# mbsrtowcs, mbsrtowcs_s

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< c‎ | string‎ | multibyte

Defined in header `<wchar.h>` |  |
| (1) |
size_t mbsrtowcs( wchar_t* dst, const char** src, size_t len, mbstate_t* ps ); |  | (since C95)
size_t mbsrtowcs( wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps); |  | (since C99)
errno_t mbsrtowcs_s( size_t *restrict retval, wchar_t *restrict dst, rsize_t dstsz, const char **restrict src, rsize_t len, mbstate_t *restrict ps); | (2) | (since C11)

1) Converts a null-terminated multibyte character sequence, which begins in the conversion state described by `*ps`, from the array whose first element is pointed to by *src to its wide character representation. If `dst` is not null, converted characters are stored in the successive elements of the wchar_t array pointed to by `dst`. No more than `len` wide characters are written to the destination array. Each multibyte character is converted as if by a call to mbrtowc. The conversion stops if:
* The multibyte null character was converted and stored. *src is set to NULL and `*ps` represents the initial shift state.
* An invalid multibyte character (according to the current C locale) was encountered. *src is set to point at the beginning of the first unconverted multibyte character.
* the next wide character to be stored would exceed `len`. *src is set to point at the beginning of the first unconverted multibyte character. This condition is not checked if dst==NULL.
2) Same as (1), except that
* the function returns its result as an out-parameter `retval`
* if no null character was written to `dst` after `len` wide characters were written, then L'\0' is stored in `dst[len]`, which means len+1 total wide characters are written
* the function clobbers the destination array from the terminating null and until `dstsz`
* If `src` and `dst` overlap, the behavior is unspecified.
* the following errors are detected at runtime and call the currently installed constraint handler function:

  - `retval`, `ps`, `src`, or *src is a null pointer
  - `dstsz` or `len` is greater than RSIZE_MAX/sizeof(wchar_t) (unless `dst` is null)
  - `dstsz` is not zero (unless `dst` is null)
  - There is no null character in the first `dstsz` multibyte characters in the *src array and `len` is greater than `dstsz` (unless `dst` is null)
  As all bounds-checked functions, `mbsrtowcs_s` is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including `wchar.h`.

### Parameters

dst | - | pointer to wide character array where the results will be stored
src | - | pointer to pointer to the first element of a null-terminated multibyte string
len | - | number of wide characters available in the array pointed to by dst
ps | - | pointer to the conversion state object
dstsz | - | max number of wide characters that will be written (size of the `dst` array)
retval | - | pointer to a size_t object where the result will be stored

### Return value

1) On success, returns the number of wide characters, excluding the terminating L'\0', written to the character array. If dst==NULL, returns the number of wide characters that would have been written given unlimited length. On conversion error (if invalid multibyte character was encountered), returns (size_t)-1, stores EILSEQ in errno, and leaves *ps in unspecified state.
2) zero on success (in which case the number of wide characters excluding terminating zero that were, or would be written to `dst`, is stored in *retval), non-sero on error. In case of a runtime constraint violation, stores (size_t)-1 in *retval (unless `retval` is null) and sets dst[0] to L'\0' (unless `dst` is null or `dstmax` is zero or greater than RSIZE_MAX)

### Example

Run this code

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

    void print_as_wide(const char* mbstr)
    {
        mbstate_t state;
        memset(&state, 0, sizeof state);
        int len = 1 + mbsrtowcs(NULL, &mbstr, 0, &state);
        wchar_t wstr[len];
        mbsrtowcs(&wstr[0], &mbstr, len, &state);
        wprintf(L"Wide string: %ls \n", wstr);
        wprintf(L"The length, including L'\\0': %d\n", len);
    }

    int main(void)
    {
        setlocale(LC_ALL, "en_US.utf8");
        print_as_wide(u8"z\u00df\u6c34\U0001f34c"); // u8"zß水🍌"
    }

Output:

    Wide string: zß水🍌
    The length, including L'\0': 5

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.29.6.4.1 The mbsrtowcs function (p: 445)

  - K.3.9.3.2.1 The mbsrtowcs_s function (p: 648-649)

- C99 standard (ISO/IEC 9899:1999):

  - 7.24.6.4.1 The mbsrtowcs function (p: 391)

### See also

mbstowcs mbstowcs_s (C11) | converts a narrow multibyte character string to wide string (function)
mbrtowc (C95) | converts the next multibyte character to wide character, given state (function)
wcsrtombs wcsrtombs_s (C95)(C11) | converts a wide string to narrow multibyte character string, given state (function)
C++ documentation for mbsrtowcs

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