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<h5 class="subsubsection">23.3.3.9 Selecting Guile Pretty-Printers</h5>

<p><a name="index-selecting-guile-pretty_002dprinters-2604"></a>
There are three sets of pretty-printers that <span class="sc">gdb</span> searches:

     <ul>
<li>Per-objfile list of pretty-printers (see <a href="Objfiles-In-Guile.html#Objfiles-In-Guile">Objfiles In Guile</a>). 
<li>Per-progspace list of pretty-printers (see <a href="Progspaces-In-Guile.html#Progspaces-In-Guile">Progspaces In Guile</a>). 
<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. 
</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><span class="sc">gdb</span> 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, <span class="sc">gdb</span> 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>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>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the pretty-printer is out of date.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>The consequences of a broken pretty-printer are severe enough that
<span class="sc">gdb</span> 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>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>.

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