1/* Printf Extension Example
2 Copyright (C) 1991-2022 Free Software Foundation, Inc.
3
4 This program is free software; you can redistribute it and/or
5 modify it under the terms of the GNU General Public License
6 as published by the Free Software Foundation; either version 2
7 of the License, or (at your option) any later version.
8
9 This program is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 GNU General Public License for more details.
13
14 You should have received a copy of the GNU General Public License
15 along with this program; if not, see <https://www.gnu.org/licenses/>.
16*/
17
18#include <stdio.h>
19#include <stdlib.h>
20#include <printf.h>
21
22/*@group*/
23typedef struct
24{
25 char *name;
26}
27Widget;
28/*@end group*/
29
30int
31print_widget (FILE *stream,
32 const struct printf_info *info,
33 const void *const *args)
34{
35 const Widget *w;
36 char *buffer;
37 int len;
38
39 /* Format the output into a string. */
40 w = *((const Widget **) (args[0]));
41 len = asprintf (ptr: &buffer, fmt: "<Widget %p: %s>", w, w->name);
42 if (len == -1)
43 return -1;
44
45 /* Pad to the minimum field width and print to the stream. */
46 len = fprintf (stream, "%*s",
47 (info->left ? -info->width : info->width),
48 buffer);
49
50 /* Clean up and return. */
51 free (ptr: buffer);
52 return len;
53}
54
55
56int
57print_widget_arginfo (const struct printf_info *info, size_t n,
58 int *argtypes)
59{
60 /* We always take exactly one argument and this is a pointer to the
61 structure.. */
62 if (n > 0)
63 argtypes[0] = PA_POINTER;
64 return 1;
65}
66
67
68int
69main (void)
70{
71 /* Make a widget to print. */
72 Widget mywidget;
73 mywidget.name = "mywidget";
74
75 /* Register the print function for widgets. */
76 register_printf_function (spec: 'W', func: print_widget, arginfo: print_widget_arginfo);
77
78 /* Now print the widget. */
79 printf (format: "|%W|\n", &mywidget);
80 printf (format: "|%35W|\n", &mywidget);
81 printf (format: "|%-35W|\n", &mywidget);
82
83 return 0;
84}
85

source code of glibc/manual/examples/rprintf.c