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<title>Selecting Guile Pretty-Printers (Debugging with GDB)</title>

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<a name="Selecting-Guile-Pretty_002dPrinters"></a>
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<a name="Selecting-Guile-Pretty_002dPrinters-1"></a>
<h4 class="subsubsection">23.3.3.9 Selecting Guile Pretty-Printers</h4>
<a name="index-selecting-guile-pretty_002dprinters"></a>

<p>There are three sets of pretty-printers that <small>GDB</small> searches:
</p>
<ul>
<li> Per-objfile list of pretty-printers (see <a href="Objfiles-In-Guile.html#Objfiles-In-Guile">Objfiles In Guile</a>).
</li><li> Per-progspace list of pretty-printers (see <a href="Progspaces-In-Guile.html#Progspaces-In-Guile">Progspaces In Guile</a>).
</li><li> The global list of pretty-printers (see <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>).
These printers are available when debugging any inferior.
</li></ul>

<p>Pretty-printer lookup is done by passing the value to be printed to the
lookup function of each enabled object in turn.
Lookup stops when a lookup function returns a non-<code>#f</code> value
or when the list is exhausted.
Lookup functions must return either a <code>&lt;gdb:pretty-printer-worker&gt;</code>
object or <code>#f</code>.  Otherwise an exception is thrown.
</p>
<p><small>GDB</small> first checks the result of <code>objfile-pretty-printers</code>
of each <code>&lt;gdb:objfile&gt;</code> in the current program space and iteratively
calls each enabled lookup function in the list for that <code>&lt;gdb:objfile&gt;</code>
until a non-<code>#f</code> object is returned.
If no pretty-printer is found in the objfile lists, <small>GDB</small> then
searches the result of <code>progspace-pretty-printers</code> of the current
program space, calling each enabled function until a non-<code>#f</code> object
is returned.
After these lists have been exhausted, it tries the global pretty-printers
list, obtained with <code>pretty-printers</code>, again calling each enabled
function until a non-<code>#f</code> object is returned.
</p>
<p>The order in which the objfiles are searched is not specified.  For a
given list, functions are always invoked from the head of the list,
and iterated over sequentially until the end of the list, or a
<code>&lt;gdb:pretty-printer-worker&gt;</code> object is returned.
</p>
<p>For various reasons a pretty-printer may not work.
For example, the underlying data structure may have changed and
the pretty-printer is out of date.
</p>
<p>The consequences of a broken pretty-printer are severe enough that
<small>GDB</small> provides support for enabling and disabling individual
printers.  For example, if <code>print frame-arguments</code> is on,
a backtrace can become highly illegible if any argument is printed
with a broken printer.
</p>
<p>Pretty-printers are enabled and disabled from Scheme by calling
<code>set-pretty-printer-enabled!</code>.
See <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>.
</p>
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