GNU Info

(libc.info)Customizing Printf

---

Next: Formatted Input Prev: Formatted Output Up: I/O on Streams

---

Enter node , (file) or (file)node

Customizing `printf'
====================

   The GNU C library lets you define your own custom conversion
specifiers for `printf' template strings, to teach `printf' clever ways
to print the important data structures of your program.

   The way you do this is by registering the conversion with the
function `register_printf_function'; see Note: Registering New
Conversions.  One of the arguments you pass to this function is a
pointer to a handler function that produces the actual output; see
Note: Defining the Output Handler, for information on how to write
this function.

   You can also install a function that just returns information about
the number and type of arguments expected by the conversion specifier.
Note: Parsing a Template String, for information about this.

   The facilities of this section are declared in the header file
`printf.h'.

Registering New Conversions

Using `register_printf_function'

to register a new output conversion.

Conversion Specifier Options

The handler must be able to get

the options specified in the template when it is called.

Defining the Output Handler

Defining the handler and arginfo

functions that are passed as arguments to `register_printf_function'.

Printf Extension Example

How to define a `printf'

handler function.

Predefined Printf Handlers

Predefined `printf' handlers.

   *Portability Note:* The ability to extend the syntax of `printf'
template strings is a GNU extension.  ISO standard C has nothing
similar.