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188 lines
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<title>Writing a Guile Pretty-Printer - Debugging with GDB</title>
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<link rel="up" href="Guile-API.html#Guile-API" title="Guile API">
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<link rel="prev" href="Selecting-Guile-Pretty_002dPrinters.html#Selecting-Guile-Pretty_002dPrinters" title="Selecting Guile Pretty-Printers">
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<div class="node">
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<a name="Writing-a-Guile-Pretty-Printer"></a>
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<a name="Writing-a-Guile-Pretty_002dPrinter"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Commands-In-Guile.html#Commands-In-Guile">Commands In Guile</a>,
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Previous: <a rel="previous" accesskey="p" href="Selecting-Guile-Pretty_002dPrinters.html#Selecting-Guile-Pretty_002dPrinters">Selecting Guile Pretty-Printers</a>,
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Up: <a rel="up" accesskey="u" href="Guile-API.html#Guile-API">Guile API</a>
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<hr>
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</div>
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<h5 class="subsubsection">23.3.3.10 Writing a Guile Pretty-Printer</h5>
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<p><a name="index-writing-a-Guile-pretty_002dprinter-2605"></a>
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A pretty-printer consists of two basic parts: a lookup function to determine
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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="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>, for details.
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<pre class="smallexample"> (define (make-my-string-printer value)
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"Print a my::string string"
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(make-pretty-printer-worker
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"string"
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(lambda (printer)
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(value-field value "_data"))
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#f))
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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"> (define (str-lookup-function pretty-printer value)
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(let ((tag (type-tag (value-type value))))
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(and tag
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(string-prefix? "std::string<" tag)
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(make-my-string-printer value))))
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</pre>
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<p>Then to register this printer in the global printer list:
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<pre class="smallexample"> (append-pretty-printer!
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(make-pretty-printer "my-string" str-lookup-function))
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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 <gdb:pretty-printer-worker> object.
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If not, it returns <code>#f</code>.
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<p>We recommend that you put your core pretty-printers into a Guile
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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="Guile-Auto_002dloading.html#Guile-Auto_002dloading">Guile 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 Guile 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>my::string</code> example,
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this code might appear in <code>(my-project my-library v1)</code>:
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<pre class="smallexample"> (use-modules (gdb))
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(define (register-printers objfile)
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(append-objfile-pretty-printer!
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(make-pretty-printer "my-string" 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"> (use-modules (gdb) (my-project my-library v1))
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(register-printers (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 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 this
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problem (see <a href="Guile-Printing-Module.html#Guile-Printing-Module">Guile Printing Module</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"> (define (make-foo-printer value)
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"Print a foo object"
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(make-pretty-printer-worker
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"foo"
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(lambda (printer)
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(format #f "a=<~a> b=<~a>"
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(value-field value "a") (value-field value "a")))
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#f))
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(define (make-bar-printer value)
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"Print a bar object"
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(make-pretty-printer-worker
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"foo"
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(lambda (printer)
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(format #f "x=<~a> y=<~a>"
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(value-field value "x") (value-field value "y")))
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#f))
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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"> (use-modules (gdb printing))
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(define (build-pretty-printer)
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(let ((pp (make-pretty-printer-collection "my-library")))
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(pp-collection-add-tag-printer "foo" make-foo-printer)
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(pp-collection-add-tag-printer "bar" make-bar-printer)
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pp))
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</pre>
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<p>And here is the autoload support:
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<pre class="smallexample"> (use-modules (gdb) (my-library))
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(append-objfile-pretty-printer! (current-objfile) (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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</body></html>
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