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245 lines
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HTML
245 lines
10 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Writing a Pretty-Printer (Debugging with GDB)</title>
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<meta name="description" content="Writing a Pretty-Printer (Debugging with GDB)">
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<meta name="keywords" content="Writing a Pretty-Printer (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Python-API.html#Python-API" rel="up" title="Python API">
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<link href="Type-Printing-API.html#Type-Printing-API" rel="next" title="Type Printing API">
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<link href="Selecting-Pretty_002dPrinters.html#Selecting-Pretty_002dPrinters" rel="prev" title="Selecting Pretty-Printers">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Writing-a-Pretty_002dPrinter"></a>
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<div class="header">
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<p>
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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> [<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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</div>
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<hr>
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<a name="Writing-a-Pretty_002dPrinter-1"></a>
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<h4 class="subsubsection">23.2.2.7 Writing a Pretty-Printer</h4>
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<a name="index-writing-a-pretty_002dprinter"></a>
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<p>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>
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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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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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 <small>GDB</small> 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>
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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>
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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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<small>GDB</small> 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, <small>GDB</small> 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>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>And then the corresponding contents of the auto-load file would be:
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</p>
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<div class="smallexample">
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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></div>
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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>
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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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<em>subprinters</em> are the printers for the individual data types.
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</p>
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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>
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<p>These are the types we are going to pretty-print:
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</p>
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<div class="smallexample">
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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></div>
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<p>Here are the printers:
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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>And here is the autoload support:
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</p>
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<div class="smallexample">
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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></div>
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<p>Finally, when this printer is loaded into <small>GDB</small>, here is the
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corresponding output of ‘<samp>info pretty-printer</samp>’:
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</p>
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<div class="smallexample">
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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></div>
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<hr>
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<div class="header">
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<p>
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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> [<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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</div>
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</body>
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</html>
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