.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH printf.h 3head 2025-05-17 "Linux man-pages (unreleased)"
.SH 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
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <printf.h>
.P
.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
.SS Callbacks
.nf
.BI "typedef int printf_function(FILE *" stream ", 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 [ n "], int " size [ n ]);
.BI "typedef void printf_va_arg_function(void *" mem ", va_list *" ap );
.fi
.SS Types
.EX
.B struct printf_info {
.BR "    int            prec;            " "// Precision"
.BR "    int            width;           " "// Width"
.BR "    wchar_t        spec;            " "// Format letter"
.BR "    unsigned int   is_long_double:1;" "// " L " or " ll " flag"
.BR "    unsigned int   is_short:1;      " "// " h " flag"
.BR "    unsigned int   is_long:1;       " "// " l " flag"
.BR "    unsigned int   alt:1;           " "// " # " flag"
.BR "    unsigned int   space:1;         " "// Space flag"
.BR "    unsigned int   left:1;          " "// " - " flag"
.BR "    unsigned int   showsign:1;      " "// " + " flag"
.BR "    unsigned int   group:1;         " "// " \[aq] " flag"
.BR "    unsigned int   extra:1;         " "// For special use"
.BR "    unsigned int   is_char:1;       " "// " hh " flag"
.BR "    unsigned int   wide:1;          " "// True for wide character streams"
.BR "    unsigned int   i18n:1;          " "// " I " flag"
.BR "    unsigned int   is_binary128:1;  " "/* Floating-point argument is"
.BR "                                    " "   ABI-compatible with"
.BR "                                    " "   IEC 60559 binary128 */"
.BR "    unsigned short user;            " "// Bits for user-installed modifiers"
.BR "    wchar_t        pad;             " "// Padding character"
.B };
.EE
.SS Constants
.EX
.BR "#define PA_FLAG_LONG_LONG    " "/* ... */"
.BR "#define PA_FLAG_LONG_DOUBLE  " "/* ... */"
.BR "#define PA_FLAG_LONG         " "/* ... */"
.BR "#define PA_FLAG_SHORT        " "/* ... */"
.BR "#define PA_FLAG_PTR          " "/* ... */"
.EE
.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.
.TP
.I spec
The character which will be used as a conversion specifier in the format string.
.TP
.I func
Callback function that will be executed by the
.BR printf (3)
family of functions
to format the input arguments into the output
.IR stream .
.RS
.TP
.I 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.
.TP
.I info
Structure that holds context information,
including the modifiers specified in the format string.
This holds the same contents as in
.IR arginfo .
.TP
.I args
Array of pointers to the arguments to the
.BR printf (3)\c
-like function.
.RE
.TP
.I arginfo
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.
.RS
.TP
.I info
Structure that holds context information,
including the modifiers specified in the format string.
This holds the same contents as in
.IR func .
.TP
.I n
Number of arguments remaining to be parsed.
.TP
.I 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
.BR printf (3)\c
-like function.
Each element should be set to a base type
.RB ( PA_ *)
from the enum above,
or a custom one,
and optionally ORed with an appropriate length modifier
.RB ( PA_FLAG_ *).
.RS
.P
The type is determined by using one of the following constants:
.TP
.B PA_INT
.IR int .
.TP
.B PA_CHAR
.IR int ,
cast to
.IR char .
.TP
.B PA_WCHAR
.IR wchar_t .
.TP
.B PA_STRING
.IR "const char\~*" ,
a \(aq\[rs]0\(aq-terminated string.
.TP
.B PA_WSTRING
.IR "const wchar_t\~*" ,
a wide character L\(aq\[rs]0\(aq-terminated string.
.TP
.B PA_POINTER
.IR void\~* .
.TP
.B PA_FLOAT
.IR float .
.TP
.B PA_DOUBLE
.IR double .
.TP
.B PA_LAST
TODO.
.RE
.TP
.I size
For user-defined types,
the size of the type (in bytes) should also be specified through this array.
Otherwise, leave it unused.
.RE
.P
.I arginfo
is called before
.IR func ,
and prepares some information needed to call
.IR func .
