`mbsrtowcs`

Article
12/02/2022

Converts a multibyte character string in the current locale to a corresponding wide character string, with the capability of restarting in the middle of a multibyte character. A more secure version of this function is available; see mbsrtowcs_s.

Syntax

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

Parameters

wcstr
Address to store the resulting converted wide character string.

mbstr
Indirect pointer to the location of the multibyte character string to convert.

count
The maximum number of characters (not bytes) to convert and store in wcstr.

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

Returns the number of characters successfully converted, not including the terminating null character, if any. Returns (size_t)(-1) if an error occurred, and sets errno to EILSEQ.

Remarks

The mbsrtowcs 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 continues for each character until either a terminating null multibyte character is encountered, a multibyte sequence that doesn't correspond to a valid character in the current locale is encountered, or until count characters have been converted. If mbsrtowcs encounters the multibyte null character ('\0') either before or when count occurs, it converts it to a 16-bit terminating null character and stops.

Thus, the wide character string at wcstr is null-terminated only if mbsrtowcs encounters a multibyte null character during conversion. If the sequences pointed to by mbstr and wcstr overlap, the behavior of mbsrtowcs is undefined. mbsrtowcs is affected by the LC_TYPE category of the current locale.

The mbsrtowcs function differs from mbstowcs, _mbstowcs_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 is used instead of mbstowcs.

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 the wcstr argument is a null pointer, the count argument is ignored, and mbsrtowcs returns the required size in wide characters for the destination string. If mbstate is a null pointer, the function uses a non-thread-safe static internal mbstate_t conversion state object. If the character sequence mbstr doesn't have a corresponding multibyte character representation, a -1 is returned, and errno is set to EILSEQ.

If mbstr is a null pointer, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, this function sets errno to EINVAL and returns -1.

In C++, this function has a template overload that invokes the newer, secure counterpart of this function. 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 function is multithread safe as long as 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`	<wchar.h>

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