This function defines the conversion specifier character spec.
Thus, if spec is
'Y', it defines the conversion `%Y'.
You can redefine the built-in conversions like `%s', but flag
characters like `#' and type modifiers like `l' can never be
used as conversions; calling
register_printf_function 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.
The handler-function is the function called by
friends when this conversion appears in a template string.
See Defining the Output Handler, for information about how to define
a function to pass as this argument. If you specify a null pointer, any
existing handler function for spec is removed.
The arginfo-function is the function called by
parse_printf_format when this conversion appears in a
template string. See Parsing a Template String, for information
Attention: In the GNU C library versions before 2.0 the
arginfo-function function did not need to be installed unless
the user used the
parse_printf_format function. This has changed.
Now a call to any of the
printf functions will call this
function when this format specifier appears in the format string.
The return value is
0 on success, and
-1 on failure
(which occurs if spec is out of range).
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.