Install Now
printf.h, register_printf_specifier, register_printf_modifier, register_printf_type, printf_function,
Contents
Description
These functions serve to extend and/or modify the behavior of the printf(3) family of functions.
register_printf_specifier()
This function registers a custom conversion specifier for the printf(3) family of functions.
spec The character which will be used as a conversion specifier in the format string.
func Callback function that will be executed by the printf(3) family of functions to format the input
arguments into the output stream.
stream Output stream where the formatted output should be printed. This stream transparently
represents the output, even in the case of functions that write to a string.
info Structure that holds context information, including the modifiers specified in the format
string. This holds the same contents as in arginfo.
args Array of pointers to the arguments to the printf(3)-like function.
arginfo
Callback function that will be executed by the printf(3) family of functions to know how many
arguments should be parsed for the custom specifier and also their types.
info Structure that holds context information, including the modifiers specified in the format
string. This holds the same contents as in func.
n Number of arguments remaining to be parsed.
argtypes
This array should be set to define the type of each of the arguments that will be parsed.
Each element in the array represents one of the arguments to be parsed, in the same order
that they are passed to the printf(3)-like function. Each element should be set to a base
type (PA_*) from the enum above, or a custom one, and optionally ORed with an appropriate
length modifier (PA_FLAG_*).
The type is determined by using one of the following constants:
PA_INTint.
PA_CHARint, cast to char.
PA_WCHARwchar_t.
PA_STRINGconstchar*, a '\0'-terminated string.
PA_WSTRINGconstwchar_t*, a wide character L'\0'-terminated string.
PA_POINTERvoid*.
PA_FLOATfloat.
PA_DOUBLEdouble.
PA_LAST
TODO.
size For user-defined types, the size of the type (in bytes) should also be specified through
this array. Otherwise, leave it unused.
arginfo is called before func, and prepares some information needed to call func.
register_printf_modifier()
TODO
register_printf_type()
TODO
Errors
EINVAL The specifier was not a valid character.
Examples
The following example program registers the 'b' and 'B' specifiers to print integers in binary format,
mirroring rules for other unsigned conversion specifiers like 'x' and 'u'. This can be used to print in
binary prior to C23.
/* This code is in the public domain */
#include <err.h>
#include <limits.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/param.h>
#include <printf.h>
#define GROUP_SEP '\''
struct Printf_Pad {
char ch;
size_t len;
};
static int b_printf(FILE *stream, const struct printf_info *info,
const void *const args[]);
static int b_arginf_sz(const struct printf_info *info,
size_t n, int argtypes[n], int size[n]);
static uintmax_t b_value(const struct printf_info *info,
const void *arg);
static size_t b_bin_repr(char bin[UINTMAX_WIDTH],
const struct printf_info *info, const void *arg);
static size_t b_bin_len(const struct printf_info *info,
ptrdiff_t min_len);
static size_t b_pad_len(const struct printf_info *info,
ptrdiff_t bin_len);
static ssize_t b_print_prefix(FILE *stream,
const struct printf_info *info);
static ssize_t b_pad_zeros(FILE *stream, const struct printf_info *info,
ptrdiff_t min_len);
static ssize_t b_print_number(FILE *stream,
const struct printf_info *info,
const char bin[UINTMAX_WIDTH],
size_t min_len, size_t bin_len);
static char pad_ch(const struct printf_info *info);
static ssize_t pad_spaces(FILE *stream, size_t pad_len);
int
main(void)
{
if (register_printf_specifier('b', b_printf, b_arginf_sz) == -1)
err(EXIT_FAILURE, "register_printf_specifier('b', ...)");
if (register_printf_specifier('B', b_printf, b_arginf_sz) == -1)
err(EXIT_FAILURE, "register_printf_specifier('B', ...)");
