<html lang="en">
<head>
<title>Emacs - Debugging with GDB</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Debugging with GDB">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="prev" href="TUI.html#TUI" title="TUI">
<link rel="next" href="GDB_002fMI.html#GDB_002fMI" title="GDB/MI">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software'' and ``Free Software Needs
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom.''-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Emacs"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="GDB_002fMI.html#GDB_002fMI">GDB/MI</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="TUI.html#TUI">TUI</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="index.html#Top">Top</a>
<hr>
</div>

<h2 class="chapter">26 Using <span class="sc">gdb</span> under <span class="sc">gnu</span> Emacs</h2>

<p><a name="index-Emacs-1998"></a><a name="index-g_t_0040sc_007bgnu_007d-Emacs-1999"></a>A special interface allows you to use <span class="sc">gnu</span> Emacs to view (and
edit) the source files for the program you are debugging with
<span class="sc">gdb</span>.

   <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
<span class="sc">gdb</span> as a subprocess of Emacs, with input and output through a newly
created Emacs buffer. 
<!-- (Do not use the @code{-tui} option to run @value{GDBN} from Emacs.) -->

   <p>Running <span class="sc">gdb</span> under Emacs can be just like running <span class="sc">gdb</span> normally except for two
things:

     <ul>
<li>All &ldquo;terminal&rdquo; input and output goes through an Emacs buffer, called
the GUD buffer.

     <p>This applies both to <span class="sc">gdb</span> commands and their output, and to the input
and output done by the program you are debugging.

     <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>All the facilities of Emacs' Shell mode are available for interacting
with your program.  In particular, you can send signals the usual
way&mdash;for example, <kbd>C-c C-c</kbd> for an interrupt, <kbd>C-c C-z</kbd> for a
stop.

     <li><span class="sc">gdb</span> displays source code through Emacs.

     <p>Each time <span class="sc">gdb</span> displays a stack frame, Emacs automatically finds the
source file for that frame and puts an arrow (&lsquo;<samp><span class="samp">=&gt;</span></samp>&rsquo;) at the
left margin of the current line.  Emacs uses a separate buffer for
source display, and splits the screen to show both your <span class="sc">gdb</span> session
and the source.

     <p>Explicit <span class="sc">gdb</span> <code>list</code> or search commands still produce output as
usual, but you probably have no reason to use them from Emacs. 
</ul>

   <p>We call this <dfn>text command mode</dfn>.  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>.

   <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 to the directory associated
with the previous buffer.  In this case, <span class="sc">gdb</span> 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
<span class="sc">gdb</span> input and output session proceeds normally, the auxiliary
buffer does not display the current source and line of execution.

   <p>The initial working directory of <span class="sc">gdb</span> is printed on the top
line of the GUD buffer and this serves as a default for the commands
that specify files for <span class="sc">gdb</span> to operate on.  See <a href="Files.html#Files">Commands to Specify Files</a>.

   <p>By default, <kbd>M-x gdb</kbd> calls the program called <samp><span class="file">gdb</span></samp>.  If you
need to call <span class="sc">gdb</span> 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>In the GUD buffer, you can use these special Emacs commands in
addition to the standard Shell mode commands:

     <dl>
<dt><kbd>C-h m</kbd><dd>Describe the features of Emacs' GUD Mode.

     <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
update the display window to show the current file and location.

     <br><dt><kbd>C-c C-n</kbd><dd>Execute to next source line in this function, skipping all function
calls, like the <span class="sc">gdb</span> <code>next</code> command.  Then update the display window
to show the current file and location.

     <br><dt><kbd>C-c C-i</kbd><dd>Execute one instruction, like the <span class="sc">gdb</span> <code>stepi</code> command; update
display window accordingly.

     <br><dt><kbd>C-c C-f</kbd><dd>Execute until exit from the selected stack frame, like the <span class="sc">gdb</span>
<code>finish</code> command.

     <br><dt><kbd>C-c C-r</kbd><dd>Continue execution of your program, like the <span class="sc">gdb</span> <code>continue</code>
command.

     <br><dt><kbd>C-c &lt;</kbd><dd>Go up the number of frames indicated by the numeric argument
(see <a href="../Emacs/Arguments.html#Arguments">Numeric Arguments</a>),
like the <span class="sc">gdb</span> <code>up</code> command.

     <br><dt><kbd>C-c &gt;</kbd><dd>Go down the number of frames indicated by the numeric argument, like the
<span class="sc">gdb</span> <code>down</code> command. 
</dl>

   <p>In any source file, the Emacs command <kbd>C-x &lt;SPC&gt;</kbd> (<code>gud-break</code>)
tells <span class="sc">gdb</span> to set a breakpoint on the source line point is on.

   <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 &lt;RET&gt; 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>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 <span class="sc">gdb</span> 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>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 <span class="sc">gdb</span>
communicates with Emacs in terms of line numbers.  If you add or
delete lines from the text, the line numbers that <span class="sc">gdb</span> knows cease
to correspond properly with the code.

   <p>A more detailed description of Emacs' interaction with <span class="sc">gdb</span> is
given in the Emacs manual (see <a href="../Emacs/Debuggers.html#Debuggers">Debuggers</a>).

<!-- The following dropped because Epoch is nonstandard.  Reactivate -->
<!-- if/when v19 does something similar. -doc@cygnus.com 19dec1990 -->
   </body></html>