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<h4 class="node-heading">gdb man</h4>
<a name="gdb-man-1"></a>
<h3 class="heading">gdb man</h3>


<p>gdb [<samp>-help</samp>] [<samp>-nh</samp>] [<samp>-nx</samp>] [<samp>-q</samp>]
[<samp>-batch</samp>] [<samp>-cd=</samp><var>dir</var>] [<samp>-f</samp>]
[<samp>-b</samp>&nbsp;<!-- /@w --><var>bps</var>]
    [<samp>-tty=</samp><var>dev</var>] [<samp>-s</samp> <var>symfile</var>]
[<samp>-e</samp>&nbsp;<!-- /@w --><var>prog</var>] [<samp>-se</samp>&nbsp;<!-- /@w --><var>prog</var>]
[<samp>-c</samp>&nbsp;<!-- /@w --><var>core</var>] [<samp>-p</samp>&nbsp;<!-- /@w --><var>procID</var>]
    [<samp>-x</samp>&nbsp;<!-- /@w --><var>cmds</var>] [<samp>-d</samp>&nbsp;<!-- /@w --><var>dir</var>]
[<var>prog</var>|<var>prog</var> <var>procID</var>|<var>prog</var> <var>core</var>]
</p>
<p>The purpose of a debugger such as <small>GDB</small> is to allow you to see what is
going on &ldquo;inside&rdquo; another program while it executes &ndash; or what another
program was doing at the moment it crashed.
</p>
<p><small>GDB</small> can do four main kinds of things (plus other things in support of
these) to help you catch bugs in the act:
</p>
<ul>
<li> Start your program, specifying anything that might affect its behavior.

</li><li> Make your program stop on specified conditions.

</li><li> Examine what has happened, when your program has stopped.

</li><li> Change things in your program, so you can experiment with correcting the
effects of one bug and go on to learn about another.
</li></ul>

<p>You can use <small>GDB</small> to debug programs written in C, C<tt>++</tt>, Fortran and
Modula-2.
</p>
<p><small>GDB</small> is invoked with the shell command <code>gdb</code>.  Once started, it reads
commands from the terminal until you tell it to exit with the <small>GDB</small>
command <code>quit</code>.  You can get online help from <small>GDB</small> itself
by using the command <code>help</code>.
</p>
<p>You can run <code>gdb</code> with no arguments or options; but the most
usual way to start <small>GDB</small> is with one argument or two, specifying an
executable program as the argument:
</p>
<div class="smallexample">
<pre class="smallexample">gdb program
</pre></div>

<p>You can also start with both an executable program and a core file specified:
</p>
<div class="smallexample">
<pre class="smallexample">gdb program core
</pre></div>

<p>You can, instead, specify a process ID as a second argument or use option
<code>-p</code>, if you want to debug a running process:
</p>
<div class="smallexample">
<pre class="smallexample">gdb program 1234
gdb -p 1234
</pre></div>

<p>would attach <small>GDB</small> to process <code>1234</code>.  With option <samp>-p</samp> you
can omit the <var>program</var> filename.
</p>
<p>Here are some of the most frequently needed <small>GDB</small> commands:
</p>
<dl compact="compact">
<dt><code>break [<var>file</var>:]<var>function</var></code></dt>
<dd><p>Set a breakpoint at <var>function</var> (in <var>file</var>).
</p>
</dd>
<dt><code>run [<var>arglist</var>]</code></dt>
<dd><p>Start your program (with <var>arglist</var>, if specified).
</p>
</dd>
<dt><code>bt</code></dt>
<dd><p>Backtrace: display the program stack.
</p>
</dd>
<dt><code>print <var>expr</var></code></dt>
<dd><p>Display the value of an expression.
</p>
</dd>
<dt><code>c</code></dt>
<dd><p>Continue running your program (after stopping, e.g. at a breakpoint).
</p>
</dd>
<dt><code>next</code></dt>
<dd><p>Execute next program line (after stopping); step <em>over</em> any
function calls in the line.
</p>
</dd>
<dt><code>edit [<var>file</var>:]<var>function</var></code></dt>
<dd><p>look at the program line where it is presently stopped.
</p>
</dd>
<dt><code>list [<var>file</var>:]<var>function</var></code></dt>
<dd><p>type the text of the program in the vicinity of where it is presently stopped.
</p>
</dd>
<dt><code>step</code></dt>
<dd><p>Execute next program line (after stopping); step <em>into</em> any
function calls in the line.
</p>
</dd>
<dt><code>help [<var>name</var>]</code></dt>
<dd><p>Show information about <small>GDB</small> command <var>name</var>, or general information
about using <small>GDB</small>.
</p>
</dd>
<dt><code>quit</code></dt>
<dd><p>Exit from <small>GDB</small>.
</p></dd>
</dl>


