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