The GNU libc also contains a concrete and useful application of the
printf handler extension. There are two functions available
which implement a special way to print floating-point numbers.
Print a given floating point number as for the format
%fexcept that there is a postfix character indicating the divisor for the number to make this less than 1000. There are two possible divisors: powers of 1024 or powers of 1000. Which one is used depends on the format character specified while registered this handler. If the character is of lower case, 1024 is used. For upper case characters, 1000 is used.
The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is:
The default precision is 3, i.e., 1024 is printed with a lower-case format character as if it were
%.3fkand will yield
Due to the requirements of
register_printf_function we must also
provide the function which returns information about the arguments.
This function will return in argtypes the information about the used parameters in the way the
vfprintfimplementation expects it. The format always takes one argument.
To use these functions both functions must be registered with a call like
register_printf_function ('B', printf_size, printf_size_info);
Here we register the functions to print numbers as powers of 1000 since
the format character
'B' is an upper-case character. If we
would additionally use
'b' in a line like
register_printf_function ('b', printf_size, printf_size_info);
we could also print using a power of 1024. Please note that all that is
different in these two lines is the format specifier. The
printf_size function knows about the difference between lower and upper
case format specifiers.
The use of
'b' is no coincidence. Rather it is
the preferred way to use this functionality since it is available on
some other systems which also use format specifiers.