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| <h5 class="subsubsection">23.2.2.6 Selecting Pretty-Printers</h5> |
| |
| <p>The Python list <code>gdb.pretty_printers</code> contains an array of |
| functions or callable objects that have been registered via addition |
| as a pretty-printer. |
| Each <code>gdb.Progspace</code> contains a <code>pretty_printers</code> attribute. |
| Each <code>gdb.Objfile</code> also contains a <code>pretty_printers</code> |
| attribute. |
| |
| <p>A function on one of these lists is passed a single <code>gdb.Value</code> |
| argument and should return a pretty-printer object conforming to the |
| interface definition above (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>). If a function |
| cannot create a pretty-printer for the value, it should return |
| <code>None</code>. |
| |
| <p><span class="sc">gdb</span> first checks the <code>pretty_printers</code> attribute of each |
| <code>gdb.Objfile</code> in the current program space and iteratively calls |
| each enabled function (see <a href="Disabling-Pretty_002dPrinters.html#Disabling-Pretty_002dPrinters">Disabling Pretty-Printers</a>) |
| in the list for that <code>gdb.Objfile</code> until it receives |
| a pretty-printer object. |
| If no pretty-printer is found in the objfile lists, <span class="sc">gdb</span> then |
| searches the pretty-printer list of the current program space, |
| calling each enabled function until an object is returned. |
| After these lists have been exhausted, it tries the global |
| <code>gdb.pretty_printers</code> list, again calling each enabled function until an |
| 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 printer |
| object is returned. |
| |
| <p>Here is an example showing how a <code>std::string</code> printer might be |
| written: |
| |
| <pre class="smallexample"> class StdStringPrinter: |
| "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 |
| regex = re.compile ("^std::basic_string<char,.*>$") |
| if lookup_tag == None: |
| return None |
| 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="Auto_002dloading.html#Auto_002dloading">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.add (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> |
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