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The GNU C Library lets you define your own custom conversion specifiers
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 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 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. See Parsing a Template String, for information about this.
The facilities of this section are declared in the header file printf.h.
Portability Note: The ability to extend the syntax of
printf template strings is a GNU extension. ISO standard C has