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<title>Debugging with GDB: Basic Guile</title>
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<a name="Basic-Guile"></a>
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<div class="header">
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<p>
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Next: <a href="Guile-Configuration.html#Guile-Configuration" accesskey="n" rel="next">Guile Configuration</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</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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<hr>
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<a name="Basic-Guile-1"></a>
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<h4 class="subsubsection">23.3.3.1 Basic Guile</h4>
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<a name="index-guile-stdout"></a>
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<a name="index-guile-pagination"></a>
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<p>At startup, <small>GDB</small> overrides Guile’s <code>current-output-port</code> and
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<code>current-error-port</code> to print using <small>GDB</small>’s output-paging streams.
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A Guile program which outputs to one of these streams may have its
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output interrupted by the user (see <a href="Screen-Size.html#Screen-Size">Screen Size</a>). In this
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situation, a Guile <code>signal</code> exception is thrown with value <code>SIGINT</code>.
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</p>
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<p>Guile’s history mechanism uses the same naming as <small>GDB</small>’s,
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namely the user of dollar-variables (e.g., $1, $2, etc.).
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The results of evaluations in Guile and in GDB are counted separately,
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<code>$1</code> in Guile is not the same value as <code>$1</code> in <small>GDB</small>.
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</p>
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<p><small>GDB</small> is not thread-safe. If your Guile program uses multiple
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threads, you must be careful to only call <small>GDB</small>-specific
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functions in the <small>GDB</small> thread.
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</p>
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<p>Some care must be taken when writing Guile code to run in
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<small>GDB</small>. Two things are worth noting in particular:
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</p>
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<ul>
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<li> <small>GDB</small> installs handlers for <code>SIGCHLD</code> and <code>SIGINT</code>.
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Guile code must not override these, or even change the options using
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<code>sigaction</code>. If your program changes the handling of these
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signals, <small>GDB</small> will most likely stop working correctly. Note
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that it is unfortunately common for GUI toolkits to install a
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<code>SIGCHLD</code> handler.
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</li><li> <small>GDB</small> takes care to mark its internal file descriptors as
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close-on-exec. However, this cannot be done in a thread-safe way on
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all platforms. Your Guile programs should be aware of this and
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should both create new file descriptors with the close-on-exec flag
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set and arrange to close unneeded file descriptors before starting a
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child process.
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</li></ul>
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<a name="index-guile-gdb-module"></a>
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<p><small>GDB</small> introduces a new Guile module, named <code>gdb</code>. All
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methods and classes added by <small>GDB</small> are placed in this module.
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<small>GDB</small> does not automatically <code>import</code> the <code>gdb</code> module,
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scripts must do this themselves. There are various options for how to
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import a module, so <small>GDB</small> leaves the choice of how the <code>gdb</code>
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module is imported to the user.
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To simplify interactive use, it is recommended to add one of the following
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to your ~/.gdbinit.
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</p>
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<div class="smallexample">
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<pre class="smallexample">guile (use-modules (gdb))
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</pre></div>
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<div class="smallexample">
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<pre class="smallexample">guile (use-modules ((gdb) #:renamer (symbol-prefix-proc 'gdb:)))
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</pre></div>
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<p>Which one to choose depends on your preference.
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The second one adds <code>gdb:</code> as a prefix to all module functions
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and variables.
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</p>
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<p>The rest of this manual assumes the <code>gdb</code> module has been imported
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without any prefix. See the Guile documentation for <code>use-modules</code>
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for more information
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(see <a href="http://www.gnu.org/software/guile/manual/html_node/Using-Guile-Modules.html#Using-Guile-Modules">Using Guile Modules</a> in <cite>GNU Guile Reference Manual</cite>).
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</p>
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<p>Example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) guile (value-type (make-value 1))
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ERROR: Unbound variable: value-type
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Error while executing Scheme code.
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(gdb) guile (use-modules (gdb))
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(gdb) guile (value-type (make-value 1))
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int
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(gdb)
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</pre></div>
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<p>The <code>(gdb)</code> module provides these basic Guile functions.
