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(libc.info)Defining the Output Handler

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Defining the Output Handler
---------------------------

   Now let's look at how to define the handler and arginfo functions
which are passed as arguments to `register_printf_function'.

   *Compatibility Note:* The interface changed in GNU libc version 2.0.
Previously the third argument was of type `va_list *'.

   You should define your handler functions with a prototype like:

     int FUNCTION (FILE *stream, const struct printf_info *info,
                         const void *const *args)

   The STREAM argument passed to the handler function is the stream to
which it should write output.

   The INFO 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.  Note: Conversion Specifier Options, for
a description of this data structure.

   The ARGS 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.

   Your handler function should return a value just like `printf' does:
it should return the number of characters it has written, or a negative
value to indicate an error.

 - Data Type: printf_function
     This is the data type that a handler function should have.

   If you are going to use `parse_printf_format' in your application,
you must also define a function to pass as the ARGINFO-FUNCTION
argument for each new conversion you install with
`register_printf_function'.

   You have to define these functions with a prototype like:

     int FUNCTION (const struct printf_info *info,
                         size_t n, int *argtypes)

   The return value from the function should be the number of arguments
the conversion expects.  The function should also fill in no more than
N elements of the ARGTYPES array with information about the types of
each of these arguments.  This information is encoded using the various
`PA_' macros.  (You will notice that this is the same calling
convention `parse_printf_format' itself uses.)

 - Data Type: printf_arginfo_function
     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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