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fprintf_s, _fprintf_s_l, fwprintf_s, _fwprintf_s_l

Print formatted data to a stream. These are versions of fprintf, _fprintf_l, fwprintf, _fwprintf_l with security enhancements as described in Security Features in the CRT.

int fprintf_s( 
   FILE *stream,
   const char *format [,
   argument ]...
int _fprintf_s_l( 
   FILE *stream,
   const char *format,
   locale_t locale [,
   argument ]...
int fwprintf_s( 
   FILE *stream,
   const wchar_t *format [,
   argument ]...
int _fwprintf_s_l( 
   FILE *stream,
   const wchar_t *format,
   locale_t locale [,
   argument ]…


Pointer to FILE structure.


Format-control string.


Optional arguments.


The locale to use.

fprintf_s returns the number of bytes written. fwprintf_s returns the number of wide characters written. Each of these functions returns a negative value instead when an output error occurs.

fprintf_s formats and prints a series of characters and values to the output stream. Each function argument (if any) is converted and output according to the corresponding format specification in format. For fprintf_s, the format argument has the same syntax and use that it has in printf_s.

fwprintf_s is a wide-character version of fprintf_s; in fwprintf_s, format is a wide-character string. These functions behave identically if the stream is opened in ANSI mode. fprintf_s doesn't currently support output into a UNICODE stream.

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

Security note Security Note

Ensure that format is not a user-defined string.

Like the non-secure versions (see fprintf, _fprintf_l, fwprintf, _fwprintf_l), these functions validate their parameters and invoke the invalid parameter handler, as described in Parameter Validation, if either stream or format is a null pointer. These functions differ from the non-secure versions in that the format string itself is also validated. If there are any unknown or badly formed formatting specifiers, these functions generate the invalid parameter exception. In all cases, If execution is allowed to continue, the functions return -1 and set errno to EINVAL. See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these, and other, error codes.

Generic-Text Routine Mappings

TCHAR.H routine

_UNICODE & _MBCS not defined

_MBCS defined

_UNICODE defined









For more information, see Format Specifications.


Required header

fprintf_s , _fprintf_s_l


fwprintf_s , _fwprintf_s_l

<stdio.h> or <wchar.h>

For additional compatibility information, see Compatibility in the Introduction.

// crt_fprintf_s.c
// This program uses fprintf_s to format various
// data and print it to the file named FPRINTF_S.OUT. It
// then displays FPRINTF_S.OUT on the screen using the system
// function to invoke the operating-system TYPE command.
#include <stdio.h>
#include <process.h>

FILE *stream;

int main( void )
   int    i = 10;
   double fp = 1.5;
   char   s[] = "this is a string";
   char   c = '\n';

   fopen_s( &stream, "fprintf_s.out", "w" );
   fprintf_s( stream, "%s%c", s, c );
   fprintf_s( stream, "%d\n", i );
   fprintf_s( stream, "%f\n", fp );
   fclose( stream );
   system( "type fprintf_s.out" );
this is a string