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<title>Writing a Pretty-Printer - Debugging with GDB</title>
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Previous:&nbsp;<a rel="previous" accesskey="p" href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters">Selecting Pretty-Printers</a>,
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<h5 class="subsubsection">23.2.2.7 Writing a Pretty-Printer</h5>
<p><a name="index-writing-a-pretty_002dprinter-2011"></a>
A pretty-printer consists of two parts: a lookup function to detect
if the type is supported, and the printer itself.
<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.
<pre class="smallexample"> class StdStringPrinter(object):
"Print a std::string"
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>
<p>And here is an example showing how a lookup function for the printer
example above might be written.
<pre class="smallexample"> def str_lookup_function(val):
lookup_tag = val.type.tag
if lookup_tag == None:
return None
regex = re.compile("^std::basic_string&lt;char,.*&gt;$")
if regex.match(lookup_tag):
return StdStringPrinter(val)
return None
</pre>
<p>The example lookup function extracts the value'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>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 <span class="sc">gdb</span> to load multiple versions of
your pretty-printers at the same time, because they will have
different names.
<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>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
<span class="sc">gdb</span> 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's
printers with a specific objfile, <span class="sc">gdb</span> will find the correct
printers for the specific version of the library used by each
inferior.
<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>:
<pre class="smallexample"> def register_printers(objfile):
objfile.pretty_printers.append(str_lookup_function)
</pre>
<p class="noindent">And then the corresponding contents of the auto-load file would be:
<pre class="smallexample"> import gdb.libstdcxx.v6
gdb.libstdcxx.v6.register_printers(gdb.current_objfile())
</pre>
<p>The previous example illustrates a basic pretty-printer.
There are a few things that can be improved on.
The printer doesn'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>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
<dfn>subprinters</dfn> are the printers for the individual data types.
<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>These are the types we are going to pretty-print:
<pre class="smallexample"> struct foo { int a, b; };
struct bar { struct foo x, y; };
</pre>
<p>Here are the printers:
<pre class="smallexample"> class fooPrinter:
"""Print a foo object."""
def __init__(self, val):
self.val = val
def to_string(self):
return ("a=&lt;" + str(self.val["a"]) +
"&gt; b=&lt;" + str(self.val["b"]) + "&gt;")
class barPrinter:
"""Print a bar object."""
def __init__(self, val):
self.val = val
def to_string(self):
return ("x=&lt;" + str(self.val["x"]) +
"&gt; y=&lt;" + str(self.val["y"]) + "&gt;")
</pre>
<p>This example doesn'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.
<pre class="smallexample"> import gdb.printing
def build_pretty_printer():
pp = gdb.printing.RegexpCollectionPrettyPrinter(
"my_library")
pp.add_printer('foo', '^foo$', fooPrinter)
pp.add_printer('bar', '^bar$', barPrinter)
return pp
</pre>
<p>And here is the autoload support:
<pre class="smallexample"> import gdb.printing
import my_library
gdb.printing.register_pretty_printer(
gdb.current_objfile(),
my_library.build_pretty_printer())
</pre>
<p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the
corresponding output of &lsquo;<samp><span class="samp">info pretty-printer</span></samp>&rsquo;:
<pre class="smallexample"> (gdb) info pretty-printer
my_library.so:
my_library
foo
bar
</pre>
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