vwprintf, vfwprintf, vswprintf, vwprintf_s, vfwprintf_s, vswprintf_s, vsnwprintf_s

Defined in header `<wchar.h>`
int vwprintf( const wchar_t* format, va_list vlist );	(1)	(since C95) (until C99)
int vwprintf( const wchar_t* restrict format, va_list vlist );	(1)		(since C99)
int vfwprintf( FILE* stream, const wchar_t* format, va_list vlist );	(2)	(since C95) (until C99)
int vfwprintf( FILE* restrict stream, const wchar_t* restrict format, va_list vlist );	(2)		(since C99)
int vswprintf( wchar_t* buffer, size_t bufsz, const wchar_t* format, va_list vlist );	(3)	(since C95) (until C99)
int vswprintf( wchar_t* restrict buffer, size_t bufsz, const wchar_t* restrict format, va_list vlist );	(3)		(since C99)
int vwprintf_s( const wchar_t* restrict format, va_list vlist);	(4)	(since C11)
int vfwprintf_s( FILE* restrict stream, const wchar_t* restrict format, va_list vlist);	(5)	(since C11)
int vswprintf_s( wchar_t* restrict buffer, rsize_t bufsz, const wchar_t* restrict format, va_list vlist);	(6)	(since C11)
int vsnwprintf_s( wchar_t* restrict buffer, rsize_t bufsz, const wchar_t* restrict format, va_list vlist);	(7)	(since C11)

Loads the data from locations, defined by vlist, converts them to wide string equivalents and writes the results to a variety of sinks.

1) Writes the results to stdout.

2) Writes the results to a file stream stream.

3) Writes the results to a wide string buffer. At most bufsz - 1 wide characters are written followed by null wide character. The resulting wide character string will be terminated with a null wide character, unless bufsz is zero.

4-6) Same as (1-3), except that the following errors are detected at runtime and call the currently installed constraint handler function:

the conversion specifier %n is present in format
any of the arguments corresponding to %s is a null pointer
format or buffer is a null pointer
bufsz is zero or greater than RSIZE_MAX / sizeof(wchar_t)
encoding errors occur in any of string and character conversion specifiers
(for vswprintf_s only), the string to be stored in buffer (including the trailing wide null) would be exceed bufsz.

7) Same as (6), except it will truncate the result to fit within the array pointed to by buffer.

As with all bounds-checked functions, vwprintf_s, vfwprintf_s, vswprintf_s, and vsnwprintf_s are only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdio.h>.

Parameters

stream	-	output wide stream to write to
buffer	-	pointer to a wide string to write to
bufsz	-	maximum number of wide characters to write
format	-	pointer to a null-terminated wide string specifying how to interpret the data
vlist	-	variable argument list containing the data to print.

The format string consists of ordinary wide characters (except %), which are copied unchanged into the output stream, and conversion specifications. Each conversion specification has the following format:

introductory % character.

(optional) flags that modify the behavior of the conversion:

-: the result of the conversion is left-justified within the field (by default it is right-justified).
0: applicable to floating-point and integer conversions only. Leading zeros are used to pad the field instead of space characters. This flag is ignored if - flag is present. For integer conversions, this flag is ignored if the precision is explicitly specified. For floating-point conversions, this flag is ignored if the value is NaN or infinity. Other conversions using this flag have undefined behavior.
+: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a plus character. This flag is ignored by other conversions.
space: applicable to floating-point and signed integer conversions only. Non-negative numbers are prefixed with a space character. This flag is ignored if + flag is present. This flag is ignored by other conversions.
#: applicable to floating-point and non-decimal integer conversions only. The alternative form of the conversion is performed. See below for the exact semantics. Other conversions using this flag have undefined behavior.

(optional) integer value or *: specifies minimum field width. The result is padded with space characters (by default), if required, on the left when right-justified, or on the right if left-justified. In the case when * is used, the width is specified by an additional argument of type int, which appears before the argument to be converted and the argument supplying precision if one is supplied. If the value of the argument is negative, then the - flag is implied and the absolute value used for minimum field width.

