The number of character to be converted.

If the *`mbstr`* argument is `NULL`, **`wcsrtombs`** returns the required size in bytes of the destination string. If *`mbstate`* is null, the internal `mbstate_t` conversion state is used. If the character sequence *`wchar`* does not have a corresponding multibyte character representation, a -1 is returned and the `errno` is set to `EILSEQ`.

The **`wcsrtombs`** function is multithread safe as long as no function in the current thread calls `setlocale` while this function is executing and the *`mbstate`* is not null.

- A wide character that cannot be converted is encountered

The destination string is always null-terminated (even in the case of an error).

If **`wcstombs_s`** successfully converts the source string, it puts the size in bytes of the converted string, including the null terminator, into *`*pReturnValue`* (provided *`pReturnValue`* is not `NULL`). This occurs even if the *`mbstr`* argument is `NULL` and provides a way to determine the required buffer size. Note that if *`mbstr`* is `NULL`, *`count`* is ignored.

If **`wcstombs_s`** encounters a wide character it cannot convert to a multibyte character, it puts 0 in *`*ReturnValue`*, sets the destination buffer to an empty string, sets `errno` to `EILSEQ`, and returns `EILSEQ`.

The **`wcstombs`** function converts the wide-character string pointed to by *`wcstr`* to the corresponding multibyte characters and stores the results in the *`mbstr`* array. The *`count`* parameter indicates the maximum number of bytes that can be stored in the multibyte output string (that is, the size of *`mbstr`*). In general, it isn't known how many bytes will be required when converting a wide-character string. Some wide characters will require only one byte in the output string; others require two. If there are two bytes in the multibyte output string for every wide character in the input string (including the wide character `NULL`), the result is guaranteed to fit.

If the `LC_CTYPE` category of the current locale does not define a mapping whose name matches the property string *`property`*, the function returns zero. Otherwise, it returns a nonzero value suitable for use as the second argument to a subsequent call to [`towctrans`](towctrans.md).

The following pairs of calls have the same behavior in all locales, but it is possible to define additional mappings even in the "C" locale:

|tolower(c)|towctrans(c, wctrans("towlower"))|

|towupper(c)|towctrans(c, wctrans("toupper"))|

The function determines a classification rule for wide-character codes. The following pairs of calls have the same behavior in all locales (but an implementation can define additional classification rules even in the "C" locale):

If successful, **`_write`** returns the number of bytes written. If the actual space remaining on the disk is less than the size of the buffer the function is trying to write to the disk, **`_write`** fails and doesn't flush any of the buffer's contents to the disk. A return value of -1 indicates an error. If invalid parameters are passed, this function invokes the invalid parameter handler, as described in [Parameter validation](../parameter-validation.md). If execution is allowed to continue, the function returns -1 and `errno` is set to one of three values: `EBADF`, which means the file descriptor is invalid or the file isn't opened for writing; `ENOSPC`, which means there isn't enough space left on the device for the operation; or `EINVAL`, which means that *`buffer`* was a null pointer or that an odd *`count`* of bytes was passed to be written to a file in Unicode mode.

When writing to files opened in text mode, **`_write`** treats a CTRL+Z character as the logical end of file. When writing to a device, **`_write`** treats a CTRL+Z character in the buffer as an output terminator.

An alternative to the ^= operator.

An alternative to the ^ operator.