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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FWPRINTF(3P) POSIX Programmer's Manual FWPRINTF(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
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, ...);
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.1‐2017 defers to the ISO C standard.
The fwprintf() function shall place output on the named output
stream. The wprintf() function shall place output on the standard
output stream stdout. The swprintf() function shall place output
followed by the null wide character in consecutive wide characters
starting at *ws; 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: ordinary wide-
characters, 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.
Conversions can be applied to the nth 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$", where n is a decimal integer in
the range [1,{NL_ARGMAX}], giving the position of the argument in
the argument list. This feature provides for the definition of
format wide-character strings that select arguments in an order
appropriate to specific languages (see the EXAMPLES section).
The format can contain either numbered argument specifications
(that is, "%n$" and "*m$"), or unnumbered argument conversion
specifications (that is, % and *), but not both. The only
exception to this is that %% can be mixed with the "%n$" 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 Nth 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$" form of
conversion specification, numbered arguments in the argument list
can be referenced from the format wide-character string as many
times as required.
In format wide-character strings containing the % form of
conversion specification, each argument in the argument list shall
be used exactly once. It is unspecified whether an encoding error
occurs if the format string contains wchar_t values that do not
correspond to members of the character set of the current locale
and the specified semantics do not require that value to be
processed by wcrtomb().
All forms of the fwprintf() function allow for the insertion of a
locale-dependent radix character in the output string, output as a
wide-character value. The radix character is defined in the
current locale (category LC_NUMERIC). In the POSIX locale, or in
a locale where the radix character is not defined, the radix
character shall default to a <period> ('.').
Each conversion specification is introduced by the '%' wide
character or by the wide-character sequence "%n$", after which the
following appear in sequence:
* Zero or more flags (in any order), which modify the meaning of
the conversion specification.
* An optional minimum field width. 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> ('*'), described below, or a
decimal integer.
* An optional precision 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> ('*'), 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 int 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. In format wide-character strings containing the
"%n$" form of a conversion specification, a field width or
precision may be indicated by the sequence "*m$", 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:
' (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.
<space> 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.
0 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. 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:
hh 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.
h 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.
l (ell) 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.
ll (ell-ell)
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.
j 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.
z 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.
t 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.
L 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:
d, i 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.
o 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.
u 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.
x 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.
X Equivalent to the x conversion specifier, except that
letters "ABCDEF" are used instead of "abcdef".
f, F 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.
e, E The double argument shall be converted in the style
"[-]d.ddde±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.
g, G 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, A A double argument representing a floating-point number
shall be converted in the style "[-]0xh.hhhhp±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.
c If no l (ell) qualifier is present, the int argument shall
be converted to a wide character as if by calling the
btowc() function and the resulting wide character shall be
written. Otherwise, the wint_t argument shall be converted
to wchar_t, and written.
s 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 mbrtowc() 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.
p 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.
n 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.
C Equivalent to lc.
S Equivalent to ls.
% Output a '%' wide character; no argument shall be
converted. The entire conversion specification shall be
%%.
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() 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.
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() or fclose() on the
same stream, or a call to exit() or abort().
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 output error was encountered, and set errno to indicate the
error.
If n or more wide characters were requested to be written,
swprintf() shall return a negative value, and set errno to
indicate the error.
For the conditions under which fwprintf() and wprintf() fail and
may fail, refer to fputwc(3p).
In addition, all forms of fwprintf() shall fail if:
EILSEQ A wide-character code that does not correspond to a valid
character has been detected.
In addition, fwprintf() and wprintf() may fail if:
ENOMEM Insufficient storage space is available.
The swprintf() shall fail if:
EOVERFLOW
The value of n is greater than {INT_MAX} or the number of
bytes needed to hold the output excluding the terminating
null is greater than {INT_MAX}.
The following sections are informative.
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.
Section 2.5, Standard I/O Streams, btowc(3p), fputwc(3p),
fwscanf(3p), mbrtowc(3p), setlocale(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 7, Locale,
inttypes.h(0p), stdio.h(0p), wchar.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FWPRINTF(3P)
Pages that refer to this page: wchar.h(0p), fwscanf(3p), swprintf(3p), vfwprintf(3p), wprintf(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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