printf.h(3head)

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)