fprintf, _fprintf_l, fwprintf, _fwprintf_l, _ftprintf, _ftprintf_l

Print formatted data to a stream. More secure versions of these functions are available; see fprintf_s, _fprintf_s_l, fwprintf_s, _fwprintf_s_l.

For _ftprintf and _ftprintf_l, see Generic-text function mappings.

Syntax

int fprintf(
   FILE *stream,
   const char *format [,
   argument ]...
);
int _fprintf_l(
   FILE *stream,
   const char *format,
   _locale_t locale [,
   argument ]...
);
int fwprintf(
   FILE *stream,
   const wchar_t *format [,
   argument ]...
);
int _fwprintf_l(
   FILE *stream,
   const wchar_t *format,
   _locale_t locale [,
   argument ]...
);

Parameters

stream
Pointer to FILE structure.

format
Format-control string.

argument
Optional arguments.

locale
The locale to use.

Return value

fprintf returns the number of bytes written. fwprintf returns the number of wide characters written. Each of these functions returns a negative value instead when an output error occurs. If stream or format is NULL, these functions invoke the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, the functions return -1 and set errno to EINVAL. The format string isn't checked for valid formatting characters as it is when using fprintf_s or fwprintf_s.

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

Remarks

fprintf 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, the format argument has the same syntax that it has in printf.

fwprintf is a wide-character version of fprintf; in fwprintf, format is a wide-character string. These functions behave identically if the stream is opened in ANSI mode. fprintf 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 thread 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`.

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	fprintf	fprintf	fwprintf
_ftprintf_l	_fprintf_l	_fprintf_l	_fwprintf_l

For more information, see Format specification syntax.

Requirements

Function	Required header
fprintf, _fprintf_l	<stdio.h>
fwprintf, _fwprintf_l	<stdio.h> or <wchar.h>

For more compatibility information, see Compatibility.

Example

// crt_fprintf.c
/* This program uses fprintf to format various
* data and print it to the file named FPRINTF.OUT. It
* then displays FPRINTF.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.out", "w" );
   fprintf( stream, "%s%c", s, c );
   fprintf( stream, "%d\n", i );
   fprintf( stream, "%f\n", fp );
   fclose( stream );
   system( "type fprintf.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
Format specification syntax: printf and wprintf functions