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<h5 class="subsubsection">23.3.3.1 Basic Guile</h5>

<p><a name="index-guile-stdout-2458"></a><a name="index-guile-pagination-2459"></a>At startup, <span class="sc">gdb</span> overrides Guile's <code>current-output-port</code> and
<code>current-error-port</code> to print using <span class="sc">gdb</span>'s output-paging streams. 
A Guile program which outputs to one of these streams may have its
output interrupted by the user (see <a href="Screen-Size.html#Screen-Size">Screen Size</a>).  In this
situation, a Guile <code>signal</code> exception is thrown with value <code>SIGINT</code>.

   <p>Guile's history mechanism uses the same naming as <span class="sc">gdb</span>'s,
namely the user of dollar-variables (e.g., $1, $2, etc.). 
The results of evaluations in Guile and in GDB are counted separately,
<code>$1</code> in Guile is not the same value as <code>$1</code> in <span class="sc">gdb</span>.

   <p><span class="sc">gdb</span> is not thread-safe.  If your Guile program uses multiple
threads, you must be careful to only call <span class="sc">gdb</span>-specific
functions in the <span class="sc">gdb</span> thread.

   <p>Some care must be taken when writing Guile code to run in
<span class="sc">gdb</span>.  Two things are worth noting in particular:

     <ul>
<li><span class="sc">gdb</span> installs handlers for <code>SIGCHLD</code> and <code>SIGINT</code>. 
Guile code must not override these, or even change the options using
<code>sigaction</code>.  If your program changes the handling of these
signals, <span class="sc">gdb</span> will most likely stop working correctly.  Note
that it is unfortunately common for GUI toolkits to install a
<code>SIGCHLD</code> handler.

     <li><span class="sc">gdb</span> takes care to mark its internal file descriptors as
close-on-exec.  However, this cannot be done in a thread-safe way on
all platforms.  Your Guile programs should be aware of this and
should both create new file descriptors with the close-on-exec flag
set and arrange to close unneeded file descriptors before starting a
child process. 
</ul>

   <p><a name="index-guile-gdb-module-2460"></a><span class="sc">gdb</span> introduces a new Guile module, named <code>gdb</code>.  All
methods and classes added by <span class="sc">gdb</span> are placed in this module. 
<span class="sc">gdb</span> does not automatically <code>import</code> the <code>gdb</code> module,
scripts must do this themselves.  There are various options for how to
import a module, so <span class="sc">gdb</span> leaves the choice of how the <code>gdb</code>
module is imported to the user. 
To simplify interactive use, it is recommended to add one of the following
to your ~/.gdbinit.

<pre class="smallexample">     guile (use-modules (gdb))
</pre>
   <pre class="smallexample">     guile (use-modules ((gdb) #:renamer (symbol-prefix-proc 'gdb:)))
</pre>
   <p>Which one to choose depends on your preference. 
The second one adds <code>gdb:</code> as a prefix to all module functions
and variables.

   <p>The rest of this manual assumes the <code>gdb</code> module has been imported
without any prefix.  See the Guile documentation for <code>use-modules</code>
for more information
(see <a href="../guile/Using-Guile-Modules.html#Using-Guile-Modules">Using Guile Modules</a>).

   <p>Example:

<pre class="smallexample">     (gdb) guile (value-type (make-value 1))
     ERROR: Unbound variable: value-type
     Error while executing Scheme code.
     (gdb) guile (use-modules (gdb))
     (gdb) guile (value-type (make-value 1))
     int
     (gdb)
</pre>
   <p>The <code>(gdb)</code> module provides these basic Guile functions.

<!-- TODO: line length -->
<div class="defun">
&mdash; Scheme Procedure: <b>execute</b><var> command </var><span class="roman">[</span><var>#:from-tty boolean</var><span class="roman">]</span> <span class="roman">[</span><var>#:to-string boolean</var><span class="roman">]</span><var><a name="index-execute-2461"></a></var><br>
<blockquote><p>Evaluate <var>command</var>, a string, as a <span class="sc">gdb</span> CLI command. 
If a <span class="sc">gdb</span> exception happens while <var>command</var> runs, it is
translated as described in
<a href="Guile-Exception-Handling.html#Guile-Exception-Handling">Guile Exception Handling</a>.

        <p><var>from-tty</var> specifies whether <span class="sc">gdb</span> ought to consider this
command as having originated from the user invoking it interactively. 
It must be a boolean value.  If omitted, it defaults to <code>#f</code>.

        <p>By default, any output produced by <var>command</var> is sent to
<span class="sc">gdb</span>'s standard output (and to the log output if logging is
turned on).  If the <var>to-string</var> parameter is
<code>#t</code>, then output will be collected by <code>execute</code> and
returned as a string.  The default is <code>#f</code>, in which case the
return value is unspecified.  If <var>to-string</var> is <code>#t</code>, the
<span class="sc">gdb</span> virtual terminal will be temporarily set to unlimited width
and height, and its pagination will be disabled; see <a href="Screen-Size.html#Screen-Size">Screen Size</a>. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>history-ref</b><var> number<a name="index-history_002dref-2462"></a></var><br>
<blockquote><p>Return a value from <span class="sc">gdb</span>'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. 
If <var>number</var> is negative, then <span class="sc">gdb</span> will take its absolute value
and count backward from the last element (i.e., the most recent element) to
find the value to return.  If <var>number</var> is zero, then <span class="sc">gdb</span> will
return the most recent element.  If the element specified by <var>number</var>
doesn't exist in the value history, a <code>gdb:error</code> exception will be
raised.

        <p>If no exception is raised, the return value is always an instance of
<code>&lt;gdb:value&gt;</code> (see <a href="Values-From-Inferior-In-Guile.html#Values-From-Inferior-In-Guile">Values From Inferior In Guile</a>).

        <p><em>Note:</em> <span class="sc">gdb</span>'s value history is independent of Guile's. 
<code>$1</code> in <span class="sc">gdb</span>'s value history contains the result of evaluating
an expression from <span class="sc">gdb</span>'s command line and <code>$1</code> from Guile's
history contains the result of evaluating an expression from Guile's
command line. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>history-append!</b><var> value<a name="index-history_002dappend_0021-2463"></a></var><br>
<blockquote><p>Append <var>value</var>, an instance of <code>&lt;gdb:value&gt;</code>, to <span class="sc">gdb</span>'s
value history.  Return its index in the history.

        <p>Putting into history values returned by Guile extensions will allow
the user convenient access to those values via CLI history
facilities. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>parse-and-eval</b><var> expression<a name="index-parse_002dand_002deval-2464"></a></var><br>
<blockquote><p>Parse <var>expression</var> as an expression in the current language,
evaluate it, and return the result as a <code>&lt;gdb:value&gt;</code>. 
The <var>expression</var> must be a string.

        <p>This function can be useful when implementing a new command
(see <a href="Commands-In-Guile.html#Commands-In-Guile">Commands In Guile</a>), as it provides a way to parse the
command's arguments as an expression. 
It is also is useful when computing values. 
For example, it is the only way to get the value of a
convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>) as a <code>&lt;gdb:value&gt;</code>. 
</p></blockquote></div>

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