fprintf_s, _fprintf_s_l, fwprintf_s, _fwprintf_s_l, _ftprintf, _ftprintf_l, _ftprintf_s, _ftprintf_s_l

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

For _ftprintf_s and _ftprintf_s_l, see Generic-text function mappings.

Syntax

int fprintf_s(
   FILE *stream,
   const char *format [,
   argument_list ]
);
int _fprintf_s_l(
   FILE *stream,
   const char *format,
   _locale_t locale [,
   argument_list ]
);
int fwprintf_s(
   FILE *stream,
   const wchar_t *format [,
   argument_list ]
);
int _fwprintf_s_l(
   FILE *stream,
   const wchar_t *format,
   _locale_t locale [,
   argument_list ]
);

Parameters

stream
Pointer to FILE structure.

format
Format-control string.

argument_list
Optional arguments to the format string.

locale
The locale to use.

Return value

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.

Remarks

fprintf_s formats and prints a series of characters and values to the output stream. Each argument in argument_list (if any) is converted and output according to the corresponding format specification in format. The format argument uses the format specification syntax for printf and wprintf functions.

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.

Important

Ensure that format is not a user-defined string.

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`.

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. 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. For more information about return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Generic-text function mappings

The function in the tchar.h column maps to the function in the other columns depending on the character set that is defined at compile time.

tchar.h function	_UNICODE and _MBCS not defined	_MBCS defined	_UNICODE defined
_ftprintf_s	fprintf_s	fprintf_s	fwprintf_s
_ftprintf_s_l	_fprintf_s_l	_fprintf_s_l	_fwprintf_s_l

For more information, see Format specification syntax.

Requirements

Function	Required header
fprintf_s, _fprintf_s_l	<stdio.h>
fwprintf_s, _fwprintf_s_l	<stdio.h> or <wchar.h>

For more compatibility information, see Compatibility.

Example

// 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
10
1.500000

See also

Stream I/O
_cprintf, _cprintf_l, _cwprintf, _cwprintf_l
fscanf, _fscanf_l, fwscanf, _fwscanf_l
sprintf, _sprintf_l, swprintf, _swprintf_l, __swprintf_l