FWPRINTF(3) Library Functions Manual FWPRINTF(3)

fwprintf swprintf wprintf — print formatted wide-character output

#include <stdio.h>
#include <wchar.h>

int fwprintf(FILE *restrict stream, const wchar_t *restrict format, ...);
int swprintf(wchar_t *restrict ws, size_t n,
       const wchar_t *restrict format, ...);
int wprintf(const wchar_t *restrict format, ...);

[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The fwprintf () function shall place output on the named output . The wprintf () function shall place output on the standard output stream . The swprintf () function shall place output followed by the null wide character in consecutive wide characters starting at * ; no more than n wide characters shall be written, including a terminating null wide character, which is always added (unless n is zero).

Each of these functions shall convert, format, and print its arguments under control of the format wide-character string. The format is composed of zero or more directives: , which are simply copied to the output stream, and conversion specifications, each of which results in the fetching of zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments are evaluated but are otherwise ignored.

[CX] Conversions can be applied to the n th argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion specifier wide character % (see below) is replaced by the sequence "%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}], giving the position of the argument in the argument list. This feature provides for the definition of format wide-character strings that select arguments in an order appropriate to specific languages (see the EXAMPLES section).

The format can contain either numbered argument conversion specifications (that is, those introduced by "% n $" and optionally containing the "* m $" forms of field width and precision), or unnumbered argument conversion specifications (that is, those introduced by the % character and optionally containing the * form of field width and precision), but not both. The only exception to this is that %% can be mixed with the "% n $" form. The results of mixing numbered and unnumbered argument specifications in a format wide-character string are undefined. When numbered argument specifications are used, specifying the N th argument requires that all the leading arguments, from the first to the ( N -1)th, are specified in the format wide-character string.

In format wide-character strings containing the "% n $" form of conversion specification, numbered arguments in the argument list can be referenced from the format wide-character string as many times as required.

In format wide-character strings containing the % form of conversion specification, each argument in the argument list shall be used exactly once. It is unspecified whether an encoding error occurs if the format string contains wchar_t values that do not correspond to members of the character set of the current locale and the specified semantics do not require that value to be processed by wcrtomb(3).

[CX] All forms of the fwprintf () function allow for the insertion of a locale-dependent radix character in the output string, output as a wide-character value. The radix character is defined in the current locale (category In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a <period> ( ‘’.).

Each conversion specification is introduced by the ‘%’ wide character [CX] or by the wide-character sequence "% n $", after which the following appear in sequence:

  • Zero or more (in any order), which modify the meaning of the conversion specification.
  • An optional minimum . If the converted value has fewer wide characters than the field width, it shall be padded with <space> characters by default on the left; it shall be padded on the right, if the left-adjustment flag ( ‘-’), described below, is given to the field width. The field width takes the form of an <asterisk> ( ‘*’), [CX] or in conversion specifications introduced by "% n $" the "* m $" string, described below, or a decimal integer.
  • An optional that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversion specifiers; the number of digits to appear after the radix character for the a, A, e, E, f, and F conversion specifiers; the maximum number of significant digits for the g and G conversion specifiers; or the maximum number of wide characters to be printed from a string in the s conversion specifiers. The precision takes the form of a <period> ( ‘’.) followed either by an <asterisk> ( ‘*’), [CX] or in conversion specifications introduced by "% n $" the "* m $" string, described below, or an optional decimal digit string, where a null digit string is treated as 0. If a precision appears with any other conversion wide character, the behavior is undefined.
  • An optional length modifier that specifies the size of the argument.
  • A conversion specifier wide character that indicates the type of conversion to be applied.

