printf.c

#include <stdio.h>

#include <stdlib.h>

#include <printf.h>

struct widget {
  size_t length;
  size_t width;
};

int print_widget(FILE *const stream, const struct printf_info *const info,
                 const void *const *args) {

  const struct widget *const mywidget = *((const struct widget **)(args[0]));

  char *buffer = NULL;

  int len = asprintf(&buffer, "<widget %p: len=%zu, width=%zu>", mywidget,
                     mywidget->length, mywidget->width);

  if (len < 0)
    return -1;

  len =
      fprintf(stream, "%*s", (info->left ? -info->width : info->width), buffer);

  free(buffer);
}

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

int main() {
  struct widget *const mywidget = &(struct widget){.length = 5, .width = 6};
  register_printf_function('W', print_widget, print_widget_arginfo);

  printf("%W\n", mywidget);
}