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| <h4 class="subsection">12.13.4 <code>printf</code> Extension Example</h4> |
| |
| <p>Here is an example showing how to define a <code>printf</code> handler function. |
| This program defines a data structure called a <code>Widget</code> and |
| defines the ‘<samp><span class="samp">%W</span></samp>’ conversion to print information about <code>Widget *</code><!-- /@w --> |
| arguments, including the pointer value and the name stored in the data |
| structure. The ‘<samp><span class="samp">%W</span></samp>’ conversion supports the minimum field width and |
| left-justification options, but ignores everything else. |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <stdlib.h> |
| #include <printf.h> |
| |
| typedef struct |
| { |
| char *name; |
| } |
| Widget; |
| |
| int |
| print_widget (FILE *stream, |
| const struct printf_info *info, |
| const void *const *args) |
| { |
| const Widget *w; |
| char *buffer; |
| int len; |
| |
| /* <span class="roman">Format the output into a string.</span> */ |
| w = *((const Widget **) (args[0])); |
| len = asprintf (&buffer, "<Widget %p: %s>", w, w->name); |
| if (len == -1) |
| return -1; |
| |
| /* <span class="roman">Pad to the minimum field width and print to the stream.</span> */ |
| len = fprintf (stream, "%*s", |
| (info->left ? -info->width : info->width), |
| buffer); |
| |
| /* <span class="roman">Clean up and return.</span> */ |
| free (buffer); |
| return len; |
| } |
| |
| |
| int |
| print_widget_arginfo (const struct printf_info *info, size_t n, |
| int *argtypes) |
| { |
| /* <span class="roman">We always take exactly one argument and this is a pointer to the |
| structure..</span> */ |
| if (n > 0) |
| argtypes[0] = PA_POINTER; |
| return 1; |
| } |
| |
| |
| int |
| main (void) |
| { |
| /* <span class="roman">Make a widget to print.</span> */ |
| Widget mywidget; |
| mywidget.name = "mywidget"; |
| |
| /* <span class="roman">Register the print function for widgets.</span> */ |
| register_printf_function ('W', print_widget, print_widget_arginfo); |
| |
| /* <span class="roman">Now print the widget.</span> */ |
| printf ("|%W|\n", &mywidget); |
| printf ("|%35W|\n", &mywidget); |
| printf ("|%-35W|\n", &mywidget); |
| |
| return 0; |
| } |
| </pre> |
| <p>The output produced by this program looks like: |
| |
| <pre class="smallexample"> |<Widget 0xffeffb7c: mywidget>| |
| | <Widget 0xffeffb7c: mywidget>| |
| |<Widget 0xffeffb7c: mywidget> | |
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