`mbsrtowcs_s`

Article
12/02/2022

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.

Syntax

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

Parameters

pReturnValue
The number of characters converted.

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

sizeInWords
The size of wcstr in words (wide characters).

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

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

mbstate
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 isn't 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 isn't 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.

Remarks

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 when there's 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 isn't a null pointer. The size is calculated even if the wcstr argument is a null pointer, which lets you determine the required buffer size. If wcstr is a null pointer, count is ignored.

If wcstr isn't 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's assigned the address just past the last multibyte character converted, if any. It 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 isn't thread-safe, we recommend that you pass your own mbstate value.

If mbsrtowcs_s encounters a multibyte character that isn't 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.

Important

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 the 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 behavior, see Global state in the CRT.

Exceptions

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 isn't a null pointer.

Requirements

Routine	Required header
`mbsrtowcs_s`	<wchar.h>