes of padding, as needed to reach the next four-byte |
| boundary within the section, and |
| </li><li> a four-byte CRC checksum, stored in the same endianness used for the |
| executable file itself. The checksum is computed on the debugging |
| information file’s full contents by the function given below, passing |
| zero as the <var>crc</var> argument. |
| </li></ul> |
| |
| <p>Any executable file format can carry a debug link, as long as it can |
| contain a section named <code>.gnu_debuglink</code> with the contents |
| described above. |
| </p> |
| <a name="index-_002enote_002egnu_002ebuild_002did-sections"></a> |
| <a name="index-build-ID-sections"></a> |
| <p>The build ID is a special section in the executable file (and in other |
| ELF binary files that <small>GDB</small> may consider). This section is |
| often named <code>.note.gnu.build-id</code>, but that name is not mandatory. |
| It contains unique identification for the built files—the ID remains |
| the same across multiple builds of the same build tree. The default |
| algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the |
| content for the build ID string. The same section with an identical |
| value is present in the original built binary with symbols, in its |
| stripped variant, and in the separate debugging information file. |
| </p> |
| <p>The debugging information file itself should be an ordinary |
| executable, containing a full set of linker symbols, sections, and |
| debugging information. The sections of the debugging information file |
| should have the same names, addresses, and sizes as the original file, |
| but they need not contain any data—much like a <code>.bss</code> section |
| in an ordinary executable. |
| </p> |
| <p>The <small>GNU</small> binary utilities (Binutils) package includes the |
| ‘<samp>objcopy</samp>’ utility that can produce |
| the separated executable / debugging information file pairs using the |
| following commands: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"><kbd>objcopy --only-keep-debug foo foo.debug</kbd> |
| <kbd>strip -g foo</kbd> |
| </pre></div> |
| |
| <p>These commands remove the debugging |
| information from the executable file <samp>foo</samp> and place it in the file |
| <samp>foo.debug</samp>. You can use the first, second or both methods to link the |
| two files: |
| </p> |
| <ul> |
| <li> The debug link method needs the following additional command to also leave |
| behind a debug link in <samp>foo</samp>: |
| |
| <div class="smallexample"> |
| <pre class="smallexample"><kbd>objcopy --add-gnu-debuglink=foo.debug foo</kbd> |
| </pre></div> |
| |
| <p>Ulrich Drepper’s <samp>elfutils</samp> package, starting with version 0.53, contains |
| a version of the <code>strip</code> command such that the command <kbd>strip foo -f |
| foo.debug</kbd> has the same functionality as the two <code>objcopy</code> commands and |
| the <code>ln -s</code> command above, together. |
| </p> |
| </li><li> Build ID gets embedded into the main executable using <code>ld --build-id</code> or |
| the <small>GCC</small> counterpart <code>gcc -Wl,--build-id</code>. Build ID support plus |
| compatibility fixes for debug files separation are present in <small>GNU</small> binary |
| utilities (Binutils) package since version 2.18. |
| </li></ul> |
| |
| |
| <a name="index-CRC-algorithm-definition"></a> |
| <p>The CRC used in <code>.gnu_debuglink</code> is the CRC-32 defined in |
| IEEE 802.3 using the polynomial: |
| </p> |
| <div class="display"> |
| <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> |
| + <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</pre></div> |
| |
| <p>The function is computed byte at a time, taking the least |
| significant bit of each byte first. The initial pattern |
| <code>0xffffffff</code> is used, to ensure leading zeros affect the CRC and |
| the final result is inverted to ensure trailing zeros also affect the |
| CRC. |
| </p> |
| <p><em>Note:</em> This is the same CRC polynomial as used in handling the |
| <em>Remote Serial Protocol</em> <code>qCRC</code> packet (see <a href="General-Query-Packets.html#qCRC-packet">qCRC packet</a>). |
| However in the case of the Remote Serial Protocol, the CRC is computed |
| <em>most</em> significant bit first, and the result is not inverted, so |
| trailing zeros have no effect on the CRC value. |
| </p> |
| <p>To complete the description, we show below the code of the function |
| which produces the CRC used in <code>.gnu_debuglink</code>. Inverting the |
| initially supplied <code>crc</code> argument means that an initial call to |
| this function passing in zero will start computing the CRC using |
| <code>0xffffffff</code>. |
| </p> |
| <a name="index-gnu_005fdebuglink_005fcrc32"></a> |
| <div class="smallexample"> |
| <pre class="smallexample">unsigned long |
| gnu_debuglink_crc32 (unsigned long crc, |
| unsigned char *buf, size_t len) |
| { |
| static const unsigned long crc32_table[256] = |
| { |
| 0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, |
| 0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, |
| 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, |
| 0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, |
| 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856, |
| 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, |
| 0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, |
| 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, |
| 0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, |
| 0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a, |
| 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, |
| 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, |
| 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, |
| 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, |
| 0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e, |
| 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01, |
| 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, |
| 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950, |
| 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, |
| 0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, |
| 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, |
| 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, |
| 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></div> |
| |
| <p>This computation does not apply to the “build ID” method. |
| </p> |
| <hr> |
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