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| <h4 class="subsection">12.13.5 Predefined <code>printf</code> Handlers</h4> |
| |
| <p>The GNU libc also contains a concrete and useful application of the |
| <code>printf</code> handler extension. There are two functions available |
| which implement a special way to print floating-point numbers. |
| |
| <!-- printf.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>printf_size</b> (<var>FILE *fp, const struct printf_info *info, const void *const *args</var>)<var><a name="index-printf_005fsize-1070"></a></var><br> |
| <blockquote><p>Print a given floating point number as for the format <code>%f</code> 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. |
| |
| <p>The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, |
| etc. The full table is: |
| |
| <p>The default precision is 3, i.e., 1024 is printed with a lower-case |
| format character as if it were <code>%.3fk</code> and will yield <code>1.000k</code>. |
| </p></blockquote></div> |
| |
| <p>Due to the requirements of <code>register_printf_function</code> we must also |
| provide the function which returns information about the arguments. |
| |
| <!-- printf.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>printf_size_info</b> (<var>const struct printf_info *info, size_t n, int *argtypes</var>)<var><a name="index-printf_005fsize_005finfo-1071"></a></var><br> |
| <blockquote><p>This function will return in <var>argtypes</var> the information about the |
| used parameters in the way the <code>vfprintf</code> implementation expects |
| it. The format always takes one argument. |
| </p></blockquote></div> |
| |
| <p>To use these functions both functions must be registered with a call like |
| |
| <pre class="smallexample"> register_printf_function ('B', printf_size, printf_size_info); |
| </pre> |
| <p>Here we register the functions to print numbers as powers of 1000 since |
| the format character <code>'B'</code> is an upper-case character. If we |
| would additionally use <code>'b'</code> in a line like |
| |
| <pre class="smallexample"> register_printf_function ('b', printf_size, printf_size_info); |
| </pre> |
| <p class="noindent">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 |
| <code>printf_size</code> function knows about the difference between lower and upper |
| case format specifiers. |
| |
| <p>The use of <code>'B'</code> and <code>'b'</code> 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. |
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