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</p>
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<dl>
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<dt><a name="index-execute"></a>Scheme Procedure: <strong>execute</strong> <em>command <span class="roman">[</span>#:from-tty boolean<span class="roman">]</span> <span class="roman">[</span>#:to-string boolean<span class="roman">]</span></em></dt>
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<dd><p>Evaluate <var>command</var>, a string, as a <small>GDB</small> CLI command.
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If a <small>GDB</small> exception happens while <var>command</var> runs, it is
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translated as described in
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<a href="Guile-Exception-Handling.html#Guile-Exception-Handling">Guile Exception Handling</a>.
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</p>
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<p><var>from-tty</var> specifies whether <small>GDB</small> ought to consider this
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command as having originated from the user invoking it interactively.
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It must be a boolean value. If omitted, it defaults to <code>#f</code>.
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</p>
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<p>By default, any output produced by <var>command</var> is sent to
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<small>GDB</small>’s standard output (and to the log output if logging is
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turned on). If the <var>to-string</var> parameter is
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<code>#t</code>, then output will be collected by <code>execute</code> and
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returned as a string. The default is <code>#f</code>, in which case the
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return value is unspecified. If <var>to-string</var> is <code>#t</code>, the
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<small>GDB</small> virtual terminal will be temporarily set to unlimited width
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and height, and its pagination will be disabled; see <a href="Screen-Size.html#Screen-Size">Screen Size</a>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-history_002dref"></a>Scheme Procedure: <strong>history-ref</strong> <em>number</em></dt>
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<dd><p>Return a value from <small>GDB</small>’s value history (see <a href="Value-History.html#Value-History">Value History</a>). The <var>number</var> argument indicates which history element to return.
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If <var>number</var> is negative, then <small>GDB</small> will take its absolute value
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and count backward from the last element (i.e., the most recent element) to
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find the value to return. If <var>number</var> is zero, then <small>GDB</small> will
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return the most recent element. If the element specified by <var>number</var>
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doesn’t exist in the value history, a <code>gdb:error</code> exception will be
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raised.
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</p>
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<p>If no exception is raised, the return value is always an instance of
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<code><gdb:value></code> (see <a href="Values-From-Inferior-In-Guile.html#Values-From-Inferior-In-Guile">Values From Inferior In Guile</a>).
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</p>
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<p><em>Note:</em> <small>GDB</small>’s value history is independent of Guile’s.
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<code>$1</code> in <small>GDB</small>’s value history contains the result of evaluating
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an expression from <small>GDB</small>’s command line and <code>$1</code> from Guile’s
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history contains the result of evaluating an expression from Guile’s
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command line.
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</p></dd></dl>
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<dl>
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<dt><a name="index-history_002dappend_0021"></a>Scheme Procedure: <strong>history-append!</strong> <em>value</em></dt>
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<dd><p>Append <var>value</var>, an instance of <code><gdb:value></code>, to <small>GDB</small>’s
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value history. Return its index in the history.
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</p>
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<p>Putting into history values returned by Guile extensions will allow
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the user convenient access to those values via CLI history
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facilities.
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</p></dd></dl>
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<dl>
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<dt><a name="index-parse_002dand_002deval"></a>Scheme Procedure: <strong>parse-and-eval</strong> <em>expression</em></dt>
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<dd><p>Parse <var>expression</var> as an expression in the current language,
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evaluate it, and return the result as a <code><gdb:value></code>.
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The <var>expression</var> must be a string.
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</p>
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<p>This function can be useful when implementing a new command
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(see <a href="Commands-In-Guile.html#Commands-In-Guile">Commands In Guile</a>), as it provides a way to parse the
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command’s arguments as an expression.
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It is also is useful when computing values.
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For example, it is the only way to get the value of a
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convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>) as a <code><gdb:value></code>.
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</p></dd></dl>
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<div class="header">
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<p>
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Next: <a href="Guile-Configuration.html#Guile-Configuration" accesskey="n" rel="next">Guile Configuration</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile API</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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