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<h3 class="section">12.13 Customizing <code>printf</code></h3>

<p><a name="index-customizing-_0040code_007bprintf_007d-1061"></a><a name="index-defining-new-_0040code_007bprintf_007d-conversions-1062"></a><a name="index-extending-_0040code_007bprintf_007d-1063"></a>
The GNU C library lets you define your own custom conversion specifiers
for <code>printf</code> template strings, to teach <code>printf</code> clever ways
to print the important data structures of your program.

   <p>The way you do this is by registering the conversion with the function
<code>register_printf_function</code>; see <a href="Registering-New-Conversions.html#Registering-New-Conversions">Registering New Conversions</a>. 
One of the arguments you pass to this function is a pointer to a handler
function that produces the actual output; see <a href="Defining-the-Output-Handler.html#Defining-the-Output-Handler">Defining the Output Handler</a>, for information on how to write this function.

   <p>You can also install a function that just returns information about the
number and type of arguments expected by the conversion specifier. 
See <a href="Parsing-a-Template-String.html#Parsing-a-Template-String">Parsing a Template String</a>, for information about this.

   <p>The facilities of this section are declared in the header file
<samp><span class="file">printf.h</span></samp>.

<ul class="menu">
<li><a accesskey="1" href="Registering-New-Conversions.html#Registering-New-Conversions">Registering New Conversions</a>:          Using <code>register_printf_function</code>
                                         to register a new output conversion. 
<li><a accesskey="2" href="Conversion-Specifier-Options.html#Conversion-Specifier-Options">Conversion Specifier Options</a>:         The handler must be able to get
                                         the options specified in the
                                         template when it is called. 
<li><a accesskey="3" href="Defining-the-Output-Handler.html#Defining-the-Output-Handler">Defining the Output Handler</a>:          Defining the handler and arginfo
                                         functions that are passed as arguments
                                         to <code>register_printf_function</code>. 
<li><a accesskey="4" href="Printf-Extension-Example.html#Printf-Extension-Example">Printf Extension Example</a>:             How to define a <code>printf</code>
                                         handler function. 
<li><a accesskey="5" href="Predefined-Printf-Handlers.html#Predefined-Printf-Handlers">Predefined Printf Handlers</a>:           Predefined <code>printf</code> handlers. 
</ul>

   <p><strong>Portability Note:</strong> The ability to extend the syntax of
<code>printf</code> template strings is a GNU extension.  ISO standard C has
nothing similar.

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