1<html lang="en"> 2<head> 3<title>Writing a Pretty-Printer - Debugging with GDB</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Debugging with GDB"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Python-API.html#Python-API" title="Python API"> 9<link rel="prev" href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" title="Selecting Pretty-Printers"> 10<link rel="next" href="Type-Printing-API.html#Type-Printing-API" title="Type Printing API"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Free Software'' and ``Free Software Needs 19Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' 20and with the Back-Cover Texts as in (a) below. 21 22(a) The FSF's Back-Cover Text is: ``You are free to copy and modify 23this GNU Manual. 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See <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>, for details on the API this class 59must provide. 60 61<pre class="smallexample"> class StdStringPrinter(object): 62 "Print a std::string" 63 64 def __init__(self, val): 65 self.val = val 66 67 def to_string(self): 68 return self.val['_M_dataplus']['_M_p'] 69 70 def display_hint(self): 71 return 'string' 72</pre> 73 <p>And here is an example showing how a lookup function for the printer 74example above might be written. 75 76<pre class="smallexample"> def str_lookup_function(val): 77 lookup_tag = val.type.tag 78 if lookup_tag == None: 79 return None 80 regex = re.compile("^std::basic_string<char,.*>$") 81 if regex.match(lookup_tag): 82 return StdStringPrinter(val) 83 return None 84</pre> 85 <p>The example lookup function extracts the value's type, and attempts to 86match it to a type that it can pretty-print. If it is a type the 87printer can pretty-print, it will return a printer object. If not, it 88returns <code>None</code>. 89 90 <p>We recommend that you put your core pretty-printers into a Python 91package. If your pretty-printers are for use with a library, we 92further recommend embedding a version number into the package name. 93This practice will enable <span class="sc">gdb</span> to load multiple versions of 94your pretty-printers at the same time, because they will have 95different names. 96 97 <p>You should write auto-loaded code (see <a href="Python-Auto_002dloading.html#Python-Auto_002dloading">Python Auto-loading</a>) such that it 98can be evaluated multiple times without changing its meaning. An 99ideal auto-load file will consist solely of <code>import</code>s of your 100printer modules, followed by a call to a register pretty-printers with 101the current objfile. 102 103 <p>Taken as a whole, this approach will scale nicely to multiple 104inferiors, each potentially using a different library version. 105Embedding a version number in the Python package name will ensure that 106<span class="sc">gdb</span> is able to load both sets of printers simultaneously. 107Then, because the search for pretty-printers is done by objfile, and 108because your auto-loaded code took care to register your library's 109printers with a specific objfile, <span class="sc">gdb</span> will find the correct 110printers for the specific version of the library used by each 111inferior. 112 113 <p>To continue the <code>std::string</code> example (see <a href="Pretty-Printing-API.html#Pretty-Printing-API">Pretty Printing API</a>), 114this code might appear in <code>gdb.libstdcxx.v6</code>: 115 116<pre class="smallexample"> def register_printers(objfile): 117 objfile.pretty_printers.append(str_lookup_function) 118</pre> 119 <p class="noindent">And then the corresponding contents of the auto-load file would be: 120 121<pre class="smallexample"> import gdb.libstdcxx.v6 122 gdb.libstdcxx.v6.register_printers(gdb.current_objfile()) 123</pre> 124 <p>The previous example illustrates a basic pretty-printer. 125There are a few things that can be improved on. 126The printer doesn't have a name, making it hard to identify in a 127list of installed printers. The lookup function has a name, but 128lookup functions can have arbitrary, even identical, names. 129 130 <p>Second, the printer only handles one type, whereas a library typically has 131several types. One could install a lookup function for each desired type 132in the library, but one could also have a single lookup function recognize 133several types. The latter is the conventional way this is handled. 134If a pretty-printer can handle multiple data types, then its 135<dfn>subprinters</dfn> are the printers for the individual data types. 136 137 <p>The <code>gdb.printing</code> module provides a formal way of solving these 138problems (see <a href="gdb_002eprinting.html#gdb_002eprinting">gdb.printing</a>). 139Here is another example that handles multiple types. 140 141 <p>These are the types we are going to pretty-print: 142 143<pre class="smallexample"> struct foo { int a, b; }; 144 struct bar { struct foo x, y; }; 145</pre> 146 <p>Here are the printers: 147 148<pre class="smallexample"> class fooPrinter: 149 """Print a foo object.""" 150 151 def __init__(self, val): 152 self.val = val 153 154 def to_string(self): 155 return ("a=<" + str(self.val["a"]) + 156 "> b=<" + str(self.val["b"]) + ">") 157 158 class barPrinter: 159 """Print a bar object.""" 160 161 def __init__(self, val): 162 self.val = val 163 164 def to_string(self): 165 return ("x=<" + str(self.val["x"]) + 166 "> y=<" + str(self.val["y"]) + ">") 167</pre> 168 <p>This example doesn't need a lookup function, that is handled by the 169<code>gdb.printing</code> module. Instead a function is provided to build up 170the object that handles the lookup. 171 172<pre class="smallexample"> import gdb.printing 173 174 def build_pretty_printer(): 175 pp = gdb.printing.RegexpCollectionPrettyPrinter( 176 "my_library") 177 pp.add_printer('foo', '^foo$', fooPrinter) 178 pp.add_printer('bar', '^bar$', barPrinter) 179 return pp 180</pre> 181 <p>And here is the autoload support: 182 183<pre class="smallexample"> import gdb.printing 184 import my_library 185 gdb.printing.register_pretty_printer( 186 gdb.current_objfile(), 187 my_library.build_pretty_printer()) 188</pre> 189 <p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the 190corresponding output of ‘<samp><span class="samp">info pretty-printer</span></samp>’: 191 192<pre class="smallexample"> (gdb) info pretty-printer 193 my_library.so: 194 my_library 195 foo 196 bar 197</pre> 198 </body></html> 199 200