(libc.info.gz) Defining the Output Handler

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 12.13.3 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.   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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