(optional) . followed by non-negative integer number or *: specifies the precision of the conversion. In the case when * is used, the precision is specified by an additional argument of type int, which appears before the argument to be converted, but after the argument supplying minimum field width if one is supplied. If the value of this argument is negative, it is ignored. If neither a number nor * is used, the precision is taken as zero. See below for the exact semantics.

(optional) length modifier that specifies the size of the argument (in combination with the conversion format specifier, it specifies the type of the corresponding argument).

conversion format specifier.

If a conversion specification is invalid, the behavior is undefined.

Non-numeric

%: Writes % character. Has no argument. No modifiers supported, full conversion specification must be %%.
p: Writes a pointer. Argument is a void*, char*, signed char*, or unsigned char*(since C23). Output format is implementation defined.
c: Writes a character. Argument type depends on the length modifier:

(none): Argument is an int, which is converted to wide character via btowc and copied to the output stream.
l: Argument is a wint_t, which is converted to a wchar_t and copied to the output stream.

s: Write a string. Argument is a pointer to the initial element of an array of characters. Argument type depends on the length modifier:

(none): Argument is a char*, signed char*, unsigned char*. String is converted to a wide string via mbrtowc (with zero-initialized conversion state), and resulting characters copied to the output stream. For the precision modifier and %n specifier, the number of characters refers to the wide string, not the converted-from narrow string.
l: Argument is a wchar_t*. Characters are copied to the output stream.

The precision specifies the maximum number of characters to write. Characters are copied until the maximum is reached or a null terminator is found. The null terminator does not contribute to the number of characters reported by %n.

Even though %c expects int argument, it is safe to pass a char because of the integer promotion that takes place when a variadic function is called.

Integer

Let SInt and UInt be respective types determined by the length specifier:

hh: signed char and unsigned char

(since C99)

h: short and unsigned short
(none): int and unsigned
l: long and unsigned long

ll: long long and unsigned long long
j: intmax_t and uintmax_t
z: signed version of size_t and size_t
t: ptrdiff_t and unsigned version of ptrdiff_t

(since C99)

wN: Any signed integer type with bit-width N and any unsigned integer type with bit-width N
wfN: int_fastN_t and uint_fastN_t

(since C23)

Then:

d/i: Writes a signed integer in decimal representation. Argument is an SInt.
u: Writes an unsigned integer in decimal representation. Argument is an UInt.
x/X: Writes an unsigned integer in hexadecimal representation. Argument is an UInt. The alternative form prefixes 0x if the argument is non-zero. If the format specifier is uppercase, the corresponding output will also be uppercase.

b: Writes an unsigned integer in binary representation. Argument is an UInt. The alternative form prefixes 0b if the argument is non-zero.

(since C23)

o: Writes an unsigned integer in octal representation. Argument is an UInt. The alternative form increases the precision as much as is needed for the first digit written to be zero.
n: Returns the number of characters written so far by this call to the function, including minimum field width padding. Argument is an SInt* to write the result to. Only supported modifier is length modifier.

The precision specifies the minimum number of digits to write, leading zeros will be written if necessary. If unspecified, a precision of 1 is used. If precision is 0 and the argument is 0, no digits will be written (the plus or space character is still written if the + or space flag is specified).

The correct conversion specifications for the fixed-width character types (int8_t, etc) are defined in the header <inttypes.h> (although PRIdMAX, PRIuMAX, etc is synonymous with %jd, %ju, etc).

The memory-writing conversion specifier %n is a common target of security exploits where format strings depend on user input and is not supported by the bounds-checked printf_s family of functions.

There is a sequence point after the action of each conversion specifier; this permits storing multiple %n results in the same variable or, as an edge case, printing a string modified by an earlier %n within the same call.

