GNU Info

(libc.info)Predefined Printf Handlers

---

Prev: Printf Extension Example Up: Customizing Printf

---

Enter node , (file) or (file)node

Predefined `printf' Handlers
----------------------------

   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.

 - Function: int printf_size (FILE *FP, const struct printf_info *INFO,
          const void *const *ARGS)
     Print a given floating point number as for the format `%f' except
     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:

     +------+--------------+--------+--------+---------------+
     |low|Multiplier|From|Upper|Multiplier|
     +------+--------------+--------+--------+---------------+
     |' '|1||' '|1|
     +------+--------------+--------+--------+---------------+
     |k|2^10 (1024)|kilo|K|10^3 (1000)|
     +------+--------------+--------+--------+---------------+
     |m|2^20|mega|M|10^6|
     +------+--------------+--------+--------+---------------+
     |g|2^30|giga|G|10^9|
     +------+--------------+--------+--------+---------------+
     |t|2^40|tera|T|10^12|
     +------+--------------+--------+--------+---------------+
     |p|2^50|peta|P|10^15|
     +------+--------------+--------+--------+---------------+
     |e|2^60|exa|E|10^18|
     +------+--------------+--------+--------+---------------+
     |z|2^70|zetta|Z|10^21|
     +------+--------------+--------+--------+---------------+
     |y|2^80|yotta|Y|10^24|
     +------+--------------+--------+--------+---------------+

     The default precision is 3, i.e., 1024 is printed with a lower-case
     format character as if it were `%.3fk' and will yield `1.000k'.

   Due to the requirements of `register_printf_function' we must also
provide the function which returns information about the arguments.

 - Function: int printf_size_info (const struct printf_info *INFO,
          size_t N, int *ARGTYPES)
     This function will return in ARGTYPES the information about the
     used parameters in the way the `vfprintf' implementation 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'' and `'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.