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102 lines
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<title>Selecting Guile Pretty-Printers - Debugging with GDB</title>
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<link rel="up" href="Guile-API.html#Guile-API" title="Guile API">
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<link rel="prev" href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API" title="Guile Pretty Printing API">
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<link rel="next" href="Writing-a-Guile-Pretty_002dPrinter.html#Writing-a-Guile-Pretty_002dPrinter" title="Writing a Guile Pretty-Printer">
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<div class="node">
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<a name="Selecting-Guile-Pretty-Printers"></a>
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<a name="Selecting-Guile-Pretty_002dPrinters"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Writing-a-Guile-Pretty_002dPrinter.html#Writing-a-Guile-Pretty_002dPrinter">Writing a Guile Pretty-Printer</a>,
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Previous: <a rel="previous" accesskey="p" href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>,
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Up: <a rel="up" accesskey="u" href="Guile-API.html#Guile-API">Guile API</a>
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</div>
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<h5 class="subsubsection">23.3.3.9 Selecting Guile Pretty-Printers</h5>
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<p><a name="index-selecting-guile-pretty_002dprinters-2604"></a>
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There are three sets of pretty-printers that <span class="sc">gdb</span> searches:
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<ul>
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<li>Per-objfile list of pretty-printers (see <a href="Objfiles-In-Guile.html#Objfiles-In-Guile">Objfiles In Guile</a>).
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<li>Per-progspace list of pretty-printers (see <a href="Progspaces-In-Guile.html#Progspaces-In-Guile">Progspaces In Guile</a>).
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<li>The global list of pretty-printers (see <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>).
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These printers are available when debugging any inferior.
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</ul>
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<p>Pretty-printer lookup is done by passing the value to be printed to the
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lookup function of each enabled object in turn.
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Lookup stops when a lookup function returns a non-<code>#f</code> value
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or when the list is exhausted.
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Lookup functions must return either a <code><gdb:pretty-printer-worker></code>
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object or <code>#f</code>. Otherwise an exception is thrown.
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<p><span class="sc">gdb</span> first checks the result of <code>objfile-pretty-printers</code>
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of each <code><gdb:objfile></code> in the current program space and iteratively
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calls each enabled lookup function in the list for that <code><gdb:objfile></code>
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until a non-<code>#f</code> object is returned.
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If no pretty-printer is found in the objfile lists, <span class="sc">gdb</span> then
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searches the result of <code>progspace-pretty-printers</code> of the current
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program space, calling each enabled function until a non-<code>#f</code> object
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is returned.
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After these lists have been exhausted, it tries the global pretty-printers
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list, obtained with <code>pretty-printers</code>, again calling each enabled
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function until a non-<code>#f</code> object is returned.
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<p>The order in which the objfiles are searched is not specified. For a
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given list, functions are always invoked from the head of the list,
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and iterated over sequentially until the end of the list, or a
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<code><gdb:pretty-printer-worker></code> object is returned.
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<p>For various reasons a pretty-printer may not work.
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For example, the underlying data structure may have changed and
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the pretty-printer is out of date.
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<p>The consequences of a broken pretty-printer are severe enough that
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<span class="sc">gdb</span> provides support for enabling and disabling individual
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printers. For example, if <code>print frame-arguments</code> is on,
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a backtrace can become highly illegible if any argument is printed
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with a broken printer.
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<p>Pretty-printers are enabled and disabled from Scheme by calling
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<code>set-pretty-printer-enabled!</code>.
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See <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>.
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</body></html>
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