.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.
.P
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
.TP
.B EINVAL
The specifier was not a valid character.
.SH STANDARDS
GNU.
.SH HISTORY
.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.
.P
.BR \%register_printf_specifier ()
supersedes
.BR \%register_printf_function (3).
.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.
.P
.\" 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  \[aq]\[rs]\[aq]\[aq]
\&
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(\[aq]b\[aq], b_printf, b_arginf_sz) == \-1)
        err(EXIT_FAILURE, "register_printf_specifier(\[aq]b\[aq], ...)");
    if (register_printf_specifier(\[aq]B\[aq], b_printf, b_arginf_sz) == \-1)
        err(EXIT_FAILURE, "register_printf_specifier(\[aq]B\[aq], ...)");
\&
    printf("....----....----....----....----\[rs]n");
    printf("%llb;\[rs]n", 0x5Ellu);
    printf("%lB;\[rs]n", 0x5Elu);
    printf("%b;\[rs]n", 0x5Eu);
    printf("%hB;\[rs]n", 0x5Eu);
    printf("%hhb;\[rs]n", 0x5Eu);
    printf("%jb;\[rs]n", (uintmax_t)0x5E);
    printf("%zb;\[rs]n", (size_t)0x5E);
    printf("....----....----....----....----\[rs]n");
    printf("%#b;\[rs]n", 0x5Eu);
    printf("%#B;\[rs]n", 0x5Eu);
    printf("....----....----....----....----\[rs]n");
    printf("%10b;\[rs]n", 0x5Eu);
    printf("%010b;\[rs]n", 0x5Eu);
    printf("%.10b;\[rs]n", 0x5Eu);
    printf("....----....----....----....----\[rs]n");
    printf("%\-10B;\[rs]n", 0x5Eu);
    printf("....----....----....----....----\[rs]n");
    printf("%\[aq]B;\[rs]n", 0x5Eu);
    printf("....----....----....----....----\[rs]n");
    printf("....----....----....----....----\[rs]n");
    printf("%#16.12b;\[rs]n", 0xAB);
    printf("%\-#\[aq]20.12b;\[rs]n", 0xAB);
    printf("%#\[aq]020B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%#020B;\[rs]n", 0xAB);
    printf("%\[aq]020B;\[rs]n", 0xAB);
    printf("%020B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%#021B;\[rs]n", 0xAB);
    printf("%\[aq]021B;\[rs]n", 0xAB);
    printf("%021B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%#022B;\[rs]n", 0xAB);
    printf("%\[aq]022B;\[rs]n", 0xAB);
    printf("%022B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%#023B;\[rs]n", 0xAB);
    printf("%\[aq]023B;\[rs]n", 0xAB);
    printf("%023B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%\-#\[aq]19.11b;\[rs]n", 0xAB);
    printf("%#\[aq]019B;\[rs]n", 0xAB);
    printf("%#019B;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]n");
    printf("%\[aq]019B;\[rs]n", 0xAB);
    printf("%019B;\[rs]n", 0xAB);
    printf("%#016b;\[rs]n", 0xAB);
    printf("....----....----....----....----\[rs]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 == \[aq] \[aq])
        pad.len = b_pad_len(info, bin_len);
\&
    /* Padding with \[aq] \[aq] (right aligned) */
    if ((pad.ch == \[aq] \[aq]) && !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 \[aq]0\[aq] */
    if (pad.ch == \[aq]0\[aq]) {
        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 \[aq] \[aq] (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] = \[aq]0\[aq];
    for (min_len = 0; val; min_len++) {
        bin[min_len] = \[aq]0\[aq] + (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(\[aq]0\[aq], 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(\[aq]0\[aq], 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(\[aq]0\[aq], 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 == \[aq] \[aq]) || info\->left)
        return \[aq] \[aq];
    return \[aq]0\[aq];
}
\&
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(\[aq] \[aq], stream) == EOF)
            return EOF;
        len++;
    }
\&
    return len;
}
.EE
.\" SRC END
.SH SEE ALSO
.BR asprintf (3),
.BR printf (3),
.BR wprintf (3)