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<title>Debugging with GDB: Writing a Pretty-Printer</title>

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<a name="Writing-a-Pretty_002dPrinter"></a>
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Next: <a href="Type-Printing-API.html#Type-Printing-API" accesskey="n" rel="next">Type Printing API</a>, Previous: <a href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" accesskey="p" rel="prev">Selecting Pretty-Printers</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<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>
<a name="Writing-a-Pretty_002dPrinter-1"></a>
<h4 class="subsubsection">23.2.2.7 Writing a Pretty-Printer</h4>
<a name="index-writing-a-pretty_002dprinter"></a>

<p>A pretty-printer consists of two parts: a lookup function to detect
if the type is supported, and the printer itself.
</p>
<p>Here is an example showing how a <code>std::string</code> printer might be
written.  See <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>, for details on the API this class
must provide.
</p>
<div class="smallexample">
<pre class="smallexample">class StdStringPrinter(object):
    &quot;Print a std::string&quot;

    def __init__(self, val):
        self.val = val

    def to_string(self):
        return self.val['_M_dataplus']['_M_p']

    def display_hint(self):
        return 'string'
</pre></div>

<p>And here is an example showing how a lookup function for the printer
example above might be written.
</p>
<div class="smallexample">
<pre class="smallexample">def str_lookup_function(val):
    lookup_tag = val.type.tag
    if lookup_tag == None:
        return None
    regex = re.compile(&quot;^std::basic_string&lt;char,.*&gt;$&quot;)
    if regex.match(lookup_tag):
        return StdStringPrinter(val)
    return None
</pre></div>

<p>The example lookup function extracts the value&rsquo;s type, and attempts to
match it to a type that it can pretty-print.  If it is a type the
printer can pretty-print, it will return a printer object.  If not, it
returns <code>None</code>.
</p>
<p>We recommend that you put your core pretty-printers into a Python
package.  If your pretty-printers are for use with a library, we
further recommend embedding a version number into the package name.
This practice will enable <small>GDB</small> to load multiple versions of
your pretty-printers at the same time, because they will have
different names.
</p>
<p>You should write auto-loaded code (see <a href="Python-Auto_002dloading.html#Python-Auto_002dloading">Python Auto-loading</a>) such that it
can be evaluated multiple times without changing its meaning.  An
ideal auto-load file will consist solely of <code>import</code>s of your
printer modules, followed by a call to a register pretty-printers with
the current objfile.
</p>
<p>Taken as a whole, this approach will scale nicely to multiple
inferiors, each potentially using a different library version.
Embedding a version number in the Python package name will ensure that
<small>GDB</small> is able to load both sets of printers simultaneously.
Then, because the search for pretty-printers is done by objfile, and
because your auto-loaded code took care to register your library&rsquo;s
printers with a specific objfile, <small>GDB</small> will find the correct
printers for the specific version of the library used by each
inferior.
</p>
<p>To continue the <code>std::string</code> example (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>),
this code might appear in <code>gdb.libstdcxx.v6</code>:
</p>
<div class="smallexample">
<pre class="smallexample">def register_printers(objfile):
    objfile.pretty_printers.append(str_lookup_function)
</pre></div>

<p>And then the corresponding contents of the auto-load file would be:
</p>
<div class="smallexample">
<pre class="smallexample">import gdb.libstdcxx.v6
gdb.libstdcxx.v6.register_printers(gdb.current_objfile())
</pre></div>

<p>The previous example illustrates a basic pretty-printer.
There are a few things that can be improved on.
The printer doesn&rsquo;t have a name, making it hard to identify in a
list of installed printers.  The lookup function has a name, but
lookup functions can have arbitrary, even identical, names.
</p>
<p>Second, the printer only handles one type, whereas a library typically has
several types.  One could install a lookup function for each desired type
in the library, but one could also have a single lookup function recognize
several types.  The latter is the conventional way this is handled.
If a pretty-printer can handle multiple data types, then its
<em>subprinters</em> are the printers for the individual data types.
</p>
<p>The <code>gdb.printing</code> module provides a formal way of solving these
problems (see <a href="gdb_002eprinting.html#gdb_002eprinting">gdb.printing</a>).
Here is another example that handles multiple types.
</p>
<p>These are the types we are going to pretty-print:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo { int a, b; };
struct bar { struct foo x, y; };
</pre></div>

<p>Here are the printers:
</p>
<div class="smallexample">
<pre class="smallexample">class fooPrinter:
    &quot;&quot;&quot;Print a foo object.&quot;&quot;&quot;

    def __init__(self, val):
        self.val = val

    def to_string(self):
        return (&quot;a=&lt;&quot; + str(self.val[&quot;a&quot;]) +
                &quot;&gt; b=&lt;&quot; + str(self.val[&quot;b&quot;]) + &quot;&gt;&quot;)

class barPrinter:
    &quot;&quot;&quot;Print a bar object.&quot;&quot;&quot;

    def __init__(self, val):
        self.val = val

    def to_string(self):
        return (&quot;x=&lt;&quot; + str(self.val[&quot;x&quot;]) +
                &quot;&gt; y=&lt;&quot; + str(self.val[&quot;y&quot;]) + &quot;&gt;&quot;)
</pre></div>

<p>This example doesn&rsquo;t need a lookup function, that is handled by the
<code>gdb.printing</code> module.  Instead a function is provided to build up
the object that handles the lookup.
</p>
<div class="smallexample">
<pre class="smallexample">import gdb.printing

def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter(
        &quot;my_library&quot;)
    pp.add_printer('foo', '^foo$', fooPrinter)
    pp.add_printer('bar', '^bar$', barPrinter)
    return pp
</pre></div>

<p>And here is the autoload support:
</p>
<div class="smallexample">
<pre class="smallexample">import gdb.printing
import my_library
gdb.printing.register_pretty_printer(
    gdb.current_objfile(),
    my_library.build_pretty_printer())
</pre></div>

<p>Finally, when this printer is loaded into <small>GDB</small>, here is the
corresponding output of &lsquo;<samp>info pretty-printer</samp>&rsquo;:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info pretty-printer
my_library.so:
  my_library
    foo
    bar
</pre></div>

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Next: <a href="Type-Printing-API.html#Type-Printing-API" accesskey="n" rel="next">Type Printing API</a>, Previous: <a href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" accesskey="p" rel="prev">Selecting Pretty-Printers</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<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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