A field width, or precision, or both, may be indicated by an <asterisk> ( ‘*’). In this case an argument of type supplies the field width or precision. Applications shall ensure that arguments specifying field width, or precision, or both appear in that order before the argument, if any, to be converted. A negative field width is taken as a ‘-’ flag followed by a positive field width. A negative precision is taken as if the precision were omitted. [CX] In format wide-character strings containing conversion specifications introduced by "% n $", in addition to being indicated by the decimal digit string, a field width may be indicated by the sequence "* m $" and precision by the sequence ".* m $", where m is a decimal integer in the range [1,{NL_ARGMAX}] giving the position in the argument list (after the format argument) of an integer argument containing the field width or precision, for example:

wprintf(L"%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The flag wide characters and their meanings are:

[CX] (The <apostrophe>.) The integer portion of the result of a decimal conversion ( %i , %d , %u , %f , %F , %g , or %G ) shall be formatted with thousands' grouping wide characters. For other conversions, the behavior is undefined. The numeric grouping wide character is used.
-
The result of the conversion shall be left-justified within the field. The conversion shall be right-justified if this flag is not specified.
The result of a signed conversion shall always begin with a sign ( '+' or '-' ). The conversion shall begin with a sign only when a negative value is converted if this flag is not specified.
If the first wide character of a signed conversion is not a sign, or if a signed conversion results in no wide characters, a <space> shall be prefixed to the result. This means that if the <space> and '+' flags both appear, the <space> flag shall be ignored.
Specifies that the value is to be converted to an alternative form. For o conversion, it shall increase the precision, if and only if necessary, to force the first digit of the result to be zero (if the value and precision are both 0, a single 0 is printed). For x or X conversion specifiers, a non-zero result shall have 0x (or 0X) prefixed to it. For a , A , e , E , f , F , g , and G conversion specifiers, the result shall always contain a radix character, even if no digits follow it. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. For g and G conversion specifiers, trailing zeros shall not be removed from the result as they normally are. For other conversion specifiers, the behavior is undefined.
For d , i , o , u , x , X , a , A , e , E , f , F , g , and G conversion specifiers, leading zeros (following any indication of sign or base) are used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the '0' and '-' flags both appear, the '0' flag shall be ignored. For d , i , o , u , x , and X conversion specifiers, if a precision is specified, the '0' flag shall be ignored. [CX] If the '0' and <apostrophe> flags both appear, the grouping wide characters are inserted before zero padding. For other conversions, the behavior is undefined.

The length modifiers and their meanings are:

Specifies that a following d , i , o , u , x , or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to signed char or unsigned char before printing); or that a following n conversion specifier applies to a pointer to a signed char argument.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to a short or unsigned short argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short or unsigned short before printing); or that a following n conversion specifier applies to a pointer to a short argument.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to a long or unsigned long argument; that a following n conversion specifier applies to a pointer to a long argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a , A , e , E , f , F , g , or G conversion specifier.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to a long long or unsigned long long argument; or that a following n conversion specifier applies to a pointer to a long long argument.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to a size_t argument.
Specifies that a following d , i , o , u , x , or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.
Specifies that a following a , A , e , E , f , F , g , or G conversion specifier applies to a long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

The int argument shall be converted to a signed decimal in the style "[-] dddd" . The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision shall be 1. The result of converting zero with an explicit precision of zero shall be no wide characters.
The unsigned argument shall be converted to unsigned octal format in the style "dddd" . The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision shall be 1. The result of converting zero with an explicit precision of zero shall be no wide characters.
The unsigned argument shall be converted to unsigned decimal format in the style "dddd" . The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision shall be 1. The result of converting zero with an explicit precision of zero shall be no wide characters.
The unsigned argument shall be converted to unsigned hexadecimal format in the style "dddd" ; the letters "abcdef" are used. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision shall be 1. The result of converting zero with an explicit precision of zero shall be no wide characters.
Equivalent to the x conversion specifier, except that letters "ABCDEF" are used instead of "abcdef" .
The double argument shall be converted to decimal notation in the style "[-] ddd.ddd" , where the number of digits after the radix character shall be equal to the precision specification. If the precision is missing, it shall be taken as 6; if the precision is explicitly zero and no '#' flag is present, no radix character shall appear. If a radix character appears, at least one digit shall appear before it. The value shall be rounded in an implementation-defined manner to the appropriate number of digits.

