mbsrtowcs, mbsrtowcs_s

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 with 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

#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);
    size_t 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': %zu\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)

mbstowcsmbstowcs_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)
wcsrtombswcsrtombs_s (C95)(C11)	converts a wide string to narrow multibyte character string, given state (function)

mbsrtowcs, mbsrtowcs_s

Parameters

Return value

Example

References

See also