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| <h4 class="subsection">12.13.3 Defining the Output Handler</h4> |
| |
| <p>Now let's look at how to define the handler and arginfo functions |
| which are passed as arguments to <code>register_printf_function</code>. |
| |
| <p><strong>Compatibility Note:</strong> The interface changed in GNU libc |
| version 2.0. Previously the third argument was of type |
| <code>va_list *</code>. |
| |
| <p>You should define your handler functions with a prototype like: |
| |
| <pre class="smallexample"> int <var>function</var> (FILE *stream, const struct printf_info *info, |
| const void *const *args) |
| </pre> |
| <p>The <var>stream</var> argument passed to the handler function is the stream to |
| which it should write output. |
| |
| <p>The <var>info</var> argument is a pointer to a structure that contains |
| information about the various options that were included with the |
| conversion in the template string. You should not modify this structure |
| inside your handler function. See <a href="Conversion-Specifier-Options.html#Conversion-Specifier-Options">Conversion Specifier Options</a>, for |
| a description of this data structure. |
| |
| <!-- The following changes some time back. -drepper@gnu, 1996/11/14 --> |
| <!-- The @code{ap_pointer} argument is used to pass the tail of the variable --> |
| <!-- argument list containing the values to be printed to your handler. --> |
| <!-- Unlike most other functions that can be passed an explicit variable --> |
| <!-- argument list, this is a @emph{pointer} to a @code{va_list}, rather than --> |
| <!-- the @code{va_list} itself. Thus, you should fetch arguments by --> |
| <!-- means of @code{va_arg (*ap_pointer, @var{type})}. --> |
| <!-- (Passing a pointer here allows the function that calls your handler --> |
| <!-- function to update its own @code{va_list} variable to account for the --> |
| <!-- arguments that your handler processes. @xref{Variadic Functions}.) --> |
| <p>The <var>args</var> is a vector of pointers to the arguments data. |
| The number of arguments was determined by calling the argument |
| information function provided by the user. |
| |
| <p>Your handler function should return a value just like <code>printf</code> |
| does: it should return the number of characters it has written, or a |
| negative value to indicate an error. |
| |
| <!-- printf.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Data Type: <b>printf_function</b><var><a name="index-printf_005ffunction-1068"></a></var><br> |
| <blockquote><p>This is the data type that a handler function should have. |
| </p></blockquote></div> |
| |
| <p>If you are going to use <code>parse_printf_format</code><!-- /@w --> in your |
| application, you must also define a function to pass as the |
| <var>arginfo-function</var> argument for each new conversion you install with |
| <code>register_printf_function</code>. |
| |
| <p>You have to define these functions with a prototype like: |
| |
| <pre class="smallexample"> int <var>function</var> (const struct printf_info *info, |
| size_t n, int *argtypes) |
| </pre> |
| <p>The return value from the function should be the number of arguments the |
| conversion expects. The function should also fill in no more than |
| <var>n</var> elements of the <var>argtypes</var> array with information about the |
| types of each of these arguments. This information is encoded using the |
| various ‘<samp><span class="samp">PA_</span></samp>’ macros. (You will notice that this is the same |
| calling convention <code>parse_printf_format</code> itself uses.) |
| |
| <!-- printf.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Data Type: <b>printf_arginfo_function</b><var><a name="index-printf_005farginfo_005ffunction-1069"></a></var><br> |
| <blockquote><p>This type is used to describe functions that return information about |
| the number and type of arguments used by a conversion specifier. |
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