A double argument representing an infinity shall be converted in one of the styles "[-]inf" or "[-]infinity"; which style is implementation-defined. A double argument representing a NaN shall be converted in one of the styles "[-]nan" or "[-]nan( n-char-sequence )"; which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces "INF", "INFINITY", or "NAN" instead of "inf", "infinity", or "nan", respectively.

The double argument shall be converted in the style "[-] d.ddd e ±dd" , where there shall be one digit before the radix character (which is non-zero if the argument is non-zero) and the number of digits after it shall be equal to the precision; if the precision is missing, it shall be taken as 6; if the precision is zero and no '#' flag is present, no radix character shall appear. The value shall be rounded in an implementation-defined manner to the appropriate number of digits. The E conversion wide character shall produce a number with 'E' instead of 'e' introducing the exponent. The exponent shall always contain at least two digits. If the value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

The double argument representing a floating-point number shall be converted in the style f or e (or in the style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let P equal the precision if non-zero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X :
  • If P > X >=-4, the conversion shall be with style f (or F) and precision P -( X +1).
  • Otherwise, the conversion shall be with style e (or E) and precision P -1.

Finally, unless the ‘#’ flag is used, any trailing zeros shall be removed from the fractional portion of the result and the decimal-point character shall be removed if there is no fractional portion remaining.

A double argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

A double argument representing a floating-point number shall be converted in the style "[-]0x h . hhhh p± d " , where there shall be one hexadecimal digit (which is non-zero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point wide character and the number of hexadecimal digits after it shall be equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision shall be sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision shall be sufficient to distinguish values of type double , except that trailing zeros may be omitted; if the precision is zero and the '#' flag is not specified, no decimal-point wide character shall appear. The letters "abcdef" are used for a conversion and the letters "ABCDEF" for A conversion. The A conversion specifier produces a number with 'X' and 'P' instead of 'x' and 'p' . The exponent shall always contain at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or F conversion specifier.

If no l (ell) qualifier is present, the int argument shall be converted to a wide character as if by calling the () function and the resulting wide character shall be written. Otherwise, the wint_t argument shall be converted to wchar_t , and written.
If no l (ell) qualifier is present, the application shall ensure that the argument is a pointer to a character array containing a character sequence beginning in the initial shift state. Characters from the array shall be converted as if by repeated calls to the () function, with the conversion state described by an mbstate_t object initialized to zero before the first character is converted, and written up to (but not including) the terminating null wide character. If the precision is specified, no more than that many wide characters shall be written. If the precision is not specified, or is greater than the size of the array, the application shall ensure that the array contains a null wide character.

If an l (ell) qualifier is present, the application shall ensure that the argument is a pointer to an array of type wchar_t. Wide characters from the array shall be written up to (but not including) a terminating null wide character. If no precision is specified, or is greater than the size of the array, the application shall ensure that the array contains a null wide character. If a precision is specified, no more than that many wide characters shall be written.

The application shall ensure that the argument is a pointer to void . The value of the pointer shall be converted to a sequence of printable wide characters in an implementation-defined manner.
The application shall ensure that the argument is a pointer to an integer into which is written the number of wide characters written to the output so far by this call to one of the fwprintf () functions. No argument shall be converted, but one shall be consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined.
[XSI] Equivalent to lc .
[XSI] Equivalent to ls .
Write a '%' wide character; no argument shall be converted. The application shall ensure that the complete conversion specification is %% .

If a conversion specification does not match one of the above forms, the behavior is undefined.

In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field shall be expanded to contain the conversion result. Characters generated by fwprintf () and wprintf () shall be printed as if fputwc(3) had been called.

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

For e, E, f, F, g, and G conversion specifiers, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded. If the number of significant decimal digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the result should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the current rounding direction.

