vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwprintf_s_l

Article
01/24/2013

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

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, as the character count.
count
Maximum number of characters to write (not including the terminating null), or _TRUNCATE.
format
Format specification.
argptr
Pointer to list of arguments.
locale
The locale to use.

For more information, see Format Specifications.

Return Value

vsnprintf_s,_vsnprintf_s and _vsnwprintf_s return the number of characters written, not including the terminating null, or a negative value if an output error occurs. vsnprintf_s is identical to _vsnprintf_s. vsnprintf_s is included for compliance to the ANSI standard. _vnsprintf is retained for backward compatibility.

If the storage required to store the data and a terminating null exceeds sizeOfBuffer, the invalid parameter handler is invoked, as described in Parameter Validation, unless count is _TRUNCATE, in which case as much of the string as will fit in buffer is written and -1 returned. If execution continues after the invalid parameter handler, these functions set buffer to an empty string, set errno to ERANGE, and return -1.

If buffer or format is a NULL pointer, or if count is less than or equal to zero, the invalid parameter handler is invoked. If execution is allowed to continue, these functions set errno to EINVAL and return -1.

Error Conditions

Condition	Return	errno
buffer is NULL	-1	EINVAL
format is NULL	-1	EINVAL
count <= 0	-1	EINVAL
sizeOfBuffer too small (and count != _TRUNCATE)	-1 (and buffer set to an empty string)	ERANGE

Remarks

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.

If count is _TRUNCATE, then these functions write as much of the string as will fit in buffer while leaving room for a terminating null. If the entire string (with terminating null) fits in buffer, then these functions return the number of characters written (not including the terminating null); otherwise, these functions return -1 to indicate that truncation occurred.

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.

Security Note
Ensure that format is not a user-defined string. For more information, see Avoiding Buffer Overruns.

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.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _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

.NET Framework Equivalent

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

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 additional compatibility information, see Compatibility in the Introduction.

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);
}

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!

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