printf("....----....----....----....----\n");
printf("%llb;\n", 0x5Ellu);
printf("%lB;\n", 0x5Elu);
printf("%b;\n", 0x5Eu);
printf("%hB;\n", 0x5Eu);
printf("%hhb;\n", 0x5Eu);
printf("%jb;\n", (uintmax_t)0x5E);
printf("%zb;\n", (size_t)0x5E);
printf("....----....----....----....----\n");
printf("%#b;\n", 0x5Eu);
printf("%#B;\n", 0x5Eu);
printf("....----....----....----....----\n");
printf("%10b;\n", 0x5Eu);
printf("%010b;\n", 0x5Eu);
printf("%.10b;\n", 0x5Eu);
printf("....----....----....----....----\n");
printf("%-10B;\n", 0x5Eu);
printf("....----....----....----....----\n");
printf("%'B;\n", 0x5Eu);
printf("....----....----....----....----\n");
printf("....----....----....----....----\n");
printf("%#16.12b;\n", 0xAB);
printf("%-#'20.12b;\n", 0xAB);
printf("%#'020B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%#020B;\n", 0xAB);
printf("%'020B;\n", 0xAB);
printf("%020B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%#021B;\n", 0xAB);
printf("%'021B;\n", 0xAB);
printf("%021B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%#022B;\n", 0xAB);
printf("%'022B;\n", 0xAB);
printf("%022B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%#023B;\n", 0xAB);
printf("%'023B;\n", 0xAB);
printf("%023B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%-#'19.11b;\n", 0xAB);
printf("%#'019B;\n", 0xAB);
printf("%#019B;\n", 0xAB);
printf("....----....----....----....----\n");
printf("%'019B;\n", 0xAB);
printf("%019B;\n", 0xAB);
printf("%#016b;\n", 0xAB);
printf("....----....----....----....----\n");
return 0;
}
static int
b_printf(FILE *stream, const struct printf_info *info,
const void *const args[])
{
char bin[UINTMAX_WIDTH];
size_t min_len, bin_len;
ssize_t len, tmp;
struct Printf_Pad pad = {0};
len = 0;
min_len = b_bin_repr(bin, info, args[0]);
bin_len = b_bin_len(info, min_len);
pad.ch = pad_ch(info);
if (pad.ch == ' ')
pad.len = b_pad_len(info, bin_len);
/* Padding with ' ' (right aligned) */
if ((pad.ch == ' ') && !info->left) {
tmp = pad_spaces(stream, pad.len);
if (tmp == EOF)
return EOF;
len += tmp;
}
/* "0b"/"0B" prefix */
if (info->alt) {
tmp = b_print_prefix(stream, info);
if (tmp == EOF)
return EOF;
len += tmp;
}
/* Padding with '0' */
if (pad.ch == '0') {
tmp = b_pad_zeros(stream, info, min_len);
if (tmp == EOF)
return EOF;
len += tmp;
}
/* Print number (including leading 0s to fill precision) */
tmp = b_print_number(stream, info, bin, min_len, bin_len);
if (tmp == EOF)
return EOF;
len += tmp;
/* Padding with ' ' (left aligned) */
if (info->left) {
tmp = pad_spaces(stream, pad.len);
if (tmp == EOF)
return EOF;
len += tmp;
}
return len;
}
static int
b_arginf_sz(const struct printf_info *info, size_t n, int argtypes[n],
[[maybe_unused]] int size[n])
{
if (n < 1)
return -1;
if (info->is_long_double)
argtypes[0] = PA_INT | PA_FLAG_LONG_LONG;
else if (info->is_long)
argtypes[0] = PA_INT | PA_FLAG_LONG;
else
argtypes[0] = PA_INT;
return 1;
}
static uintmax_t
b_value(const struct printf_info *info, const void *arg)
{
if (info->is_long_double)
return *(const unsigned long long *)arg;
if (info->is_long)
return *(const unsigned long *)arg;
/* short and char are both promoted to int */
return *(const unsigned int *)arg;
}
static size_t
b_bin_repr(char bin[UINTMAX_WIDTH],
const struct printf_info *info, const void *arg)
{
size_t min_len;
uintmax_t val;
val = b_value(info, arg);
bin[0] = '0';
for (min_len = 0; val; min_len++) {
bin[min_len] = '0' + (val % 2);
val >>= 1;
}
return MAX(min_len, 1);
}
static size_t
b_bin_len(const struct printf_info *info, ptrdiff_t min_len)
{
return MAX(info->prec, min_len);
}
static size_t
b_pad_len(const struct printf_info *info, ptrdiff_t bin_len)
{
ptrdiff_t pad_len;
pad_len = info->width - bin_len;
if (info->alt)
pad_len -= 2;
if (info->group)
pad_len -= (bin_len - 1) / 4;
return MAX(pad_len, 0);
}
static ssize_t
b_print_prefix(FILE *stream, const struct printf_info *info)
{
ssize_t len;
len = 0;
if (fputc('0', stream) == EOF)
return EOF;
len++;
if (fputc(info->spec, stream) == EOF)
return EOF;
len++;
return len;
}
static ssize_t
