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 printf and 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 about this.

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.