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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)

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          /* ... */

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_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.

   register_printf_modifier()
       TODO

   register_printf_type()
       TODO

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.

ERRORS

       EINVAL The specifier was not a valid character.

STANDARDS

       GNU.

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).

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;
       }

SEE ALSO

       asprintf(3), printf(3), wprintf(3)