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| <h5 class="subsubsection">23.3.3.10 Writing a Guile Pretty-Printer</h5> |
| |
| <p><a name="index-writing-a-Guile-pretty_002dprinter-2605"></a> |
| A pretty-printer consists of two basic parts: a lookup function to determine |
| if the type is supported, and the printer itself. |
| |
| <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. |
| |
| <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> |
| <p>And here is an example showing how a lookup function for the printer |
| example above might be written. |
| |
| <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> |
| <p>Then to register this printer in the global printer list: |
| |
| <pre class="smallexample"> (append-pretty-printer! |
| (make-pretty-printer "my-string" str-lookup-function)) |
| </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 <gdb:pretty-printer-worker> object. |
| If not, it returns <code>#f</code>. |
| |
| <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 <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="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>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 |
| <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>my::string</code> example, |
| this code might appear in <code>(my-project my-library v1)</code>: |
| |
| <pre class="smallexample"> (use-modules (gdb)) |
| (define (register-printers objfile) |
| (append-objfile-pretty-printer! |
| (make-pretty-printer "my-string" str-lookup-function))) |
| </pre> |
| <p class="noindent">And then the corresponding contents of the auto-load file would be: |
| |
| <pre class="smallexample"> (use-modules (gdb) (my-project my-library v1)) |
| (register-printers (current-objfile)) |
| </pre> |
| <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 |
| <dfn>subprinters</dfn> are the printers for the individual data types. |
| |
| <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>These are the types we are going to pretty-print: |
| |
| <pre class="smallexample"> struct foo { int a, b; }; |
| struct bar { struct foo x, y; }; |
| </pre> |
| <p>Here are the printers: |
| |
| <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> |
| <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. |
| |
| <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> |
| <p>And here is the autoload support: |
| |
| <pre class="smallexample"> (use-modules (gdb) (my-library)) |
| (append-objfile-pretty-printer! (current-objfile) (build-pretty-printer)) |
| </pre> |
| <p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the |
| corresponding output of ‘<samp><span class="samp">info pretty-printer</span></samp>’: |
| |
| <pre class="smallexample"> (gdb) info pretty-printer |
| my_library.so: |
| my-library |
| foo |
| bar |
| </pre> |
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