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<title>Writing a Guile Pretty-Printer (Debugging with GDB)</title>
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<a name="Writing-a-Guile-Pretty_002dPrinter"></a>
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<p>
Next: <a href="Commands-In-Guile.html#Commands-In-Guile" accesskey="n" rel="next">Commands In Guile</a>, Previous: <a href="Selecting-Guile-Pretty_002dPrinters.html#Selecting-Guile-Pretty_002dPrinters" accesskey="p" rel="prev">Selecting Guile Pretty-Printers</a>, Up: <a href="Guile-API.html#Guile-API" accesskey="u" rel="up">Guile 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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<hr>
<a name="Writing-a-Guile-Pretty_002dPrinter-1"></a>
<h4 class="subsubsection">23.3.3.10 Writing a Guile Pretty-Printer</h4>
<a name="index-writing-a-Guile-pretty_002dprinter"></a>
<p>A pretty-printer consists of two basic parts: a lookup function to determine
if the type is supported, and the printer itself.
</p>
<p>Here is an example showing how a <code>std::string</code> printer might be
written. See <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>, for details.
</p>
<div class="smallexample">
<pre class="smallexample">(define (make-my-string-printer value)
"Print a my::string string"
(make-pretty-printer-worker
"string"
(lambda (printer)
(value-field value "_data"))
#f))
</pre></div>
<p>And here is an example showing how a lookup function for the printer
example above might be written.
</p>
<div class="smallexample">
<pre class="smallexample">(define (str-lookup-function pretty-printer value)
(let ((tag (type-tag (value-type value))))
(and tag
(string-prefix? "std::string<" tag)
(make-my-string-printer value))))
</pre></div>
<p>Then to register this printer in the global printer list:
</p>
<div class="smallexample">
<pre class="smallexample">(append-pretty-printer!
(make-pretty-printer "my-string" str-lookup-function))
</pre></div>
<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 <gdb:pretty-printer-worker> object.
If not, it returns <code>#f</code>.
</p>
<p>We recommend that you put your core pretty-printers into a Guile
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 <small>GDB</small> to load multiple versions of
your pretty-printers at the same time, because they will have
different names.
</p>
<p>You should write auto-loaded code (see <a href="Guile-Auto_002dloading.html#Guile-Auto_002dloading">Guile 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>
<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 Guile package name will ensure that
<small>GDB</small> 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, <small>GDB</small> will find the correct
printers for the specific version of the library used by each
inferior.
</p>
<p>To continue the <code>my::string</code> example,
this code might appear in <code>(my-project my-library v1)</code>:
</p>
<div class="smallexample">
<pre class="smallexample">(use-modules (gdb))
(define (register-printers objfile)
(append-objfile-pretty-printer!
(make-pretty-printer "my-string" str-lookup-function)))
</pre></div>
<p>And then the corresponding contents of the auto-load file would be:
</p>
<div class="smallexample">
<pre class="smallexample">(use-modules (gdb) (my-project my-library v1))
(register-printers (current-objfile))
</pre></div>
<p>The previous example illustrates a basic pretty-printer.
There are a few things that can be improved on.
The printer only handles one type, whereas a library typically has
several types. One could install a lookup function for each desired type
in the library, but one could also have a single lookup function recognize
several types. The latter is the conventional way this is handled.
If a pretty-printer can handle multiple data types, then its
<em>subprinters</em> are the printers for the individual data types.
</p>
<p>The <code>(gdb printing)</code> module provides a formal way of solving this
problem (see <a href="Guile-Printing-Module.html#Guile-Printing-Module">Guile Printing Module</a>).
Here is another example that handles multiple types.
</p>
<p>These are the types we are going to pretty-print:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo { int a, b; };
struct bar { struct foo x, y; };
</pre></div>
<p>Here are the printers:
</p>
<div class="smallexample">
<pre class="smallexample">(define (make-foo-printer value)
"Print a foo object"
(make-pretty-printer-worker
"foo"
(lambda (printer)
(format #f "a=<~a> b=<~a>"
(value-field value "a") (value-field value "a")))
#f))
(define (make-bar-printer value)
"Print a bar object"
(make-pretty-printer-worker
"foo"
(lambda (printer)
(format #f "x=<~a> y=<~a>"
(value-field value "x") (value-field value "y")))
#f))
</pre></div>
<p>This example doesn’t need a lookup function, that is handled by the
<code>(gdb printing)</code> module. Instead a function is provided to build up
the object that handles the lookup.
</p>
<div class="smallexample">
<pre class="smallexample">(use-modules (gdb printing))
(define (build-pretty-printer)
(let ((pp (make-pretty-printer-collection "my-library")))
(pp-collection-add-tag-printer "foo" make-foo-printer)
(pp-collection-add-tag-printer "bar" make-bar-printer)
pp))
</pre></div>
<p>And here is the autoload support:
</p>
<div class="smallexample">
<pre class="smallexample">(use-modules (gdb) (my-library))
(append-objfile-pretty-printer! (current-objfile) (build-pretty-printer))
</pre></div>
<p>Finally, when this printer is loaded into <small>GDB</small>, here is the
corresponding output of ‘<samp>info pretty-printer</samp>’:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) info pretty-printer
my_library.so:
my-library
foo
bar
</pre></div>
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<p>
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