Convert a multibyte character string in the current locale to its wide character string representation. A version of mbsrtowcs with security enhancements as described in Security Features in the CRT.


errno_t mbsrtowcs_s(
   size_t * pReturnValue,
   wchar_t * wcstr,
   size_t sizeInWords,
   const char ** mbstr,
   size_t count,
   mbstate_t * mbstate
template <size_t size>
errno_t mbsrtowcs_s(
   size_t * pReturnValue,
   wchar_t (&wcstr)[size],
   const char ** mbstr,
   size_t count,
   mbstate_t * mbstate
); // C++ only


The number of characters converted.

Address of buffer to store the resulting converted wide character string.

The size of wcstr in words (wide characters).

Indirect pointer to the location of the multibyte character string to be converted.

The maximum number of wide characters to store in the wcstr buffer, not including the terminating null, or _TRUNCATE.

A pointer to an mbstate_t conversion state object. If this value is a null pointer, a static internal conversion state object is used. Because the internal mbstate_t object is not thread-safe, we recommend that you always pass your own mbstate parameter.

Return Value

Zero if conversion is successful, or an error code on failure.

Error condition Return value and errno
wcstr is a null pointer and sizeInWords > 0 EINVAL
mbstr is a null pointer EINVAL
The string indirectly pointed to by mbstr contains a multibyte sequence that is not valid for the current locale. EILSEQ
The destination buffer is too small to contain the converted string (unless count is _TRUNCATE; for more information, see Remarks) ERANGE

If any one of these conditions occurs, the invalid parameter exception is invoked as described in Parameter Validation . If execution is allowed to continue, the function returns an error code and sets errno as indicated in the table.


The mbsrtowcs_s function converts a string of multibyte characters indirectly pointed to by mbstr into wide characters stored in the buffer pointed to by wcstr, by using the conversion state contained in mbstate. The conversion will continue for each character until one of these conditions is met:

  • A multibyte null character is encountered

  • An invalid multibyte character is encountered

  • The number of wide characters stored in the wcstr buffer equals count.

The destination string wcstr is always null-terminated, even in the case of an error, unless wcstr is a null pointer.

If count is the special value _TRUNCATE, mbsrtowcs_s converts as much of the string as will fit into the destination buffer, while still leaving room for a null terminator.

If mbsrtowcs_s successfully converts the source string, it puts the size in wide characters of the converted string and the null terminator into *pReturnValue, provided pReturnValue is not a null pointer. This occurs even if the wcstr argument is a null pointer and lets you determine the required buffer size. Note that if wcstr is a null pointer, count is ignored.

If wcstr is not a null pointer, the pointer object pointed to by mbstr is assigned a null pointer if conversion stopped because a terminating null character was reached. Otherwise, it is assigned the address just past the last multibyte character converted, if any. This allows a subsequent function call to restart conversion where this call stopped.

If mbstate is a null pointer, the library internal mbstate_t conversion state static object is used. Because this internal static object is not thread-safe, we recommend that you pass your own mbstate value.

If mbsrtowcs_s encounters a multibyte character that is not valid in the current locale, it puts -1 in *pReturnValue, sets the destination buffer wcstr to an empty string, sets errno to EILSEQ, and returns EILSEQ.

If the sequences pointed to by mbstr and wcstr overlap, the behavior of mbsrtowcs_s is undefined. mbsrtowcs_s is affected by the LC_TYPE category of the current locale.


Ensure that wcstr and mbstr do not overlap, and that count correctly reflects the number of multibyte characters to convert.

The mbsrtowcs_s function differs from mbstowcs_s, _mbstowcs_s_l by its restartability. The conversion state is stored in mbstate for subsequent calls to the same or other restartable functions. Results are undefined when mixing the use of restartable and nonrestartable functions. For example, an application should use mbsrlen instead of mbslen, if a subsequent call to mbsrtowcs_s is used instead of mbstowcs_s.

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

By default, this function's global state is scoped to the application. To change this, see Global state in the CRT.


The mbsrtowcs_s function is multithread safe if no function in the current thread calls setlocale as long as this function is executing and the mbstate argument is not a null pointer.


Routine Required header
mbsrtowcs_s <wchar.h>

See also

Data Conversion
Interpretation of Multibyte-Character Sequences
mbtowc, _mbtowc_l
mbstowcs_s, _mbstowcs_s_l