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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
printf.h(3head)printf.h(3head)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
Standard C library (libc,-lc)
#include <printf.h>int register_printf_specifier(intspec, printf_functionfunc,printf_arginfo_size_functionarginfo);int register_printf_modifier(const wchar_t *str);int register_printf_type(printf_va_arg_functionfct);Callbackstypedef int printf_function(FILE *stream, const struct printf_info *info,const void *constargs[]);typedef int printf_arginfo_size_function(const struct printf_info *info,size_tn, intargtypes[n], intsize[n]);typedef void printf_va_arg_function(void *mem, va_list *ap);Typesstruct printf_info {int prec;// Precisionint width;// Widthwchar_t spec;// Format letterunsigned int is_long_double:1;//Lorllflagunsigned int is_short:1;//hflagunsigned int is_long:1;//lflagunsigned int alt:1;//#flagunsigned int space:1;// Space flagunsigned int left:1;//-flagunsigned int showsign:1;//+flagunsigned int group:1;//'flagunsigned int extra:1;// For special useunsigned int is_char:1;//hhflagunsigned int wide:1;// True for wide character streamsunsigned int i18n:1;//Iflagunsigned int is_binary128:1;/* Floating-point argument is ABI-compatible with IEC 60559 binary128 */unsigned short user;// Bits for user-installed modifierswchar_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/* ... */These functions serve to extend and/or modify the behavior of theprintf(3) family of functions.register_printf_specifier() This function registers a custom conversion specifier for theprintf(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 theprintf(3) family of functions to format the input arguments into the outputstream.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 inarginfo.args Array of pointers to the arguments to theprintf(3)-like function.arginfo Callback function that will be executed by theprintf(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 infunc.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 theprintf(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_INTint.PA_CHARint, cast tochar.PA_WCHARwchar_t.PA_STRINGconst char *, a '\0'-terminated string.PA_WSTRINGconst wchar_t *, a wide character L'\0'-terminated string.PA_POINTERvoid *.PA_FLOATfloat.PA_DOUBLEdouble.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 beforefunc, and prepares some information needed to callfunc.register_printf_modifier() TODOregister_printf_type() TODO
register_printf_specifier(),register_printf_modifier(), andregister_printf_type() return zero on success, or -1 on error.Callbacks The callback of typeprintf_function should return the number of characters written, or -1 on error. The callback of typeprintf_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 throughargtypes. On error, it should return -1.
EINVALThe specifier was not a valid character.
GNU.
register_printf_function(3) is an older function similar toregister_printf_specifier(), and is now deprecated. That function can't handle user-defined types.register_printf_specifier() supersedesregister_printf_function(3).
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; }asprintf(3),printf(3),wprintf(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17printf.h(3head)HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |