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199 lines
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<title>Writing a Pretty-Printer - Debugging with GDB</title>
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<a name="Writing-a-Pretty-Printer"></a>
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<a name="Writing-a-Pretty_002dPrinter"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Type-Printing-API.html#Type-Printing-API">Type Printing API</a>,
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Previous: <a rel="previous" accesskey="p" href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters">Selecting Pretty-Printers</a>,
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Up: <a rel="up" accesskey="u" href="Python-API.html#Python-API">Python API</a>
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<h5 class="subsubsection">23.2.2.7 Writing a Pretty-Printer</h5>
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<p><a name="index-writing-a-pretty_002dprinter-2011"></a>
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A pretty-printer consists of two parts: a lookup function to detect
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if the type is supported, and the printer itself.
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<p>Here is an example showing how a <code>std::string</code> printer might be
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written. See <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>, for details on the API this class
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must provide.
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<pre class="smallexample"> class StdStringPrinter(object):
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"Print a std::string"
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def __init__(self, val):
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self.val = val
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def to_string(self):
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return self.val['_M_dataplus']['_M_p']
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def display_hint(self):
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return 'string'
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</pre>
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<p>And here is an example showing how a lookup function for the printer
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example above might be written.
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<pre class="smallexample"> def str_lookup_function(val):
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lookup_tag = val.type.tag
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if lookup_tag == None:
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return None
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regex = re.compile("^std::basic_string<char,.*>$")
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if regex.match(lookup_tag):
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return StdStringPrinter(val)
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return None
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</pre>
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<p>The example lookup function extracts the value's type, and attempts to
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match it to a type that it can pretty-print. If it is a type the
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printer can pretty-print, it will return a printer object. If not, it
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returns <code>None</code>.
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<p>We recommend that you put your core pretty-printers into a Python
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package. If your pretty-printers are for use with a library, we
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further recommend embedding a version number into the package name.
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This practice will enable <span class="sc">gdb</span> to load multiple versions of
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your pretty-printers at the same time, because they will have
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different names.
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<p>You should write auto-loaded code (see <a href="Python-Auto_002dloading.html#Python-Auto_002dloading">Python Auto-loading</a>) such that it
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can be evaluated multiple times without changing its meaning. An
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ideal auto-load file will consist solely of <code>import</code>s of your
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printer modules, followed by a call to a register pretty-printers with
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the current objfile.
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<p>Taken as a whole, this approach will scale nicely to multiple
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inferiors, each potentially using a different library version.
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Embedding a version number in the Python package name will ensure that
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<span class="sc">gdb</span> is able to load both sets of printers simultaneously.
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Then, because the search for pretty-printers is done by objfile, and
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because your auto-loaded code took care to register your library's
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printers with a specific objfile, <span class="sc">gdb</span> will find the correct
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printers for the specific version of the library used by each
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inferior.
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<p>To continue the <code>std::string</code> example (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>),
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this code might appear in <code>gdb.libstdcxx.v6</code>:
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<pre class="smallexample"> def register_printers(objfile):
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objfile.pretty_printers.append(str_lookup_function)
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</pre>
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<p class="noindent">And then the corresponding contents of the auto-load file would be:
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<pre class="smallexample"> import gdb.libstdcxx.v6
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gdb.libstdcxx.v6.register_printers(gdb.current_objfile())
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</pre>
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<p>The previous example illustrates a basic pretty-printer.
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There are a few things that can be improved on.
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The printer doesn't have a name, making it hard to identify in a
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list of installed printers. The lookup function has a name, but
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lookup functions can have arbitrary, even identical, names.
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<p>Second, the printer only handles one type, whereas a library typically has
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several types. One could install a lookup function for each desired type
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in the library, but one could also have a single lookup function recognize
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several types. The latter is the conventional way this is handled.
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If a pretty-printer can handle multiple data types, then its
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<dfn>subprinters</dfn> are the printers for the individual data types.
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<p>The <code>gdb.printing</code> module provides a formal way of solving these
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problems (see <a href="gdb_002eprinting.html#gdb_002eprinting">gdb.printing</a>).
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Here is another example that handles multiple types.
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<p>These are the types we are going to pretty-print:
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<pre class="smallexample"> struct foo { int a, b; };
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struct bar { struct foo x, y; };
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</pre>
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<p>Here are the printers:
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<pre class="smallexample"> class fooPrinter:
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"""Print a foo object."""
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def __init__(self, val):
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self.val = val
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def to_string(self):
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return ("a=<" + str(self.val["a"]) +
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"> b=<" + str(self.val["b"]) + ">")
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class barPrinter:
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"""Print a bar object."""
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def __init__(self, val):
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self.val = val
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def to_string(self):
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return ("x=<" + str(self.val["x"]) +
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"> y=<" + str(self.val["y"]) + ">")
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</pre>
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<p>This example doesn't need a lookup function, that is handled by the
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<code>gdb.printing</code> module. Instead a function is provided to build up
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the object that handles the lookup.
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<pre class="smallexample"> import gdb.printing
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def build_pretty_printer():
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pp = gdb.printing.RegexpCollectionPrettyPrinter(
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"my_library")
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pp.add_printer('foo', '^foo$', fooPrinter)
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pp.add_printer('bar', '^bar$', barPrinter)
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return pp
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</pre>
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<p>And here is the autoload support:
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<pre class="smallexample"> import gdb.printing
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import my_library
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gdb.printing.register_pretty_printer(
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gdb.current_objfile(),
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my_library.build_pretty_printer())
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</pre>
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<p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the
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corresponding output of ‘<samp><span class="samp">info pretty-printer</span></samp>’:
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<pre class="smallexample"> (gdb) info pretty-printer
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my_library.so:
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my_library
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foo
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bar
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</pre>
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