Floating-point

The argument type is determined by the length specifier:

(none): double

l: double

(since C99)

L: long double

H: _Decimal32
D: _Decimal64
DD: _Decimal128

(since C23)

Then:

If the argument is infinity, then inf or infinity is written. Which one is used is implementation defined.
If the argument is not-a-number, then nan or nan(char_sequence) is written. Which one is used is implementation defined.

Otherwise:

f/F: Writes a floating-point in decimal notation [-]ddd.ddd. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
e/E: Writes a floating-point in decimal exponent notation [-]d.ddde±dd. If the exponent is less than 10, it is written with a leading zero. The precision specifies the number of decimal places, trailing zeros will be written if necessary. If unspecified, a precision of 6 is used.
g/G: Writes a floating-point in decimal or decimal exponent notation, depending on the argument and precision.

Let P = max(0, precision - 1), or 5 if precision is unspecified.

Let EXP be the exponent written if %e were the specification.

If -4 ≤ EXP ≤ P, then the argument is written as if the f/F format specifier was used with precision P - EXP.
Otherwise, the argument is written as if the e/E format specifier was used with precision P.

a/A: Writes a floating-point in hexadecimal exponent notation [-]0xh.hhhp±d. The precision specifies the number of digits following the decimal point, trailing zeros will be written if necessary.

If the argument type is double or long double:

If the precision is unspecified:

If FLT_RADIX is a power of 2, the exact value is printed.
Otherwise, enough digits are printed to distinguish the number from any other value of the argument type, except that trailing zeros may be omitted.

Otherwise, for decimal type arguments:

Let q_min,q_max be:

-101, 90 if argument type is _Decimal32
-398, 369 if argument type is _Decimal64
-6176, 6111 if argument type is _Decimal128

Let the argument be expressed with integers s,c,q as s × c × 10^q where:

c ≥ 0,
s is -1 or 1,
q_min ≤ q ≤ q_max.

Let n be the number of significant digits in c.

Let P be the precision.

If P is specified and 0 < P < n, then the argument is rounded to P decimal digits, with unbounded exponent range, such that c is a P-digit integer
If 0 ≤ -q ≤ n + 5, then the argument is written with the style of the f/F format specifier used with precision -q.
Otherwise, the argument is written with the style of the e/E format specifier used with precision n - 1, except that:

If c = 0, then q is written for the exponent.
No leading zero is written for the exponent.

(since C23)

There is exactly one digit before the decimal place, but it is unspecified how this digit is chosen. For example, 0x1.fp+6 and 0xf.6p+3 are both valid outputs for an argument of 123.0 with precision 1.

(since C99)

The alternative form writes a decimal point even if precision is 0. If the format specifier is uppercase, the corresponding output will also be uppercase.

Return value

1,4) The number of wide characters written if successful or negative value if an error occurred.

3) The number of wide characters written if successful or negative value if an error occurred. If the resulting string gets truncated due to bufsz limit, function returns the total number of characters (not including the terminating null wide character) which would have been written, if the limit were not imposed.

2,5) number of wide characters transmitted to the output stream or negative value if an output error, a runtime constraints violation error, or an encoding error occurred.

6) number of wide characters written to buffer, not counting the null wide character (which is always written as long as buffer is not a null pointer and bufsz is not zero and not greater than RSIZE_MAX/sizeof(wchar_t)), or zero on runtime constraint violations, and negative value on encoding errors.

7) number of wide characters not including the terminating null character (which is always written as long as buffer is not a null pointer and bufsz is not zero and not greater than RSIZE_MAX/sizeof(wchar_t)), which would have been written to buffer if bufsz was ignored, or a negative value if a runtime constraints violation or an encoding error occurred.

Notes

All these functions invoke va_arg at least once, the value of arg is indeterminate after the return. These functions to not invoke va_end, and it must be done by the caller.

