vsprintf_s, _vsprintf_s_l, vswprintf_s, _vswprintf_s_l

Write formatted output using a pointer to a list of arguments. These functions are versions of vsprintf, _vsprintf_l, vswprintf, _vswprintf_l, __vswprintf_l with security enhancements as described in Security Features in the CRT.

Syntax

int vsprintf_s(
   char *buffer,
   size_t numberOfElements,
   const char *format,
   va_list argptr
);
int _vsprintf_s_l(
   char *buffer,
   size_t numberOfElements,
   const char *format,
   locale_t locale,
   va_list argptr
);
int vswprintf_s(
   wchar_t *buffer,
   size_t numberOfElements,
   const wchar_t *format,
   va_list argptr
);
int _vswprintf_s_l(
   wchar_t *buffer,
   size_t numberOfElements,
   const wchar_t *format,
   locale_t locale,
   va_list argptr
);
template <size_t size>
int vsprintf_s(
   char (&buffer)[size],
   const char *format,
   va_list argptr
); // C++ only
template <size_t size>
int vswprintf_s(
   wchar_t (&buffer)[size],
   const wchar_t *format,
   va_list argptr
); // C++ only

Parameters

buffer
Storage location for output.

numberOfElements
Size of buffer in characters.

format
Format specification.

argptr
Pointer to list of arguments.

locale
The locale to use.

Return Value

vsprintf_s and vswprintf_s return the number of characters written, not including the terminating null character, or a negative value if an output error occurs. If buffer or format is a null pointer, if numberOfElements is zero, or if the format string contains invalid formatting characters, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, the functions return -1 and set errno to EINVAL.

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

Remarks

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

vswprintf_s conforms to the ISO C Standard for vswprintf, which requires the second parameter, count, of type size_t.

These functions differ from the non-secure versions only in that the secure versions support positional parameters. For more information, see printf_p Positional Parameters.

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.

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 non-secure functions with their secure counterparts. For more information, see Secure Template Overloads.

Important

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`.

Generic-Text Routine Mappings

TCHAR.H routine _UNICODE & _MBCS not defined _MBCS defined _UNICODE defined
_vstprintf_s vsprintf_s vsprintf_s vswprintf_s
_vstprintf_s_l _vsprintf_s_l _vsprintf_s_l _vswprintf_s_l

Requirements

Routine Required header Optional headers
vsprintf_s, _vsprintf_s_l <stdio.h> and <stdarg.h> <varargs.h>*
vswprintf_s, _vswprintf_s_l <stdio.h> or <wchar.h>, and <stdarg.h> <varargs.h>*

* Required for UNIX V compatibility.

For additional compatibility information, see Compatibility.

Example

// crt_vsprintf_s.c
// Compile with: cl /W4 crt_vsprintf_s.c
// This program uses vsprintf_s to write to a buffer.
// The size of the buffer is determined by _vscprintf.

#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>

void test( char const * const format, ... )
{
   va_list args;
   int len;
   char * buffer;

   va_start( args, format );
   len = _vscprintf( format, args ) // _vscprintf doesn't count
                               + 1; // terminating '\0'
   buffer = (char *) malloc( len * sizeof(char) );
   if ( NULL != buffer )
   {
      vsprintf_s( buffer, len, format, args );
      puts( buffer );
      free( buffer );
   }
   va_end( args );
}

int main( void )
{
   test( "%d %c %d", 123, '<', 456 );
   test( "%s", "This is a string" );
}
123 < 456
This is a string

See also

Stream I/O
vprintf Functions
Format Specification Syntax: printf and wprintf 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