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<title>gdb man (Debugging with GDB)</title>
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Man-Pages.html#Man-Pages" rel="up" title="Man Pages">
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<link href="gdbserver-man.html#gdbserver-man" rel="next" title="gdbserver man">
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</head>
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<body lang="en">
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<a name="gdb-man"></a>
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<div class="header">
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<p>
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Next: <a href="gdbserver-man.html#gdbserver-man" accesskey="n" rel="next">gdbserver man</a>, Up: <a href="Man-Pages.html#Man-Pages" accesskey="u" rel="up">Man Pages</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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</div>
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<hr>
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<h4 class="node-heading">gdb man</h4>
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<a name="gdb-man-1"></a>
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<h3 class="heading">gdb man</h3>
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<p>gdb [<samp>-help</samp>] [<samp>-nh</samp>] [<samp>-nx</samp>] [<samp>-q</samp>]
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[<samp>-batch</samp>] [<samp>-cd=</samp><var>dir</var>] [<samp>-f</samp>]
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[<samp>-b</samp> <!-- /@w --><var>bps</var>]
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[<samp>-tty=</samp><var>dev</var>] [<samp>-s</samp> <var>symfile</var>]
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[<samp>-e</samp> <!-- /@w --><var>prog</var>] [<samp>-se</samp> <!-- /@w --><var>prog</var>]
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[<samp>-c</samp> <!-- /@w --><var>core</var>] [<samp>-p</samp> <!-- /@w --><var>procID</var>]
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[<samp>-x</samp> <!-- /@w --><var>cmds</var>] [<samp>-d</samp> <!-- /@w --><var>dir</var>]
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[<var>prog</var>|<var>prog</var> <var>procID</var>|<var>prog</var> <var>core</var>]
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</p>
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<p>The purpose of a debugger such as <small>GDB</small> is to allow you to see what is
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going on “inside” another program while it executes – or what another
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program was doing at the moment it crashed.
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</p>
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<p><small>GDB</small> can do four main kinds of things (plus other things in support of
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these) to help you catch bugs in the act:
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</p>
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<ul>
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<li> Start your program, specifying anything that might affect its behavior.
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</li><li> Make your program stop on specified conditions.
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</li><li> Examine what has happened, when your program has stopped.
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</li><li> Change things in your program, so you can experiment with correcting the
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effects of one bug and go on to learn about another.
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</li></ul>
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<p>You can use <small>GDB</small> to debug programs written in C, C<tt>++</tt>, Fortran and
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Modula-2.
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</p>
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<p><small>GDB</small> is invoked with the shell command <code>gdb</code>. Once started, it reads
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commands from the terminal until you tell it to exit with the <small>GDB</small>
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command <code>quit</code>. You can get online help from <small>GDB</small> itself
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by using the command <code>help</code>.
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</p>
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<p>You can run <code>gdb</code> with no arguments or options; but the most
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usual way to start <small>GDB</small> is with one argument or two, specifying an
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executable program as the argument:
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</p>
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<div class="smallexample">
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<pre class="smallexample">gdb program
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</pre></div>
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<p>You can also start with both an executable program and a core file specified:
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</p>
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<div class="smallexample">
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<pre class="smallexample">gdb program core
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</pre></div>
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<p>You can, instead, specify a process ID as a second argument, if you want
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to debug a running process:
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</p>
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<div class="smallexample">
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<pre class="smallexample">gdb program 1234
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gdb -p 1234
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</pre></div>
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<p>would attach <small>GDB</small> to process <code>1234</code> (unless you also have a file
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named <samp>1234</samp>; <small>GDB</small> does check for a core file first).
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With option <samp>-p</samp> you can omit the <var>program</var> filename.
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</p>
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<p>Here are some of the most frequently needed <small>GDB</small> commands:
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</p>
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<dl compact="compact">
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<dt><code>break [<var>file</var>:]<var>function</var></code></dt>
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<dd><p>Set a breakpoint at <var>function</var> (in <var>file</var>).
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</p>
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</dd>
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<dt><code>run [<var>arglist</var>]</code></dt>
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<dd><p>Start your program (with <var>arglist</var>, if specified).
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</p>
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</dd>
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<dt><code>bt</code></dt>
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<dd><p>Backtrace: display the program stack.
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</p>
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</dd>
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<dt><code>print <var>expr</var></code></dt>
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<dd><p>Display the value of an expression.
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</p>
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</dd>
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<dt><code>c</code></dt>
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<dd><p>Continue running your program (after stopping, e.g. at a breakpoint).
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</p>
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</dd>
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<dt><code>next</code></dt>
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<dd><p>Execute next program line (after stopping); step <em>over</em> any
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function calls in the line.
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</p>
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</dd>
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<dt><code>edit [<var>file</var>:]<var>function</var></code></dt>
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<dd><p>look at the program line where it is presently stopped.
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</p>
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</dd>
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<dt><code>list [<var>file</var>:]<var>function</var></code></dt>
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<dd><p>type the text of the program in the vicinity of where it is presently stopped.
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</p>
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</dd>
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<dt><code>step</code></dt>
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<dd><p>Execute next program line (after stopping); step <em>into</em> any
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function calls in the line.
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</p>
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</dd>
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<dt><code>help [<var>name</var>]</code></dt>
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<dd><p>Show information about <small>GDB</small> command <var>name</var>, or general information
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about using <small>GDB</small>.
