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vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwprintf_s_l

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Write formatted output using a pointer to a list of arguments. These functions are versions of vsnprintf, _vsnprintf, _vsnprintf_l, _vsnwprintf, _vsnwprintf_l with security enhancements as described in Security features in the CRT.

Syntax

int vsnprintf_s(
   char *buffer,
   size_t sizeOfBuffer,
   size_t count,
   const char *format,
   va_list argptr
);

int _vsnprintf_s(
   char *buffer,
   size_t sizeOfBuffer,
   size_t count,
   const char *format,
   va_list argptr
);

int _vsnprintf_s_l(
   char *buffer,
   size_t sizeOfBuffer,
   size_t count,
   const char *format,
   _locale_t locale,
   va_list argptr
);

int _vsnwprintf_s(
   wchar_t *buffer,
   size_t sizeOfBuffer,
   size_t count,
   const wchar_t *format,
   va_list argptr
);

int _vsnwprintf_s_l(
   wchar_t *buffer,
   size_t sizeOfBuffer,
   size_t count,
   const wchar_t *format,
   _locale_t locale,
   va_list argptr
);

template <size_t size>
int _vsnprintf_s(
   char (&buffer)[size],
   size_t count,
   const char *format,
   va_list argptr
); // C++ only

template <size_t size>
int _vsnwprintf_s(
   wchar_t (&buffer)[size],
   size_t count,
   const wchar_t *format,
   va_list argptr
); // C++ only

Parameters

buffer
Storage location for output.

sizeOfBuffer
The size of the buffer for output. Size in bytes for the functions that take char, and words for those that take wchar_t.

count
Maximum number of characters to write not including the terminating NULL. For the functions that take wchar_t, it's the number of wide characters to write. Or _TRUNCATE.

format
Format specification.

argptr
Pointer to list of arguments.

locale
The locale to use when formatting the output.

For more information, see Format specifications.

Return value

The number of characters written, not including the terminating NULL, or a negative value if an output error occurs.

See Behavior summary for details.

Remarks

vsnprintf_s is identical to _vsnprintf_s and is included for conformance to the ANSI standard. _vnsprintf is retained for backward compatibility.

Each of these functions takes a pointer to an argument list, then formats and writes up to count characters of the given data to the memory pointed to by buffer and appends a terminating null.

The versions of these functions with the _l suffix are identical except that they use the locale parameter passed in instead of the current thread locale.

Behavior summary

For the following table:

  • Let len be the size of the formatted data. If the function takes a char buffer, the size is in bytes. If the function takes a wchar_t buffer, the size specifies the number of 16-bit words.
  • Characters refer to char characters for functions that take a char buffer, and to wchar_t characters for functions that take a wchar_t buffer.
  • For more information about the invalid parameter handler, see Parameter Validation.
Condition Behavior Return value errno Invokes invalid parameter handler
Success Writes the characters into the buffer using the specified format string The number of characters written N/A No
Encoding error during formatting If processing string specifier s, S, or Z, format specification processing stops. -1 EILSEQ (42) No
Encoding error during formatting If processing character specifier c or C, the invalid character is skipped. The number of characters written isn't incremented for the skipped character, nor is any data written for it. Processing the format specification continues after skipping the specifier with the encoding error. The number of characters written, not including the terminating NULL. EILSEQ (42) No
buffer == NULL and sizeOfBuffer == 0 and count == 0 No data is written. 0 N/A No
buffer == NULL and either sizeOfBuffer != 0 or count != 0 If execution continues after invalid parameter handler executes, sets errno and returns a negative value. -1 EINVAL (22) Yes
buffer != NULL and sizeOfBuffer == 0 No data is written. If execution continues after invalid parameter handler executes, sets errno and returns a negative value. -1 EINVAL (22) Yes
count == 0 Doesn't write any data and returns the number of characters that would have been written, not including the terminating NULL. The number of characters that would have been written not including the terminating NULL. N/A No
count < 0 Unsafe: the value is treated as unsigned, likely creating a large value that results in overwriting the memory that follows the buffer. The number of characters written, not including the terminating NULL. N/A No
count < sizeOfBuffer and len <= count All of the data is written and a terminating NULL is appended. The number of characters written. N/A No
count < sizeOfBuffer and len > count The first count characters are written. -1 N/A No
count >= sizeOfBuffer and len < sizeOfBuffer All of the data is written with a terminating NULL. The number of characters written, not including the terminating NULL. N/A No
count >= sizeOfBuffer and len >= sizeOfBuffer and count != _TRUNCATE If execution continues after invalid parameter handler executes, sets errno, sets buffer[0] == NULL, and returns a negative value. -1 ERANGE (34) Yes
count == _TRUNCATE and len >= sizeOfBuffer Writes as much of the string as fits in buffer, including the terminating NULL. -1 N/A No
count == _TRUNCATE and len < sizeOfBuffer Writes the entire string into buffer with terminating NULL. Number of characters written. N/A No
format == NULL No data is written. If execution continues after invalid parameter handler executes, sets errno and returns a negative value. -1 EINVAL (22) Yes

