`snprintf`, `_snprintf`, `_snprintf_l`, `_snwprintf`, `_snwprintf_l`

Artikel
07/03/2023

Writes formatted data to a string. More secure versions of these functions are available; see _snprintf_s, _snprintf_s_l, _snwprintf_s, _snwprintf_s_l.

Syntax

int snprintf(
   char *buffer,
   size_t count,
   const char *format [,
   argument] ...
);

int _snprintf(
   char *buffer,
   size_t count,
   const char *format [,
   argument] ...
);

int _snprintf_l(
   char *buffer,
   size_t count,
   const char *format,
   _locale_t locale [,
   argument] ...
);

int _snwprintf(
   wchar_t *buffer,
   size_t count,
   const wchar_t *format [,
   argument] ...
);

int _snwprintf_l(
   wchar_t *buffer,
   size_t count,
   const wchar_t *format,
   _locale_t locale [,
   argument] ...
);

template <size_t size>
int _snprintf(
   char (&buffer)[size],
   size_t count,
   const char *format [,
   argument] ...
); // C++ only

template <size_t size>
int _snprintf_l(
   char (&buffer)[size],
   size_t count,
   const char *format,
   _locale_t locale [,
   argument] ...
); // C++ only

template <size_t size>
int _snwprintf(
   wchar_t (&buffer)[size],
   size_t count,
   const wchar_t *format [,
   argument] ...
); // C++ only

template <size_t size>
int _snwprintf_l(
   wchar_t (&buffer)[size],
   size_t count,
   const wchar_t *format,
   _locale_t locale [,
   argument] ...
); // C++ only

Parameters

buffer
Storage location for the output.

count
Maximum number of characters to write. For the functions that take wchar_t, it's the maximum number of wide characters to write.

format
Format-control string.

argument
Optional arguments.

locale
The locale to use to format the output.

For more information, see Format specification syntax: printf and wprintf functions.

Return value

The number of characters that which would have been written to the buffer if count was ignored. The count doesn't include the terminating NULL character.

Let len be the length of the formatted data string, not including the terminating NULL.
For all functions, if len < count, then len characters are stored in buffer, a null-terminator is appended, and the number of characters written, not including the terminating NULL, is returned.

The wide character versions of these functions return the number of wide characters written, not including the terminating NULL.

See Behavior summary for details.

Remarks

Beginning with the UCRT in Visual Studio 2015 and Windows 10, snprintf is no longer identical to _snprintf. The snprintf behavior is now C99 standard conformant. The difference is that if you run out of buffer, snprintf null-terminates the end of the buffer and returns the number of characters that would have been required whereas _snprintf doesn't null-terminate the buffer and returns -1. Also, snprintf() includes one more character in the output because it doesn't null-terminate the buffer.

snprintf and the _snprintf family of functions format and store count or fewer characters in buffer.
snprintf always stores a terminating NULL character, truncating the output if necessary.
If snprintf returns a value > count - 1, the output has been truncated.
The _snprintf family of functions only appends a terminating NULL character if the formatted string length is strictly less than count characters.
Each argument (if any) is converted and is 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. If copying occurs between strings that overlap, the behavior is undefined.

Behavior summary

For the following table:

Let sizeOfBuffer be the size of buffer. 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.
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, a `NULL` is placed at the beginning of the buffer.	-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 `count != 0`	If execution continues after invalid parameter handler executes, sets `errno` and returns a negative value.	-1	`EINVAL` (22)	Yes
`count == 0`	The number of characters that would have been written, not including the terminating `NULL`. You can use this result to allocate sufficient buffer space for the string and a terminating `NULL`, and then call the function again to fill the buffer.	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	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, not including the terminating `NULL`.	N/A	No
`count < sizeOfBuffer` and `len > count`	The first `count-1` characters are written followed by a null-terminator.	The number of characters that would have been written had `count` matched the number of characters to output, not including the null-terminator.	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`	Unsafe: Overwrites the memory that follows the buffer.	The number of characters written, not including the terminating `NULL`.	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. Because the _snprintf functions do not guarantee null termination—in particular, when the return value is count—make sure that they are followed by code that adds the null terminator. 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.

_snwprintf is a wide-character version of _snprintf; the pointer arguments to _snwprintf are wide-character strings. Detection of encoding errors in _snwprintf might differ from the detection in _snprintf. _snwprintf, just like swprintf, writes output to a string instead of a destination of type FILE.

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

In C++, these functions have template overloads that invoke the newer, more secure counterparts. For more information, see Secure template overloads.

