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| <h4 class="subsection">12.13.1 Registering New Conversions</h4> |
| |
| <p>The function to register a new output conversion is |
| <code>register_printf_function</code>, declared in <samp><span class="file">printf.h</span></samp>. |
| <a name="index-printf_002eh-1064"></a> |
| <!-- printf.h --> |
| <!-- GNU --> |
| |
| <div class="defun"> |
| — Function: int <b>register_printf_function</b> (<var>int spec, printf_function handler-function, printf_arginfo_function arginfo-function</var>)<var><a name="index-register_005fprintf_005ffunction-1065"></a></var><br> |
| <blockquote><p>This function defines the conversion specifier character <var>spec</var>. |
| Thus, if <var>spec</var> is <code>'Y'</code>, it defines the conversion ‘<samp><span class="samp">%Y</span></samp>’. |
| You can redefine the built-in conversions like ‘<samp><span class="samp">%s</span></samp>’, but flag |
| characters like ‘<samp><span class="samp">#</span></samp>’ and type modifiers like ‘<samp><span class="samp">l</span></samp>’ can never be |
| used as conversions; calling <code>register_printf_function</code> for those |
| characters has no effect. It is advisable not to use lowercase letters, |
| since the ISO C standard warns that additional lowercase letters may be |
| standardized in future editions of the standard. |
| |
| <p>The <var>handler-function</var> is the function called by <code>printf</code> and |
| friends when this conversion appears in a template string. |
| See <a href="Defining-the-Output-Handler.html#Defining-the-Output-Handler">Defining the Output Handler</a>, for information about how to define |
| a function to pass as this argument. If you specify a null pointer, any |
| existing handler function for <var>spec</var> is removed. |
| |
| <p>The <var>arginfo-function</var> is the function called by |
| <code>parse_printf_format</code> when this conversion appears in a |
| template string. See <a href="Parsing-a-Template-String.html#Parsing-a-Template-String">Parsing a Template String</a>, for information |
| about this. |
| |
| <!-- The following is not true anymore. The `parse_printf_format' function --> |
| <!-- is now also called from `vfprintf' via `parse_one_spec'. --> |
| <!-- drepper@gnu, 1996/11/14 --> |
| <!-- Normally, you install both functions for a conversion at the same time, --> |
| <!-- but if you are never going to call @code{parse_printf_format}, you do --> |
| <!-- not need to define an arginfo function. --> |
| <p><strong>Attention:</strong> In the GNU C library versions before 2.0 the |
| <var>arginfo-function</var> function did not need to be installed unless |
| the user used the <code>parse_printf_format</code> function. This has changed. |
| Now a call to any of the <code>printf</code> functions will call this |
| function when this format specifier appears in the format string. |
| |
| <p>The return value is <code>0</code> on success, and <code>-1</code> on failure |
| (which occurs if <var>spec</var> is out of range). |
| |
| <p>You can redefine the standard output conversions, but this is probably |
| not a good idea because of the potential for confusion. Library routines |
| written by other people could break if you do this. |
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