[CX] The last data modification and last file status change timestamps of the file shall be marked for update between the call to a successful execution of fwprintf () or wprintf () and the next successful completion of a call to fflush(3) or fclose(3) on the same stream, or a call to exit(3) or abort(3).

Upon successful completion, these functions shall return the number of wide characters transmitted, excluding the terminating null wide character in the case of swprintf (), or a negative value if an error was encountered, [CX] and set errno to indicate the error.

If n or more wide characters were requested to be written, swprintf () shall return a negative value, [CX] and set errno to indicate the error.

For the conditions under which fwprintf () and wprintf () fail and may fail, refer to fputwc(3).

In addition, all forms of fwprintf () shall fail if:

[CX] A wide-character code that does not correspond to a valid character has been detected.
[CX] The value to be returned is greater than {INT_MAX}.

In addition, fwprintf () and wprintf () may fail if:

[CX] Insufficient storage space is available.

The swprintf () shall fail if:

[CX] The number of wide characters requested to be written was n or more.

To print the language-independent date and time format, the following statement could be used:

wprintf(format, weekday, month, day, hour, min);

For American usage, format could be a pointer to the wide-character string:

L"%s, %s %d, %d:%.2d\n"

producing the message:

Sunday, July 3, 10:02

whereas for German usage, format could be a pointer to the wide-character string:

L"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

producing the message:

Sonntag, 3. Juli, 10:02

None.

If an implementation detects that there are insufficient arguments for the format, it is recommended that the function should fail and report an [EINVAL] error.

None.

V2_chap02(3), btowc(3), fputwc(3), fwscanf(3), mbrtowc(3), setlocale(3)

XBD V1_chap07(7), <inttypes.h>, <stdio.h>, <wchar.h>

First released in Issue 5. Included for alignment with ISO/IEC 9899/AMD1:1995 (“ISO C90, Amendment 1”) (E).

The Open Group Corrigendum U040/1 is applied to the RETURN VALUE section, describing the case if n or more wide characters are requested to be written using swprintf ().

The normative text is updated to avoid use of the term "must" for application requirements.

The following changes are made for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard:

  • The prototypes for fwprintf (), swprintf (), and wprintf () are updated.
  • The DESCRIPTION is updated.
  • The hh, ll, j, t, and z length modifiers are added.
  • The a, A, and F conversion characters are added.
  • XSI shading is removed from the description of character string representations of infinity and NaN floating-point values.

The DESCRIPTION is updated to use the terms "conversion specifier" and "conversion specification" consistently.

ISO/IEC 9899:1999 (“ISO C99”) standard, Technical Corrigendum 1 is incorporated.

Austin Group Interpretation 1003.1-2001 #161 is applied, updating the DESCRIPTION of the 0 flag.

Austin Group Interpretation 1003.1-2001 #170 is applied.

ISO/IEC 9899:1999 (“ISO C99”) standard, Technical Corrigendum 2 #68 (SD5-XSH-ERN-70) is applied, revising the description of g and G.

Functionality relating to the "%n$" form of conversion specification and the <apostrophe> flag is moved from the XSI option to the Base.

The [EOVERFLOW] error is added for swprintf ().

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0226 [302] and XSH/TC1-2008/0227 [14] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0144 [73], XSH/TC2-2008/0145 [894], XSH/TC2-2008/0146 [557], and XSH/TC2-2008/0147 [936] are applied.

Austin Group Defect 1021 is applied, changing "output error" to "error" in the RETURN VALUE section and changing the requirements for [EOVERFLOW].

Austin Group Defect 1137 is applied, clarifying the use of "% n $" and "* m $" in conversion specifications.

Austin Group Defect 1205 is applied, changing the description of the % conversion specifier.

Austin Group Defect 1219 is applied, changing the swprintf ()-specific [EOVERFLOW] error.

June 14, 2024 posix.fail