printf.h(3head) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON

printf.h(3head)                                          printf.h(3head)

NAME         top

       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         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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         top

       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         top

       EINVAL The specifier was not a valid character.

STANDARDS         top

       GNU.

HISTORY         top

       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         top

       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         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                printf.h(3head)