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</p>
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</dd>
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<dt><code>quit</code></dt>
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<dd><p>Exit from <small>GDB</small>.
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</p></dd>
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</dl>
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<p>Any arguments other than options specify an executable
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file and core file (or process ID); that is, the first argument
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encountered with no
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associated option flag is equivalent to a <samp>-se</samp> option, and the second,
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if any, is equivalent to a <samp>-c</samp> option if it’s the name of a file.
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Many options have
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both long and short forms; both are shown here. The long forms are also
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recognized if you truncate them, so long as enough of the option is
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present to be unambiguous. (If you prefer, you can flag option
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arguments with <samp>+</samp> rather than <samp>-</samp>, though we illustrate the
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more usual convention.)
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</p>
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<p>All the options and command line arguments you give are processed
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in sequential order. The order makes a difference when the <samp>-x</samp>
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option is used.
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</p>
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<dl compact="compact">
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<dt><code>-help</code></dt>
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<dt><code>-h</code></dt>
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<dd><p>List all options, with brief explanations.
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</p>
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</dd>
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<dt><code>-symbols=<var>file</var></code></dt>
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<dt><code>-s <var>file</var></code></dt>
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<dd><p>Read symbol table from file <var>file</var>.
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</p>
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</dd>
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<dt><code>-write</code></dt>
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<dd><p>Enable writing into executable and core files.
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</p>
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</dd>
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<dt><code>-exec=<var>file</var></code></dt>
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<dt><code>-e <var>file</var></code></dt>
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<dd><p>Use file <var>file</var> as the executable file to execute when
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appropriate, and for examining pure data in conjunction with a core
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dump.
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</p>
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</dd>
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<dt><code>-se=<var>file</var></code></dt>
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<dd><p>Read symbol table from file <var>file</var> and use it as the executable
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file.
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</p>
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</dd>
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<dt><code>-core=<var>file</var></code></dt>
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<dt><code>-c <var>file</var></code></dt>
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<dd><p>Use file <var>file</var> as a core dump to examine.
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</p>
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</dd>
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<dt><code>-command=<var>file</var></code></dt>
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<dt><code>-x <var>file</var></code></dt>
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<dd><p>Execute <small>GDB</small> commands from file <var>file</var>.
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</p>
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</dd>
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<dt><code>-ex <var>command</var></code></dt>
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<dd><p>Execute given <small>GDB</small> <var>command</var>.
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</p>
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</dd>
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<dt><code>-directory=<var>directory</var></code></dt>
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<dt><code>-d <var>directory</var></code></dt>
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<dd><p>Add <var>directory</var> to the path to search for source files.
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</p>
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</dd>
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<dt><code>-nh</code></dt>
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<dd><p>Do not execute commands from <samp>~/.gdbinit</samp>.
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</p>
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</dd>
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<dt><code>-nx</code></dt>
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<dt><code>-n</code></dt>
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<dd><p>Do not execute commands from any <samp>.gdbinit</samp> initialization files.
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</p>
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</dd>
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<dt><code>-quiet</code></dt>
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<dt><code>-q</code></dt>
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<dd><p>“Quiet”. Do not print the introductory and copyright messages. These
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messages are also suppressed in batch mode.
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</p>
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</dd>
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<dt><code>-batch</code></dt>
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<dd><p>Run in batch mode. Exit with status <code>0</code> after processing all the command
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files specified with <samp>-x</samp> (and <samp>.gdbinit</samp>, if not inhibited).
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Exit with nonzero status if an error occurs in executing the <small>GDB</small>
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commands in the command files.
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</p>
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<p>Batch mode may be useful for running <small>GDB</small> as a filter, for example to
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download and run a program on another computer; in order to make this
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more useful, the message
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</p>
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<div class="smallexample">
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<pre class="smallexample">Program exited normally.
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</pre></div>
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<p>(which is ordinarily issued whenever a program running under <small>GDB</small> control
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terminates) is not issued when running in batch mode.
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</p>
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</dd>
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<dt><code>-cd=<var>directory</var></code></dt>
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<dd><p>Run <small>GDB</small> using <var>directory</var> as its working directory,
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instead of the current directory.
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</p>
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</dd>
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<dt><code>-fullname</code></dt>
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<dt><code>-f</code></dt>
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<dd><p>Emacs sets this option when it runs <small>GDB</small> as a subprocess. It tells
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<small>GDB</small> to output the full file name and line number in a standard,
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recognizable fashion each time a stack frame is displayed (which
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includes each time the program stops). This recognizable format looks
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like two ‘<samp>\032</samp>’ characters, followed by the file name, line number
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and character position separated by colons, and a newline. The
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Emacs-to-<small>GDB</small> interface program uses the two ‘<samp>\032</samp>’
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characters as a signal to display the source code for the frame.
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</p>
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</dd>
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<dt><code>-b <var>bps</var></code></dt>
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<dd><p>Set the line speed (baud rate or bits per second) of any serial
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interface used by <small>GDB</small> for remote debugging.
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</p>
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</dd>
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<dt><code>-tty=<var>device</var></code></dt>
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<dd><p>Run using <var>device</var> for your program’s standard input and output.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="gdbserver-man.html#gdbserver-man" accesskey="n" rel="next">gdbserver man</a>, Up: <a href="Man-Pages.html#Man-Pages" accesskey="u" rel="up">Man Pages</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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</div>
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