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12.13.4 printf Extension Example

Here is an example showing how to define a printf handler function. This program defines a data structure called a Widget and defines the ‘%W’ conversion to print information about Widget * arguments, including the pointer value and the name stored in the data structure. The ‘%W’ conversion supports the minimum field width and left-justification options, but ignores everything else.

#include <stdio.h>
#include <stdlib.h>
#include <printf.h>

typedef struct
  char *name;

print_widget (FILE *stream,
              const struct printf_info *info,
              const void *const *args)
  const Widget *w;
  char *buffer;
  int len;

  /* Format the output into a string. */
  w = *((const Widget **) (args[0]));
  len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
  if (len == -1)
    return -1;

  /* Pad to the minimum field width and print to the stream. */
  len = fprintf (stream, "%*s",
                 (info->left ? -info->width : info->width),

  /* Clean up and return. */
  free (buffer);
  return len;

print_widget_arginfo (const struct printf_info *info, size_t n,
                      int *argtypes)
  /* We always take exactly one argument and this is a pointer to the
     structure.. */
  if (n > 0)
    argtypes[0] = PA_POINTER;
  return 1;

main (void)
  /* Make a widget to print. */
  Widget mywidget; = "mywidget";

  /* Register the print function for widgets. */
  register_printf_function ('W', print_widget, print_widget_arginfo);

  /* Now print the widget. */
  printf ("|%W|\n", &mywidget);
  printf ("|%35W|\n", &mywidget);
  printf ("|%-35W|\n", &mywidget);

  return 0;

The output produced by this program looks like:

|<Widget 0xffeffb7c: mywidget>|
|      <Widget 0xffeffb7c: mywidget>|
|<Widget 0xffeffb7c: mywidget>      |