title: "_mbsnbcat_s, _mbsnbcat_s_l"

ms.date: "4/2/2020"

Appends to a multibyte character string, at most, the first **n** bytes of another multibyte-character string. These are versions of [_mbsnbcat, _mbsnbcat_l](mbsnbcat-mbsnbcat-l.md) that have security enhancements, as described in [Security Features in the CRT](../../c-runtime-library/security-features-in-the-crt.md).

*dest*<br/>

*sizeInBytes*<br/>

Size of the *dest* buffer in bytes.

*src*<br/>

*count*<br/>

Number of bytes from *src* to append to *dest*.

*locale*<br/>

|**Dest**|*sizeInBytes*|*src*|Return value|

|**NULL**|any|any|**EINVAL**|

|Any|<= 0|any|**EINVAL**|

|Any|any|**NULL**|**EINVAL**|

If any of the error conditions occurs, the function generates an invalid parameter error, as described in [Parameter Validation](../../c-runtime-library/parameter-validation.md). If the error is handled, the function returns **EINVAL** and sets **errno** to **EINVAL**.

The **_mbsnbcat_s** function appends to *dest*, at most, the first *count* bytes of *src*. If the byte that immediately precedes the null character in *dest* is a lead byte, it is overwritten by the initial byte of *src*. Otherwise, the initial byte of *src* overwrites the terminating null character of *dest*. If a null byte appears in *src* before *count* bytes are appended, **_mbsnbcat_s** appends all bytes from *src*, up to the null character. If *count* is greater than the length of *src*, the length of *src* is used in place of *count*. The resulting string is terminated by a null character. If copying takes place between strings that overlap, the behavior is undefined.

The output value is affected by the setting of the **LC_CTYPE** category setting of the locale; see [setlocale, _wsetlocale](setlocale-wsetlocale.md) for more information. The versions of these functions are identical, except that the ones that don't have the **_l** suffix use the current locale and the ones that do have the **_l** suffix instead use the locale parameter that's passed in. For more information, see [Locale](../../c-runtime-library/locale.md).

In C++, the use of these functions is simplified by template overloads; the overloads can infer buffer length automatically and thereby eliminate the need to specify a size argument, and they can automatically use their newer, more secure functions to replace older, less-secure functions. For more information, see [Secure Template Overloads](../../c-runtime-library/secure-template-overloads.md).

The debug library versions of these functions first fill the buffer with 0xFE. To disable this behavior, use [_CrtSetDebugFillThreshold](crtsetdebugfillthreshold.md).

|Tchar.h routine|_UNICODE and _MBCS not defined|_MBCS defined|_UNICODE defined|

|**_tcsncat**|[strncat](strncat-strncat-l-wcsncat-wcsncat-l-mbsncat-mbsncat-l.md)|**_mbsnbcat_s**|[wcsncat](strncat-strncat-l-wcsncat-wcsncat-l-mbsncat-mbsncat-l.md)|

|**_tcsncat_s_l**|**_strncat_s_l**|**_mbsnbcat_s_l**|**_wcsncat_s_l**|

|**_mbsnbcat_s**|\<mbstring.h>|

|**_mbsnbcat_s_l**|\<mbstring.h>|

[String Manipulation](../../c-runtime-library/string-manipulation-crt.md)<br/>

[_mbsnbcmp, _mbsnbcmp_l](mbsnbcmp-mbsnbcmp-l.md)<br/>

[_strncnt, _wcsncnt, _mbsnbcnt, _mbsnbcnt_l, _mbsnccnt, _mbsnccnt_l](strncnt-wcsncnt-mbsnbcnt-mbsnbcnt-l-mbsnccnt-mbsnccnt-l.md)<br/>

[_mbsnbcpy, _mbsnbcpy_l](mbsnbcpy-mbsnbcpy-l.md)<br/>

[_mbsnbcpy_s, _mbsnbcpy_s_l](mbsnbcpy-s-mbsnbcpy-s-l.md)<br/>

[_mbsnbset, _mbsnbset_l](mbsnbset-mbsnbset-l.md)<br/>

[strncat, _strncat_l, wcsncat, _wcsncat_l, _mbsncat, _mbsncat_l](strncat-strncat-l-wcsncat-wcsncat-l-mbsncat-mbsncat-l.md)<br/>

[strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l](strncat-s-strncat-s-l-wcsncat-s-wcsncat-s-l-mbsncat-s-mbsncat-s-l.md)<br/>