crtsetdebugfillthreshold.md

title	description	ms.date	api_name	api_location	api_type	topic_type	f1_keywords	helpviewer_keywords
_CrtSetDebugFillThreshold	Use the _CrtSetDebugFillThreshold function to set the maximum amount of buffer to fill in secure CRT functions.	04/10/2025	_CrtSetDebugFillThreshold	msvcrt.dll msvcr80.dll msvcr90.dll msvcr100.dll msvcr100_clr0400.dll msvcr110.dll msvcr110_clr0400.dll msvcr120.dll msvcr120_clr0400.dll ucrtbase.dll	DLLExport	apiref	_CrtSetDebugFillThreshold CrtSetDebugFillThreshold	debug, buffer-filling behavior CrtSetDebugFillThreshold function _CrtSetDebugFillThreshold function buffer-filling behavior 0xFE

_CrtSetDebugFillThreshold

Retrieves or modifies the threshold controlling buffer-filling behavior in debug functions.

Syntax

size_t _CrtSetDebugFillThreshold( size_t newThreshold );

Parameters

newThreshold
New threshold size in bytes.

Return value

The previous threshold value.

Remarks

The debug versions of some security-enhanced CRT functions fill the buffer passed to them with a special character (0xFE). This fill character helps to find cases where the incorrect size was passed to the function. Unfortunately, it also reduces performance. To improve performance, use _CrtSetDebugFillThreshold to disable buffer-filling for buffers larger than the newThreshold threshold. A newThreshold value of 0 disables it for all buffers.

The default threshold is SIZE_T_MAX.

Here's a list of the affected functions:

• asctime_s, _wasctime_s
• _cgets_s, _cgetws_s
• ctime_s, _ctime32_s, _ctime64_s, _wctime_s, _wctime32_s, _wctime64_s
• _ecvt_s
• _fcvt_s
• _gcvt_s
• _itoa_s, _ltoa_s, _ultoa_s, _i64toa_s, _ui64toa_s, _itow_s, _ltow_s, _ultow_s, _i64tow_s, _ui64tow_s
• _makepath_s, _wmakepath_s
• _mbsnbcat_s, _mbsnbcat_s_l
• _mbsnbcpy_s, _mbsnbcpy_s_l
• _mbsnbset_s, _mbsnbset_s_l
• _mktemp_s, _wmktemp_s
• _splitpath_s, _wsplitpath_s
• strcat_s, wcscat_s, _mbscat_s
• strcpy_s, wcscpy_s, _mbscpy_s
• _strdate_s, _wstrdate_s
• strerror_s, _strerror_s, _wcserror_s, __wcserror_s
• _strlwr_s, _strlwr_s_l, _mbslwr_s, _mbslwr_s_l, _wcslwr_s, _wcslwr_s_l
• strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l
• strncpy_s, _strncpy_s_l, wcsncpy_s, _wcsncpy_s_l, _mbsncpy_s, _mbsncpy_s_l
• _strnset_s, _strnset_s_l, _wcsnset_s, _wcsnset_s_l, _mbsnset_s, _mbsnset_s_l
• _strset_s, _strset_s_l, _wcsset_s, _wcsset_s_l, _mbsset_s, _mbsset_s_l
• _strtime_s, _wstrtime_s
• _strupr_s, _strupr_s_l, _mbsupr_s, _mbsupr_s_l, _wcsupr_s, _wcsupr_s_l
• vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwpr`intf_s_l

Requirements

Routine	Required header
_CrtSetDebugFillThreshold	<crtdbg.h>

This function is Microsoft-specific. For more compatibility information, see Compatibility.

Libraries

Debug versions of the C run-time libraries only.

Example

// crt_crtsetdebugfillthreshold.c
// compile with: cl /MTd crt_crtsetdebugfillthreshold.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <crtdbg.h>

void Clear( char buff[], size_t size )
{
   for( int i=0; i<size; ++i )
      buff[i] = 0;
}

void Print( char buff[], size_t size )
{
   for( int i=0; i<size; ++i )
      printf( "%02x  %c\n", (unsigned char)buff[i], buff[i] );
}

int main( void )
{
   char buff[10];

   printf( "With buffer-filling on:\n" );
   strcpy_s( buff, _countof(buff), "howdy" );
   Print( buff, _countof(buff) );

   _CrtSetDebugFillThreshold( 0 );

   printf( "With buffer-filling off:\n" );
   Clear( buff, _countof(buff) );
   strcpy_s( buff, _countof(buff), "howdy" );
   Print( buff, _countof(buff) );
}

With buffer-filling on:
68  h
6f  o
77  w
64  d
79  y
00
fe  ■
fe  ■
fe  ■
fe  ■
With buffer-filling off:
68  h
6f  o
77  w
64  d
79  y
00
00
00
00
00

See also

Debug routines