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175 lines
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<title>Emacs - Debugging with GDB</title>
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<a name="Emacs"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="GDB_002fMI.html#GDB_002fMI">GDB/MI</a>,
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</div>
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<h2 class="chapter">26 Using <span class="sc">gdb</span> under <span class="sc">gnu</span> Emacs</h2>
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<p><a name="index-Emacs-2948"></a><a name="index-g_t_0040sc_007bgnu_007d-Emacs-2949"></a>A special interface allows you to use <span class="sc">gnu</span> Emacs to view (and
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edit) the source files for the program you are debugging with
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<span class="sc">gdb</span>.
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<p>To use this interface, use the command <kbd>M-x gdb</kbd> in Emacs. Give the
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executable file you want to debug as an argument. This command starts
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<span class="sc">gdb</span> as a subprocess of Emacs, with input and output through a newly
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created Emacs buffer.
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<!-- (Do not use the @code{-tui} option to run @value{GDBN} from Emacs.) -->
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<p>Running <span class="sc">gdb</span> under Emacs can be just like running <span class="sc">gdb</span> normally except for two
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things:
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<ul>
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<li>All “terminal” input and output goes through an Emacs buffer, called
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the GUD buffer.
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<p>This applies both to <span class="sc">gdb</span> commands and their output, and to the input
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and output done by the program you are debugging.
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<p>This is useful because it means that you can copy the text of previous
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commands and input them again; you can even use parts of the output
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in this way.
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<p>All the facilities of Emacs' Shell mode are available for interacting
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with your program. In particular, you can send signals the usual
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way—for example, <kbd>C-c C-c</kbd> for an interrupt, <kbd>C-c C-z</kbd> for a
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stop.
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<li><span class="sc">gdb</span> displays source code through Emacs.
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<p>Each time <span class="sc">gdb</span> displays a stack frame, Emacs automatically finds the
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source file for that frame and puts an arrow (‘<samp><span class="samp">=></span></samp>’) at the
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left margin of the current line. Emacs uses a separate buffer for
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source display, and splits the screen to show both your <span class="sc">gdb</span> session
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and the source.
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<p>Explicit <span class="sc">gdb</span> <code>list</code> or search commands still produce output as
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usual, but you probably have no reason to use them from Emacs.
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</ul>
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<p>We call this <dfn>text command mode</dfn>. Emacs 22.1, and later, also uses
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a graphical mode, enabled by default, which provides further buffers
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that can control the execution and describe the state of your program.
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See <a href="../Emacs/GDB-Graphical-Interface.html#GDB-Graphical-Interface">GDB Graphical Interface</a>.
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<p>If you specify an absolute file name when prompted for the <kbd>M-x
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gdb</kbd> argument, then Emacs sets your current working directory to where
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your program resides. If you only specify the file name, then Emacs
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sets your current working directory to the directory associated
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with the previous buffer. In this case, <span class="sc">gdb</span> may find your
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program by searching your environment's <code>PATH</code> variable, but on
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some operating systems it might not find the source. So, although the
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<span class="sc">gdb</span> input and output session proceeds normally, the auxiliary
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buffer does not display the current source and line of execution.
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<p>The initial working directory of <span class="sc">gdb</span> is printed on the top
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line of the GUD buffer and this serves as a default for the commands
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that specify files for <span class="sc">gdb</span> to operate on. See <a href="Files.html#Files">Commands to Specify Files</a>.
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<p>By default, <kbd>M-x gdb</kbd> calls the program called <samp><span class="file">gdb</span></samp>. If you
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need to call <span class="sc">gdb</span> by a different name (for example, if you
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keep several configurations around, with different names) you can
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customize the Emacs variable <code>gud-gdb-command-name</code> to run the
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one you want.
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<p>In the GUD buffer, you can use these special Emacs commands in
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addition to the standard Shell mode commands:
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<dl>
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<dt><kbd>C-h m</kbd><dd>Describe the features of Emacs' GUD Mode.
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<br><dt><kbd>C-c C-s</kbd><dd>Execute to another source line, like the <span class="sc">gdb</span> <code>step</code> command; also
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update the display window to show the current file and location.
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<br><dt><kbd>C-c C-n</kbd><dd>Execute to next source line in this function, skipping all function
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calls, like the <span class="sc">gdb</span> <code>next</code> command. Then update the display window
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to show the current file and location.
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<br><dt><kbd>C-c C-i</kbd><dd>Execute one instruction, like the <span class="sc">gdb</span> <code>stepi</code> command; update
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display window accordingly.
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<br><dt><kbd>C-c C-f</kbd><dd>Execute until exit from the selected stack frame, like the <span class="sc">gdb</span>
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<code>finish</code> command.
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<br><dt><kbd>C-c C-r</kbd><dd>Continue execution of your program, like the <span class="sc">gdb</span> <code>continue</code>
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command.
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<br><dt><kbd>C-c <</kbd><dd>Go up the number of frames indicated by the numeric argument
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(see <a href="../Emacs/Arguments.html#Arguments">Numeric Arguments</a>),
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like the <span class="sc">gdb</span> <code>up</code> command.
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<br><dt><kbd>C-c ></kbd><dd>Go down the number of frames indicated by the numeric argument, like the
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<span class="sc">gdb</span> <code>down</code> command.
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</dl>
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<p>In any source file, the Emacs command <kbd>C-x <SPC></kbd> (<code>gud-break</code>)
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tells <span class="sc">gdb</span> to set a breakpoint on the source line point is on.
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<p>In text command mode, if you type <kbd>M-x speedbar</kbd>, Emacs displays a
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separate frame which shows a backtrace when the GUD buffer is current.
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Move point to any frame in the stack and type <RET> to make it
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become the current frame and display the associated source in the
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source buffer. Alternatively, click <kbd>Mouse-2</kbd> to make the
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selected frame become the current one. In graphical mode, the
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speedbar displays watch expressions.
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<p>If you accidentally delete the source-display buffer, an easy way to get
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it back is to type the command <code>f</code> in the <span class="sc">gdb</span> buffer, to
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request a frame display; when you run under Emacs, this recreates
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the source buffer if necessary to show you the context of the current
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frame.
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<p>The source files displayed in Emacs are in ordinary Emacs buffers
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which are visiting the source files in the usual way. You can edit
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the files with these buffers if you wish; but keep in mind that <span class="sc">gdb</span>
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communicates with Emacs in terms of line numbers. If you add or
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delete lines from the text, the line numbers that <span class="sc">gdb</span> knows cease
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to correspond properly with the code.
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<p>A more detailed description of Emacs' interaction with <span class="sc">gdb</span> is
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given in the Emacs manual (see <a href="../Emacs/Debuggers.html#Debuggers">Debuggers</a>).
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</body></html>
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