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strcat_s, wcscat_s, _mbscat_s

Append a string. These are versions of strcat, wcscat, _mbscat with security enhancements as described in Security Features in the CRT.

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

strDestination

Null-terminated destination string buffer.

numberOfElements

Size of the destination string buffer.

strSource

Null-terminated source string buffer.

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

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

Routine

Required header

strcat_s

<string.h>

wcscat_s

<string.h> or <wchar.h>

_mbscat_s

<mbstring.h>

For additional compatibility information, see Compatibility in the Introduction.

See the code example in strcpy_s, wcscpy_s, _mbscpy_s.

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