strcat_s, wcscat_s, _mbscat_s

Article
04/28/2015

Appends a string. These versions of strcat, wcscat, _mbscat have security enhancements, as described in Security Features in the CRT.

Important

_mbscat_s cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported with /ZW.

errno_t strcat_s(
   char *strDestination,
   size_t numberOfElements,
   const char *strSource 
);
errno_t wcscat_s(
   wchar_t *strDestination,
   size_t numberOfElements,
   const wchar_t *strSource 
);
errno_t _mbscat_s(
   unsigned char *strDestination,
   size_t numberOfElements,
   const unsigned char *strSource 
);
template <size_t size>
errno_t strcat_s(
   char (&strDestination)[size],
   const char *strSource 
); // C++ only
template <size_t size>
errno_t wcscat_s(
   wchar_t (&strDestination)[size],
   const wchar_t *strSource 
); // C++ only
template <size_t size>
errno_t _mbscat_s(
   unsigned char (&strDestination)[size],
   const unsigned char *strSource 
); // C++ only

Parameters

strDestination
Null-terminated destination string buffer.
numberOfElements
Size of the destination string buffer.
strSource
Null-terminated source string buffer.

Return Value

Zero if successful; an error code on failure.

Error Conditions

strDestination	numberOfElements	strSource	Return value	Contents of strDestination
NULL or unterminated	any	any	EINVAL	not modified
any	any	NULL	EINVAL	strDestination[0] set to 0
any	0, or too small	any	ERANGE	strDestination[0] set to 0

Remarks

The strcat_s function appends strSource to strDestination and terminates the resulting string with a null character. The initial character of strSource overwrites the terminating null character of strDestination. The behavior of strcat_s is undefined if the source and destination strings overlap.

Note that the second parameter is the total size of the buffer, not the remaining size:

char buf[16];
strcpy_s(buf, 16, "Start");
strcat_s(buf, 16, " End");               // Correct
strcat_s(buf, 16 – strlen(buf), " End"); // Incorrect

wcscat_s and _mbscat_s are wide-character and multibyte-character versions of strcat_s. The arguments and return value of wcscat_s are wide-character strings; those of _mbscat_s are multibyte-character strings. These three functions behave identically otherwise.

If strDestination is a null pointer, or is not null-terminated, or if strSource is a NULL pointer, or if the destination string is too small, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions return EINVAL and set errno to EINVAL.

In C++, using these functions is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with their newer, secure counterparts. For more information, see Secure Template Overloads.

The debug versions of these functions first fill the buffer with 0xFD. To disable this behavior, use _CrtSetDebugFillThreshold.

Generic-Text Routine Mappings

TCHAR.H routine	_UNICODE & _MBCS not defined	_MBCS defined	_UNICODE defined
_tcscat_s	strcat_s	_mbscat_s	wcscat_s

Requirements

Routine	Required header
strcat_s	<string.h>
wcscat_s	<string.h> or <wchar.h>
_mbscat_s	<mbstring.h>

For additional compatibility information, see Compatibility.

Example

See the code example in strcpy_s, wcscpy_s, _mbscpy_s.

.NET Framework Equivalent

System::String::Concat

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