b_pad_zeros(FILE *stream, const struct printf_info *info,
ptrdiff_t min_len)
{
ssize_t len;
ptrdiff_t tmp;
len = 0;
tmp = info->width - (info->alt * 2);
if (info->group)
tmp -= tmp / 5 - !(tmp % 5);
for (ptrdiff_t i = tmp - 1; i > min_len - 1; i--) {
if (fputc('0', stream) == EOF)
return EOF;
len++;
if (!info->group || (i % 4))
continue;
if (fputc(GROUP_SEP, stream) == EOF)
return EOF;
len++;
}
return len;
}
static ssize_t
b_print_number(FILE *stream, const struct printf_info *info,
const char bin[UINTMAX_WIDTH],
size_t min_len, size_t bin_len)
{
ssize_t len;
len = 0;
/* Print leading zeros to fill precision */
for (size_t i = bin_len - 1; i > min_len - 1; i--) {
if (fputc('0', stream) == EOF)
return EOF;
len++;
if (!info->group || (i % 4))
continue;
if (fputc(GROUP_SEP, stream) == EOF)
return EOF;
len++;
}
/* Print number */
for (size_t i = min_len - 1; i < min_len; i--) {
if (fputc(bin[i], stream) == EOF)
return EOF;
len++;
if (!info->group || (i % 4) || !i)
continue;
if (fputc(GROUP_SEP, stream) == EOF)
return EOF;
len++;
}
return len;
}
static char
pad_ch(const struct printf_info *info)
{
if ((info->prec != -1) || (info->pad == ' ') || info->left)
return ' ';
return '0';
}
static ssize_t
pad_spaces(FILE *stream, size_t pad_len)
{
ssize_t len;
len = 0;
for (size_t i = pad_len - 1; i < pad_len; i--) {
if (fputc(' ', stream) == EOF)
return EOF;
len++;
}
return len;
}
History
register_printf_function(3) is an older function similar to register_printf_specifier(), and is now deprecated. That function can't handle user-defined types. register_printf_specifier() supersedes register_printf_function(3).
Library
Standard C library (libc, -lc)
Name
printf.h, register_printf_specifier, register_printf_modifier, register_printf_type, printf_function,
printf_arginfo_size_function, printf_va_arg_function, printf_info, PA_INT, PA_CHAR, PA_WCHAR, PA_STRING,
PA_WSTRING, PA_POINTER, PA_FLOAT, PA_DOUBLE, PA_LAST, PA_FLAG_LONG_LONG, PA_FLAG_LONG_DOUBLE,
PA_FLAG_LONG, PA_FLAG_SHORT, PA_FLAG_PTR - define custom behavior for printf-like functions
Return Value
register_printf_specifier(), register_printf_modifier(), and register_printf_type() return zero on
success, or -1 on error.
Callbacks
The callback of type printf_function should return the number of characters written, or -1 on error.
The callback of type printf_arginfo_size_function should return the number of arguments to be parsed by
this specifier. It also passes information about the type of those arguments to the caller through
argtypes. On error, it should return -1.
See Also
asprintf(3), printf(3), wprintf(3) Linux man-pages 6.9.1 2024-06-15 printf.h(3head)
Standards
GNU.
Synopsis
#include<printf.h>intregister_printf_specifier(intspec,printf_functionfunc,printf_arginfo_size_functionarginfo);intregister_printf_modifier(constwchar_t*str);intregister_printf_type(printf_va_arg_functionfct);Callbackstypedefintprintf_function(FILE*stream,conststructprintf_info*info,constvoid*constargs[]);typedefintprintf_arginfo_size_function(conststructprintf_info*info,size_tn,intargtypes[n],intsize[n]);typedefvoidprintf_va_arg_function(void*mem,va_list*ap);Typesstructprintf_info{intprec; // Precision
intwidth; // Width
wchar_tspec; // Format letter
unsignedintis_long_double:1;// L or ll flag
unsignedintis_short:1; // h flag
unsignedintis_long:1; // l flag
unsignedintalt:1; // # flag
unsignedintspace:1; // Space flag
unsignedintleft:1; // - flag
unsignedintshowsign:1; // + flag
unsignedintgroup:1; // ' flag
unsignedintextra:1; // For special use
unsignedintis_char:1; // hh flag
unsignedintwide:1; // True for wide character streams
unsignedinti18n:1; // I flag
unsignedintis_binary128:1; /* Floating-point argument is
ABI-compatible with
IEC 60559 binary128 */
unsignedshortuser; // Bits for user-installed modifiers
wchar_tpad; // Padding character
};Constants#definePA_FLAG_LONG_LONG /* ... */
#definePA_FLAG_LONG_DOUBLE /* ... */
#definePA_FLAG_LONG /* ... */
#definePA_FLAG_SHORT /* ... */
#definePA_FLAG_PTR /* ... */
PNG Icons
Emojis