While narrow strings provide vsnprintf, which makes it possible to determine the required output buffer size, there is no equivalent for wide strings (until C11's vsnwprintf_s), and in order to determine the buffer size, the program may need to call vswprintf, check the result value, and reallocate a larger buffer, trying again until successful.

vsnwprintf_s, unlike vswprintf_s, will truncate the result to fit within the array pointed to by buffer, even though truncation is treated as an error by most bounds-checked functions.

Example

Run this code

#include <locale.h>
#include <stdarg.h>
#include <stddef.h>
#include <stdio.h>
#include <time.h>
#include <wchar.h>

void debug_wlog(const wchar_t* fmt, ...)
{
    struct timespec ts;
    timespec_get(&ts, TIME_UTC);
    char time_buf[100];
    size_t rc = strftime(time_buf, sizeof time_buf, "%D %T", gmtime(&ts.tv_sec));
    snprintf(time_buf + rc, sizeof time_buf - rc, ".%06ld UTC", ts.tv_nsec / 1000);

    va_list args;
    va_start(args, fmt);
    wchar_t buf[1024];
    int rc2 = vswprintf(buf, sizeof buf / sizeof *buf, fmt, args);
    va_end(args);

    if (rc2 > 0)
       wprintf(L"%s [debug]: %ls\n", time_buf, buf);
    else
       wprintf(L"%s [debug]: (string too long)\n", time_buf);
}

int main(void)
{
    setlocale(LC_ALL, "");
    debug_wlog(L"Logging, %d, %d, %d", 1, 2, 3);
}

Possible output:

02/20/15 22:12:38.476575 UTC [debug]: Logging, 1, 2, 3

References

C23 standard (ISO/IEC 9899:2024):

7.29.2.5 The vfwprintf function (p: TBD)

7.29.2.7 The vswprintf function (p: TBD)

7.29.2.9 The vwprintf function (p: TBD)

K.3.9.1.6 The vfwprintf_s function (p: TBD)

K.3.9.1.8 The vsnwprintf_s function (p: TBD)

K.3.9.1.9 The vswprintf_s function (p: TBD)

K.3.9.1.11 The vwprintf_s function (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.29.2.5 The vfwprintf function (p: TBD)

7.29.2.7 The vswprintf function (p: TBD)

7.29.2.9 The vwprintf function (p: TBD)

K.3.9.1.6 The vfwprintf_s function (p: TBD)

K.3.9.1.8 The vsnwprintf_s function (p: TBD)

K.3.9.1.9 The vswprintf_s function (p: TBD)

K.3.9.1.11 The vwprintf_s function (p: TBD)

C11 standard (ISO/IEC 9899:2011):

7.29.2.5 The vfwprintf function (p: 417-418)

7.29.2.7 The vswprintf function (p: 419)

7.29.2.9 The vwprintf function (p: 420)

K.3.9.1.6 The vfwprintf_s function (p: 632)

K.3.9.1.8 The vsnwprintf_s function (p: 633-634)

K.3.9.1.9 The vswprintf_s function (p: 634-635)

K.3.9.1.11 The vwprintf_s function (p: 636)

C99 standard (ISO/IEC 9899:1999):

7.24.2.5 The vfwprintf function (p: 363)

7.24.2.7 The vswprintf function (p: 364)

7.24.2.9 The vwprintf function (p: 365)

Compiler support
Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index

vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (C99)(C11)(C11)(C11)(C11)	prints formatted output to stdout, a file stream or a buffer using variable argument list (function) [edit]
wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)	prints formatted wide character output to stdout, a file stream or a buffer (function) [edit]
C++ documentation for vwprintf, vfwprintf, vswprintf

cppreference.com

Search

Namespaces

Variants

Views

Actions

vwprintf, vfwprintf, vswprintf, vwprintf_s, vfwprintf_s, vswprintf_s, vsnwprintf_s

Contents

Parameters

Non-numeric

Integer

Floating-point

Return value

Notes

Example

References

See also

Navigation

Tools