Generic-text routine mappings

`Tchar.h` routine	`_UNICODE` and `_MBCS` not defined	`_MBCS` defined	`_UNICODE` defined
`_sntprintf`	`_snprintf`	`_snprintf`	`_snwprintf`
`_sntprintf_l`	`_snprintf_l`	`_snprintf_l`	`_snwprintf_l`

Requirements

Routine	Required header
`snprintf`, `_snprintf`, `_snprintf_l`	`<stdio.h>`
`_snwprintf`, `_snwprintf_l`	`<stdio.h>` or `<wchar.h>`

For more compatibility information, see Compatibility.

Example

// crt_snprintf.c
// compile with: /W3
#include <stdio.h>
#include <stdlib.h>

#if !defined(__cplusplus)
typedef int bool;
const bool true = 1;
const bool false = 0;
#endif

#define FAIL 0 // change to 1 and see what happens

int main(void)
{
   char buffer[200];
   const static char s[] = "computer"
#if FAIL
"computercomputercomputercomputercomputercomputercomputercomputer"
"computercomputercomputercomputercomputercomputercomputercomputer"
"computercomputercomputercomputercomputercomputercomputercomputer"
"computercomputercomputercomputercomputercomputercomputercomputer"
#endif
   ;
   const char c = 'l';
   const int i = 35;
#if FAIL
   const double fp = 1e300; // doesn't fit in the buffer
#else
   const double fp = 1.7320534;
#endif
   /* !subtract one to prevent "squeezing out" the terminal null! */
   const int bufferSize = sizeof(buffer)/sizeof(buffer[0]) - 1;
   int bufferUsed = 0;
   int bufferLeft = bufferSize - bufferUsed;
   bool bSuccess = true;
   buffer[0] = 0;

   /* Format and print various data: */

   if (bufferLeft > 0)
   {
      int perElementBufferUsed = _snprintf(&buffer[bufferUsed],
      bufferLeft, "   String: %s\n", s ); // C4996
      // Note: _snprintf is deprecated; consider _snprintf_s instead
      if (bSuccess = (perElementBufferUsed >= 0))
      {
         bufferUsed += perElementBufferUsed;
         bufferLeft -= perElementBufferUsed;
         if (bufferLeft > 0)
         {
            int perElementBufferUsed = _snprintf(&buffer[bufferUsed],
            bufferLeft, "   Character: %c\n", c ); // C4996
            if (bSuccess = (perElementBufferUsed >= 0))
            {
               bufferUsed += perElementBufferUsed;
               bufferLeft -= perElementBufferUsed;
               if (bufferLeft > 0)
               {
                  int perElementBufferUsed = _snprintf(&buffer
                  [bufferUsed], bufferLeft, "   Integer: %d\n", i ); // C4996
                  if (bSuccess = (perElementBufferUsed >= 0))
                  {
                     bufferUsed += perElementBufferUsed;
                     bufferLeft -= perElementBufferUsed;
                     if (bufferLeft > 0)
                     {
                        int perElementBufferUsed = _snprintf(&buffer
                        [bufferUsed], bufferLeft, "   Real: %f\n", fp ); // C4996
                        if (bSuccess = (perElementBufferUsed >= 0))
                        {
                           bufferUsed += perElementBufferUsed;
                        }
                     }
                  }
               }
            }
         }
      }
   }

   if (!bSuccess)
   {
      printf("%s\n", "failure");
   }
   else
   {
      /* !store null because _snprintf doesn't necessarily (if the string
       * fits without the terminal null, but not with it)!
       * bufferUsed might be as large as bufferSize, which normally is
       * like going one element beyond a buffer, but in this case
       * subtracted one from bufferSize, so we're ok.
       */
      buffer[bufferUsed] = 0;
      printf( "Output:\n%s\ncharacter count = %d\n", buffer, bufferUsed );
   }
   return EXIT_SUCCESS;
}

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

character count = 69

Delen via

`snprintf`, `_snprintf`, `_snprintf_l`, `_snwprintf`, `_snwprintf_l`

Syntax

Parameters

Return value

Remarks

Behavior summary

Generic-text routine mappings

Requirements

Example

See also

Feedback

Aanvullende resources

Delen via

snprintf, _snprintf, _snprintf_l, _snwprintf, _snwprintf_l

Syntax

Parameters

Return value

Remarks

Behavior summary

Generic-text routine mappings

Requirements

Example

See also

Feedback

Aanvullende resources

`snprintf`, `_snprintf`, `_snprintf_l`, `_snwprintf`, `_snwprintf_l`