sprintf_s, _sprintf_s_l, swprintf_s, _swprintf_s_l

Write formatted data to a string. These functions are versions of sprintf, _sprintf_l, swprintf, _swprintf_l, __swprintf_l with security enhancements as described in Security features in the CRT.

Syntax

int sprintf_s(
   char *buffer,
   size_t sizeOfBuffer,
   const char *format,
   ...
);
int _sprintf_s_l(
   char *buffer,
   size_t sizeOfBuffer,
   const char *format,
   _locale_t locale,
   ...
);
int swprintf_s(
   wchar_t *buffer,
   size_t sizeOfBuffer,
   const wchar_t *format,
   ...
);
int _swprintf_s_l(
   wchar_t *buffer,
   size_t sizeOfBuffer,
   const wchar_t *format,
   _locale_t locale,
   ...
);
template <size_t size>
int sprintf_s(
   char (&buffer)[size],
   const char *format,
   ...
); // C++ only
template <size_t size>
int swprintf_s(
   wchar_t (&buffer)[size],
   const wchar_t *format,
   ...
); // C++ only

Parameters

buffer
Storage location for output

sizeOfBuffer
Maximum number of characters to store.

format
Format-control string

...
Optional arguments to be formatted

locale
The locale to use.

For more information, see Format specification syntax.

Return value

The number of characters written, or -1 if an error occurred. If buffer or format is a null pointer, sprintf_s and swprintf_s return -1 and set errno to EINVAL.

sprintf_s returns the number of bytes stored in buffer, not counting the terminating null character. swprintf_s returns the number of wide characters stored in buffer, not counting the terminating null wide character.

Remarks

The sprintf_s function formats and stores a series of characters and values in buffer. Each argument (if any) is converted and output according to the corresponding format specification in format. The format consists of ordinary characters and has the same form and function as the format argument for printf. A null character is appended after the last character written. If copying occurs between strings that overlap, the behavior is undefined.

One main difference between sprintf_s and sprintf is that sprintf_s checks the format string for valid formatting characters, whereas sprintf only checks if the format string or buffer are NULL pointers. If either check fails, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, the function returns -1 and sets errno to EINVAL.

The other main difference between sprintf_s and sprintf is that sprintf_s takes a length parameter specifying the size of the output buffer in characters. If the buffer is too small for the formatted text, including the terminating null, then the buffer is set to an empty string by placing a null character at buffer[0], and the invalid parameter handler is invoked. Unlike _snprintf, sprintf_s guarantees that the buffer will be null-terminated unless the buffer size is zero.

swprintf_s is a wide-character version of sprintf_s; the pointer arguments to swprintf_s are wide-character strings. Detection of encoding errors in swprintf_s may differ from the detection in sprintf_s. 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++, use of these functions is simplified by template overloads. The overloads can infer buffer length automatically, which eliminates the need to specify a size argument. And, they can automatically replace older, non-secure functions with newer, more secure counterparts. For more information, see Secure template overloads.

There are versions of sprintf_s that offer more control over what happens if the buffer is too small. For more information, see _snprintf_s, _snprintf_s_l, _snwprintf_s, _snwprintf_s_l.

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 and _MBCS not defined	_MBCS defined	_UNICODE defined
_stprintf_s	sprintf_s	sprintf_s	swprintf_s
_stprintf_s_l	_sprintf_s_l	_sprintf_s_l	_swprintf_s_l

Requirements

Routine	Required header
sprintf_s, _sprintf_s_l	C: <stdio.h> C++: <cstdio> or <stdio.h>
swprintf_s, _swprintf_s_l	C: <stdio.h> or <wchar.h> C++: <cstdio>, <cwchar>, <stdio.h> or <wchar.h>

For more compatibility information, see Compatibility.

Example: Use sprintf_s to format data

// crt_sprintf_s.c
// This program uses sprintf_s to format various
// data and place them in the string named buffer.
//

#include <stdio.h>

int main( void )
{
   char  buffer[200], s[] = "computer", c = 'l';
   int   i = 35, j;
   float fp = 1.7320534f;

   // Format and print various data:
   j  = sprintf_s( buffer, 200,     "   String:    %s\n", s );
   j += sprintf_s( buffer + j, 200 - j, "   Character: %c\n", c );
   j += sprintf_s( buffer + j, 200 - j, "   Integer:   %d\n", i );
   j += sprintf_s( buffer + j, 200 - j, "   Real:      %f\n", fp );

   printf_s( "Output:\n%s\ncharacter count = %d\n", buffer, j );
}

Output:
   String:    computer
   Character: l
   Integer:   35
   Real:      1.732053

character count = 79

Example: Error code handling

// crt_swprintf_s.c
// wide character example
// also demonstrates swprintf_s returning error code
#include <stdio.h>

int main( void )
{
   wchar_t buf[100];
   int len = swprintf_s( buf, 100, L"%s", L"Hello world" );
   printf( "wrote %d characters\n", len );
   len = swprintf_s( buf, 100, L"%s", L"Hello\xffff world" );
   // swprintf_s fails because string contains WEOF (\xffff)
   printf( "wrote %d characters\n", len );
}

wrote 11 characters
wrote -1 characters

See also

Stream I/O
fprintf, _fprintf_l, fwprintf, _fwprintf_l
printf, _printf_l, wprintf, _wprintf_l
sprintf, _sprintf_l, swprintf, _swprintf_l, __swprintf_l
scanf, _scanf_l, wscanf, _wscanf_l
sscanf, _sscanf_l, swscanf, _swscanf_l
vprintf functions