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


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


The size in bytes of the converted string, including the null terminator.

The address of a buffer for the resulting converted multibyte character string.

The size in bytes of the mbstr buffer.

Points to the wide character string to be converted.

The maximum number of bytes to be stored in the mbstr buffer, or _TRUNCATE.

A pointer to an mbstate_t conversion state object.

Return Value

Zero if successful, an error code on failure.

Error condition Return value and errno
mbstr is NULL and sizeInBytes > 0 EINVAL
wcstr is NULL EINVAL
The destination buffer is too small to contain the converted string (unless count is _TRUNCATE; see Remarks below) ERANGE

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

  • A null wide character is encountered

  • A wide character that cannot be converted is encountered

  • The number of bytes stored in the mbstr buffer equals count.

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

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

If wcsrtombs_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 wcsrtombs_s encounters a wide character it cannot convert to a multibyte character, it puts -1 in *pReturnValue, sets the destination buffer to an empty string, sets errno to EILSEQ, and returns EILSEQ.

If the sequences pointed to by wcstr and mbstr overlap, the behavior of wcsrtombs_s is undefined. wcsrtombs_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 wide characters to convert.

The wcsrtombs_s function differs from wcstombs_s, _wcstombs_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 these functions 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, see Global state in the CRT.


The wcsrtombs_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.


// crt_wcsrtombs_s.cpp
// This code example converts a wide
// character string into a multibyte
// character string.

#include <stdio.h>
#include <memory.h>
#include <wchar.h>
#include <errno.h>

#define MB_BUFFER_SIZE 100

int main()
    const wchar_t   wcString[] =
                    {L"Every good boy does fine."};
    const wchar_t   *wcsIndirectString = wcString;
    char            mbString[MB_BUFFER_SIZE];
    size_t          countConverted;
    errno_t         err;
    mbstate_t       mbstate;

    // Reset to initial shift state
    ::memset((void*)&mbstate, 0, sizeof(mbstate));

    err = wcsrtombs_s(&countConverted, mbString, MB_BUFFER_SIZE,
                      &wcsIndirectString, MB_BUFFER_SIZE, &mbstate);
    if (err == EILSEQ)
        printf( "An encoding error was detected in the string.\n" );
        printf( "The string was successfully converted.\n" );
The string was successfully converted.


Routine Required header
wcsrtombs_s <wchar.h>

See also

Data Conversion
Interpretation of Multibyte-Character Sequences
wctomb, _wctomb_l
wcstombs, _wcstombs_l