<p>Any arguments other than options specify an executable
file and core file (or process ID); that is, the first argument
encountered with no
associated option flag is equivalent to a <samp>-se</samp> option, and the second,
if any, is equivalent to a <samp>-c</samp> option if it&rsquo;s the name of a file.
Many options have
both long and short forms; both are shown here.  The long forms are also
recognized if you truncate them, so long as enough of the option is
present to be unambiguous.  (If you prefer, you can flag option
arguments with <samp>+</samp> rather than <samp>-</samp>, though we illustrate the
more usual convention.)
</p>
<p>All the options and command line arguments you give are processed
in sequential order.  The order makes a difference when the <samp>-x</samp>
option is used.
</p>
<dl compact="compact">
<dt><code>-help</code></dt>
<dt><code>-h</code></dt>
<dd><p>List all options, with brief explanations.
</p>
</dd>
<dt><code>-symbols=<var>file</var></code></dt>
<dt><code>-s <var>file</var></code></dt>
<dd><p>Read symbol table from file <var>file</var>.
</p>
</dd>
<dt><code>-write</code></dt>
<dd><p>Enable writing into executable and core files.
</p>
</dd>
<dt><code>-exec=<var>file</var></code></dt>
<dt><code>-e <var>file</var></code></dt>
<dd><p>Use file <var>file</var> as the executable file to execute when
appropriate, and for examining pure data in conjunction with a core
dump.
</p>
</dd>
<dt><code>-se=<var>file</var></code></dt>
<dd><p>Read symbol table from file <var>file</var> and use it as the executable
file.
</p>
</dd>
<dt><code>-core=<var>file</var></code></dt>
<dt><code>-c <var>file</var></code></dt>
<dd><p>Use file <var>file</var> as a core dump to examine.
</p>
</dd>
<dt><code>-command=<var>file</var></code></dt>
<dt><code>-x <var>file</var></code></dt>
<dd><p>Execute <small>GDB</small> commands from file <var>file</var>.
</p>
</dd>
<dt><code>-ex <var>command</var></code></dt>
<dd><p>Execute given <small>GDB</small> <var>command</var>.
</p>
</dd>
<dt><code>-directory=<var>directory</var></code></dt>
<dt><code>-d <var>directory</var></code></dt>
<dd><p>Add <var>directory</var> to the path to search for source files.
</p>
</dd>
<dt><code>-nh</code></dt>
<dd><p>Do not execute commands from <samp>~/.gdbinit</samp>.
</p>
</dd>
<dt><code>-nx</code></dt>
<dt><code>-n</code></dt>
<dd><p>Do not execute commands from any <samp>.gdbinit</samp> initialization files.
</p>
</dd>
<dt><code>-quiet</code></dt>
<dt><code>-q</code></dt>
<dd><p>&ldquo;Quiet&rdquo;.  Do not print the introductory and copyright messages.  These
messages are also suppressed in batch mode.
</p>
</dd>
<dt><code>-batch</code></dt>
<dd><p>Run in batch mode.  Exit with status <code>0</code> after processing all the command
files specified with <samp>-x</samp> (and <samp>.gdbinit</samp>, if not inhibited).
Exit with nonzero status if an error occurs in executing the <small>GDB</small>
commands in the command files.
</p>
<p>Batch mode may be useful for running <small>GDB</small> as a filter, for example to
download and run a program on another computer; in order to make this
more useful, the message
</p>
<div class="smallexample">
<pre class="smallexample">Program exited normally.
</pre></div>

<p>(which is ordinarily issued whenever a program running under <small>GDB</small> control
terminates) is not issued when running in batch mode.
</p>
</dd>
<dt><code>-cd=<var>directory</var></code></dt>
<dd><p>Run <small>GDB</small> using <var>directory</var> as its working directory,
instead of the current directory.
</p>
</dd>
<dt><code>-fullname</code></dt>
<dt><code>-f</code></dt>
<dd><p>Emacs sets this option when it runs <small>GDB</small> as a subprocess.  It tells
<small>GDB</small> to output the full file name and line number in a standard,
recognizable fashion each time a stack frame is displayed (which
includes each time the program stops).  This recognizable format looks
like two &lsquo;<samp>\032</samp>&rsquo; characters, followed by the file name, line number
and character position separated by colons, and a newline.  The
Emacs-to-<small>GDB</small> interface program uses the two &lsquo;<samp>\032</samp>&rsquo;
characters as a signal to display the source code for the frame.
</p>
</dd>
<dt><code>-b <var>bps</var></code></dt>
<dd><p>Set the line speed (baud rate or bits per second) of any serial
interface used by <small>GDB</small> for remote debugging.
</p>
</dd>
<dt><code>-tty=<var>device</var></code></dt>
<dd><p>Run using <var>device</var> for your program&rsquo;s standard input and output.
</p></dd>
</dl>


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