`wcrtomb_s`

Article
02/09/2023

Convert a wide character into its multibyte character representation. A version of wcrtomb with security enhancements as described in Security features in the CRT.

Syntax

errno_t wcrtomb_s(
   size_t *pReturnValue,
   char *mbchar,
   size_t sizeOfmbchar,
   wchar_t *wchar,
   mbstate_t *mbstate
);
template <size_t size>
errno_t wcrtomb_s(
   size_t *pReturnValue,
   char (&mbchar)[size],
   wchar_t *wchar,
   mbstate_t *mbstate
); // C++ only

Parameters

pReturnValue
Returns the number of bytes written or -1 if an error occurred.

mbchar
The resulting multibyte converted character.

sizeOfmbchar
The size of the mbchar variable in bytes.

wchar
A wide character to convert.

mbstate
A pointer to an mbstate_t object.

Return value

Returns zero or an errno value if an error occurs.

Remarks

The wcrtomb_s function converts a wide character, beginning in the specified conversion state contained in mbstate, from the value contained in wchar, into the address represented by mbchar. The pReturnValue value will be the number of bytes converted, but no more than MB_CUR_MAX bytes, or an -1 if an error occurred.

If mbstate is null, the internal mbstate_t conversion state is used. If the character contained in wchar doesn't have a corresponding multibyte character, the value of pReturnValue is -1, and the function returns the errno value of EILSEQ.

The wcrtomb_s function differs from wctomb_s, _wctomb_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 would use wcsrlen rather than wcslen, if a subsequent call to wcsrtombs_s were used instead of wcstombs_s.

In C++, using this function is simplified by template overloads; the overloads can infer buffer length automatically (eliminating the need to specify a size argument) and they can automatically replace older, non-secure functions with 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 behavior, see Global state in the CRT.

Exceptions

The wcrtomb_s function is multithread safe as long as no function in the current thread calls setlocale while this function is executing and the mbstate is null.

Example

// crt_wcrtomb_s.c
// This program converts a wide character
// to its corresponding multibyte character.
//

#include <string.h>
#include <stdio.h>
#include <wchar.h>

int main( void )
{
    errno_t     returnValue;
    size_t      pReturnValue;
    mbstate_t   mbstate;
    size_t      sizeOfmbStr = 1;
    char        mbchar = 0;
    wchar_t*    wchar = L"Q\0";

    // Reset to initial conversion state
    memset(&mbstate, 0, sizeof(mbstate));

    returnValue = wcrtomb_s(&pReturnValue, &mbchar, sizeof(char),
                            *wchar, &mbstate);
    if (returnValue == 0) {
        printf("The corresponding wide character \"");
        wprintf(L"%s\"", wchar);
        printf(" was converted to a the \"%c\" ", mbchar);
        printf("multibyte character.\n");
    }
    else
    {
        printf("No corresponding multibyte character "
               "was found.\n");
    }
}

The corresponding wide character "Q" was converted to a the "Q" multibyte character.

Requirements

Routine	Required header
`wcrtomb_s`	<wchar.h>

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