printf.h(3head)

🌸 printf.h — bestie edition ✨

okay so this is printf.h, from section 3head of the manual 💖 it lives in the Standard C library (libc, -lc), and it's the whole entire plug-in system for the printf(3) family. yes. you can teach printf new tricks. it's giving main character energy fr fr 👑

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✨ 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 🦋

📚 LIBRARY 📚

Standard C library (libc, -lc).

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✨ SYNOPSIS ✨

#include <printf.h>

int register_printf_specifier(int spec, printf_function func,
                              printf_arginfo_size_function arginfo);
int register_printf_modifier(const wchar_t *str);
int register_printf_type(printf_va_arg_function fct);

🎀 Callbacks

typedef int printf_function(FILE *stream, const struct printf_info *info,
                              const void *const args[]);
typedef int printf_arginfo_size_function(const struct printf_info *info,
                              size_t n, int argtypes[n], int size[n]);
typedef void printf_va_arg_function(void *mem, va_list *ap);

🎀 Types

struct printf_info {
    int            prec;            // Precision
    int            width;           // Width
    wchar_t        spec;            // Format letter
    unsigned int   is_long_double:1;// L or ll flag
    unsigned int   is_short:1;      // h flag
    unsigned int   is_long:1;       // l flag
    unsigned int   alt:1;           // # flag
    unsigned int   space:1;         // Space flag
    unsigned int   left:1;          // - flag
    unsigned int   showsign:1;      // + flag
    unsigned int   group:1;         // ' flag
    unsigned int   extra:1;         // For special use
    unsigned int   is_char:1;       // hh flag
    unsigned int   wide:1;          // True for wide character streams
    unsigned int   i18n:1;          // I flag
    unsigned int   is_binary128:1;  /* Floating-point argument is
                                       ABI-compatible with
                                       IEC 60559 binary128 */
    unsigned short user;            // Bits for user-installed modifiers
    wchar_t        pad;             // Padding character
};

🎀 Constants

#define PA_FLAG_LONG_LONG    /* ... */
#define PA_FLAG_LONG_DOUBLE  /* ... */
#define PA_FLAG_LONG         /* ... */
#define PA_FLAG_SHORT        /* ... */
#define PA_FLAG_PTR          /* ... */

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💅 DESCRIPTION 💅

These functions serve to extend and/or modify the behavior of the printf(3) family of functions. yeah. you're not just using printf, you're redesigning it. iconic behavior tbh 💅

🌸 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. (yes bestie, even string-writers get the same deal 🫶)
  • 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_INT — int.
• PA_CHAR — int, cast to char.
• PA_WCHAR — wchar_t.
• PA_STRING — const char *, a '\0'-terminated string.
• PA_WSTRING — const wchar_t *, a wide character L'\0'-terminated string.
• PA_POINTER — void *.
• PA_FLOAT — float.
• PA_DOUBLE — double.
• 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. so it's the little pre-game ritual, okay? arginfo first, func second, no skipping 🎀

🌸 register_printf_modifier()

TODO

🌸 register_printf_type()

TODO

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🔥 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.

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💀 ERRORS 💀

• EINVAL — the specifier was not a valid character. like, it literally can't be that. nope 🚫

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👑 STANDARDS 👑

GNU.

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📜 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. rip bozo 💀 (affectionate)

register_printf_specifier() supersedes register_printf_function(3).

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🧚‍♀️✨ EXAMPLES ✨🧚‍♀️

okay bestie sit DOWN because this example is genuinely iconic 💅 it's a whole program that registers the 'b' and 'B' specifiers so you can print integers in binary format — and it mirrors the rules for all the other unsigned conversion specifiers like 'x' and 'u', no cap. Which means it's lowkey useful if you wanna print in binary before C23 shows up with it built in 👑

🫶 quick aside, transcribed straight from the source: /* This code is in the public domain */ — it's giving generous, it's giving free real estate 🌈

here's the whole thing, first the includes + declarations + main():

/* 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;
}

and now the ✨ helpers ✨, i.e. the actual braincell of the operation 🧠💖

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;
}

like… the b_bin_repr() one builds the binary digits backwards into bin[] and then b_print_number() walks it back from the end, and the padding/precision/grouping all get their own little moment on stage 😭💜 every single specifier, every #, every ', every width and precision in those printf() calls is doing its thing and it EATS. periodt. 🔥

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🔍💫 SEE ALSO 💫🔍

bestie these are your ride-or-dies, the siblings in the same universe 🌸

• asprintf(3)
• printf(3)
• wprintf(3)

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and that's the whole page, we made it 🎀 tysm for reading to the end, mwah 🫶 just so you know, this page came from Linux man-pages 6.20 and it's dated 2025-05-17 — like, that's the receipt, that's the timestamp, screenshot it for the group chat 💅✨