snprintf, _snprintf, _snprintf_l, _snwprintf, _snwprintf_l

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06/05/2017

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The latest version of this topic can be found at snprintf, _snprintf, _snprintf_l, _snwprintf, _snwprintf_l.

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 store.

format
Format-control string.

argument
Optional arguments.

locale
The locale to use.

For more information, see Format Specification Syntax: printf and wprintf Functions.

Return Value

Let len be the length of the formatted data string, not including the terminating null. Both len and count are in bytes for snprintf and _snprintf, wide characters for _snwprintf.

For all functions, if len < count, len characters are stored in buffer, a null-terminator is appended, and len is returned.

The snprintf function truncates the output when len is greater than or equal to count, by placing a null-terminator at buffer[count-1]. The value returned is len, the number of characters that would have been output if count was large enough. The snprintf function returns a negative value if an encoding error occurs.

For all functions other than snprintf, if len = count, len characters are stored in buffer, no null-terminator is appended, and len is returned. If len > count, count characters are stored in buffer, no null-terminator is appended, and a negative value is returned.

If buffer is a null pointer and count is zero, len is returned as the count of characters required to format the output, not including the terminating null. To make a successful call with the same argument and locale parameters, allocate a buffer holding at least len + 1 characters.

If buffer is a null pointer and count is nonzero, or if format is a null pointer, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions return -1 and set errno to EINVAL.

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

Remarks

The snprintf function and the _snprintf family of functions format and store count or fewer characters in buffer. The snprintf function always stores a terminating null character, truncating the output if necessary. 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.

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.

Beginning with the UCRT in Visual Studio 2015 and Windows 10, snprintf is no longer identical to _snprintf. The snprintf function behavior is now C99 standard compliant.

_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 that 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 their 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 nul! */  
   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 nul because _snprintf doesn't necessarily (if the string   
       * fits without the terminal nul, 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

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