glibc/manual/examples/rprintf.c

/* Printf Extension Example
   Copyright (C) 1991-2023 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License
   as published by the Free Software Foundation; either version 2
   of the License, or (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, see <https://www.gnu.org/licenses/>.
*/

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

/*@group*/
typedef struct
{
  char *name;
}
Widget;
/*@end group*/

int
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),
		 buffer);

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


int
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;
}


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