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authorUlrich Drepper <drepper@redhat.com>1997-04-19 11:22:31 +0000
committerUlrich Drepper <drepper@redhat.com>1997-04-19 11:22:31 +0000
commit29bb8719cfc14ccaadfdf371c036c33da14a2746 (patch)
tree68024b4195b105603d4dba293e317efe08e82751 /manual
parent307bcd77838a5546a4fa65e7a88258a6ea78898c (diff)
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Add description of printf_size and printf_size_info.
Diffstat (limited to 'manual')
-rw-r--r--manual/stdio.texi74
1 files changed, 74 insertions, 0 deletions
diff --git a/manual/stdio.texi b/manual/stdio.texi
index dd85554..97c3f58 100644
--- a/manual/stdio.texi
+++ b/manual/stdio.texi
@@ -1968,6 +1968,7 @@ The facilities of this section are declared in the header file
to @code{register_printf_function}.
* Printf Extension Example:: How to define a @code{printf}
handler function.
+* Predefined Printf Handlers:: Predefined @code{printf} handlers.
@end menu
@strong{Portability Note:} The ability to extend the syntax of
@@ -2212,6 +2213,79 @@ The output produced by this program looks like:
|<Widget 0xffeffb7c: mywidget> |
@end smallexample
+@node Predefined Printf Handlers
+@subsection Predefined @code{printf} Handlers
+
+The GNU libc also contains a concrete and useful application of the
+@code{printf} handler extension. There are two functions available
+which implement a special way to print floating-point numbers.
+
+@comment printf.h
+@comment GNU
+@deftypefun int printf_size (FILE *@var{fp}, const struct printf_info *@var{info}, const void *const *@var{args})
+Print a given floating point number as for the format @code{%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 to 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:
+
+@multitable {' '} {2^10 (1024)} {zetta} {Upper} {10^24 (1000)}
+@item low @tab Multiplier @tab From @tab Upper @tab Multiplier
+@item ' ' @tab 1 @tab @tab ' ' @tab 1
+@item k @tab 2^10 (1024) @tab kilo @tab K @tab 10^3 (1000)
+@item m @tab 2^20 @tab mega @tab M @tab 10^6
+@item g @tab 2^30 @tab giga @tab G @tab 10^9
+@item t @tab 2^40 @tab tera @tab T @tab 10^12
+@item p @tab 2^50 @tab peta @tab P @tab 10^15
+@item e @tab 2^60 @tab exa @tab E @tab 10^18
+@item z @tab 2^70 @tab zetta @tab Z @tab 10^21
+@item y @tab 2^80 @tab yotta @tab Y @tab 10^24
+@end multitable
+
+The default precision is 3, i.e., 1024 is printed with a lower-case
+format character as if it were @code{%.3fk} and will yield @code{1.000k}.
+@end deftypefun
+
+Due to the requirements of @code{register_printf_function} we must also
+provide the function which return information about the arguments.
+
+@comment printf.h
+@comment GNU
+@deftypefun int printf_size_info (const struct printf_info *@var{info}, size_t @var{n}, int *@var{argtypes})
+This function will return in @var{argtypes} the information about the
+used parameters in the way the @code{vfprintf} implementation expects
+it. The format always takes one argument.
+@end deftypefun
+
+To use these functions both functions must be registered with a call like
+
+@smallexample
+register_printf_function ('B', printf_size, printf_size_info);
+@end smallexample
+
+Here we register the functions to print numbers as powers of 1000 since
+the format character @code{'B'} is an upper-case characeter. If we
+would additionally use @code{'b'} in a line like
+
+@smallexample
+register_printf_function ('b', printf_size, printf_size_info);
+@end smallexample
+
+@noindent
+we could also print using power of 1024. Please note that all what is
+different in these both lines in the format specifier. The
+@code{printf_size} function knows about the difference of low and upper
+case format specifiers.
+
+The use of @code{'B'} and @code{'b'} is no coincidence. Rather it is
+the preferred way to use this functionality since it is available on
+some other systems also available using the format specifiers.
+
@node Formatted Input
@section Formatted Input