diff --git a/man3/register_printf_modifier.3 b/man3/register_printf_modifier.3
new file mode 100644
index 000000000..296a92dd3
--- /dev/null
+++ b/man3/register_printf_modifier.3
@@ -0,0 +1 @@
+.so man3/register_printf_specifier.3
diff --git a/man3/register_printf_speficier.3 b/man3/register_printf_speficier.3
new file mode 100644
index 000000000..7e9d3e700
--- /dev/null
+++ b/man3/register_printf_speficier.3
@@ -0,0 +1,439 @@
+.\" Copyright (C) 2022 Alejandro Colomar <alx.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH register_printf_specifier 3 2022-09-18 "Linux man-pages (unreleased)"
+.SH NAME
+register_printf_specifier,
+register_printf_modifier,
+register_printf_type
+\- define custom behavior for printf-like functions
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <printf.h>
+.PP
+.BI "typedef int printf_function(FILE *" stream ,
+.BI "                            const struct printf_info *" info ,
+.BI "                            const void *const *" args );
+.BI "typedef int printf_arginfo_size_function(const struct printf_info *" info ,
+.BI "                            size_t " n ", int *" argtypes ", int *" size );
+.BI "typedef void printf_va_arg_function(void *" mem ", va_list *" ap );
+.PP
+.BI "int register_printf_specifier(int " spec ", printf_function " func ,
+.BI "                            printf_arginfo_size_function " arginfo );
+.BI "int register_printf_modifier(const wchar_t *" str );
+.BI "int register_printf_type(printf_va_arg_function " fct );
+.fi
+.SH DESCRIPTION
+These functions serve to extend and/or modify the behavior of the
+.BR printf (3)
+family of functions.
+.SS register_printf_specifier()
+This function registers a custom conversion specifier for the
+.BR printf (3)
+family of functions.
+.PP
+.I spec
+is a character,
+which will be used as a conversion specifier in the format string.
+.PP
+.I func
+is a callback function that will be
+executed by the
+.BR printf (3)
+family of functions
+to format the input arguments into the output
+.I stream
+(transparently including the case of an output string).
+.PP
+.I arginfo
+is a callback function that will be executed by the
+.BR printf (3)
+family of functions
+to know how many arguments should be parsed for the custom specifier
+and also their types.
+.SS register_printf_modifier()
+TODO
+.SS register_printf_type()
+TODO
+.SH RETURN VALUE
+.BR \%register_printf_specifier (),
+.BR \%register_printf_modifier (),
+and
+.BR \%register_printf_type ()
+return zero on success, or \-1 on error.
+.SS Callbacks
+The callback of type
+.I printf_function
+should return the number of characters written,
+or \-1 on error.
+.PP
+The callback of type
+.I \%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
+.IR argtypes .
+On error, it should return \-1.
+.SH ERRORS
+TODO
+.SH VERSIONS
+.BR \%register_printf_function (3)
+is an older function similar to
+.BR \%register_printf_specifier,
+and is now deprecated.
+That function can't handle user-defined types.
+.PP
+.BR \%register_printf_specifier ()
+superseeds
+.BR \%register_printf_function (3).
+.SH STANDARDS
+These nonstandard functions are present in glibc.
+.SH 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.
+.PP
+.\" SRC BEGIN (register_printf_specifier.c)
+.EX
+/* 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 printf_b_init(void);
+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);
+
+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 (printf_b_init() == \-1)
+        err(EXIT_FAILURE, "printf_b_init");
+
+    printf("....----....----....----....----\en");
+    printf("%llb;\en", 0x5Ellu);
+    printf("%lB;\en", 0x5Elu);
+    printf("%b;\en", 0x5Eu);
+    printf("%hB;\en", 0x5Eu);
+    printf("%hhb;\en", 0x5Eu);
+    printf("%jb;\en", (uintmax_t)0x5E);
+    printf("%zb;\en", (size_t)0x5E);
+    printf("....----....----....----....----\en");
+    printf("%#b;\en", 0x5Eu);
+    printf("%#B;\en", 0x5Eu);
+    printf("....----....----....----....----\en");
+    printf("%10b;\en", 0x5Eu);
+    printf("%010b;\en", 0x5Eu);
+    printf("%.10b;\en", 0x5Eu);
+    printf("....----....----....----....----\en");
+    printf("%\-10B;\en", 0x5Eu);
+    printf("....----....----....----....----\en");
+    printf("%'B;\en", 0x5Eu);
+    printf("....----....----....----....----\en");   
+    printf("....----....----....----....----\en");
+    printf("%#16.12b;\en", 0xAB);
+    printf("%\-#'20.12b;\en", 0xAB);
+    printf("%#'020B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%#020B;\en", 0xAB);
+    printf("%'020B;\en", 0xAB);
+    printf("%020B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%#021B;\en", 0xAB);
+    printf("%'021B;\en", 0xAB);
+    printf("%021B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%#022B;\en", 0xAB);
+    printf("%'022B;\en", 0xAB);
+    printf("%022B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%#023B;\en", 0xAB);
+    printf("%'023B;\en", 0xAB);
+    printf("%023B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%\-#'19.11b;\en", 0xAB);
+    printf("%#'019B;\en", 0xAB);
+    printf("%#019B;\en", 0xAB);
+    printf("....----....----....----....----\en");
+    printf("%'019B;\en", 0xAB);
+    printf("%019B;\en", 0xAB);
+    printf("%#016b;\en", 0xAB);
+    printf("....----....----....----....----\en");
+
+    return 0;
+}
+
+static int
+printf_b_init(void)
+{
+    if (register_printf_specifier('b', b_printf, b_arginf_sz))
+        return -1;
+    if (register_printf_specifier('B', b_printf, b_arginf_sz))
+        return -1;
+    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([[maybe_unused]] const struct printf_info *info,
+            size_t n, int argtypes[n], [[maybe_unused]] int *size)
+{
+    if (n < 1)
+        return -1;
+
+    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;
+
+    /* 'h' and 'hh' 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;
+}
+.EE
+.\" SRC END
+.SH SEE ALSO
+TODO
diff --git a/man3/register_printf_type.3 b/man3/register_printf_type.3
new file mode 100644
index 000000000..296a92dd3
--- /dev/null
+++ b/man3/register_printf_type.3
@@ -0,0 +1 @@
+.so man3/register_printf_specifier.3