For information about these and other error codes, see _doserrno, errno, _sys_errlist, and _sys_nerr.

Important

Ensure that format is not a user-defined string. For more information, see Avoiding buffer overruns. Starting in Windows 10 version 2004 (build 19041), the printf family of functions prints exactly representable floating point numbers according to the IEEE 754 rules for rounding. In previous versions of Windows, exactly representable floating point numbers ending in '5' would always round up. IEEE 754 states that they must round to the closest even digit (also known as "Banker's Rounding"). For example, both printf("%1.0f", 1.5) and printf("%1.0f", 2.5) should round to 2. Previously, 1.5 would round to 2 and 2.5 would round to 3. This change only affects exactly representable numbers. For example, 2.35 (which, when represented in memory, is closer to 2.35000000000000008) continues to round up to 2.4. Rounding done by these functions now also respects the floating point rounding mode set by fesetround. Previously, rounding always chose FE_TONEAREST behavior. This change only affects programs built using Visual Studio 2019 version 16.2 and later. To use the legacy floating point rounding behavior, link with 'legacy_stdio_float_rounding.obj`.

Note

To ensure that there is room for the terminating null, be sure that count is strictly less than the buffer length, or use _TRUNCATE.

In C++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with their newer, secure counterparts. For more information, see Secure template overloads.

Tip

If you get an undefined external _vsnprintf_s error and are using the Universal C Runtime, add legacy_stdio_definitions.lib to the set of libraries to link. The Universal C Runtime doesn't export this function directly and is instead defined inline in <stdio.h>. For more information, see Overview of potential upgrade issues and Visual Studio 2015 conformance changes.

Generic-text routine mappings

TCHAR.H routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_vsntprintf_s _vsnprintf_s _vsnprintf_s _vsnwprintf_s
_vsntprintf_s_l _vsnprintf_s_l _vsnprintf_s_l _vsnwprintf_s_l

Requirements

Routine Required header Optional headers
vsnprintf_s <stdio.h> and <stdarg.h> <varargs.h>*
_vsnprintf_s, _vsnprintf_s_l <stdio.h> and <stdarg.h> <varargs.h>*
_vsnwprintf_s, _vsnwprintf_s_l <stdio.h> or <wchar.h>, and <stdarg.h> <varargs.h>*

* Required for UNIX V compatibility.

For more compatibility information, see Compatibility.

Example

// crt_vsnprintf_s.cpp
#include <stdio.h>
#include <wtypes.h>

void FormatOutput(LPCSTR formatstring, ...)
{
   int nSize = 0;
   char buff[10];
   memset(buff, 0, sizeof(buff));
   va_list args;
   va_start(args, formatstring);
   nSize = vsnprintf_s( buff, _countof(buff), _TRUNCATE, formatstring, args);
   printf("nSize: %d, buff: %s\n", nSize, buff);
   va_end(args);
}

int main() {
   FormatOutput("%s %s", "Hi", "there");
   FormatOutput("%s %s", "Hi", "there!");
   FormatOutput("%s %s", "Hi", "there!!");
}

nSize: 8, buff: Hi there
nSize: 9, buff: Hi there!
nSize: -1, buff: Hi there!

See also

Stream I/O
vprintf functions
fprintf, _fprintf_l, fwprintf, _fwprintf_l
printf, _printf_l, wprintf, _wprintf_l
sprintf, _sprintf_l, swprintf, _swprintf_l, __swprintf_l
va_arg, va_copy, va_end, va_start