mbsrtowcs, mbsrtowcs_s

 

C

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Headers

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Program utilities

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Error handling

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Date and time utilities

Strings library

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Numerics

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Atomic operations (C11)

Thread support (C11)

Technical Specifications

 

Strings library

Byte strings

Multibyte strings

Wide strings

 

Null-terminated multibyte strings

Wide/multibyte conversions
mbsinit (C95)
mbtowc mbstowcsmbstowcs_s (C11) btowc (C95) mbrtowc (C95) mbsrtowcsmbsrtowcs_s (C95)(C11) mbrtoc16 (C11) c16rtomb (C11) c32rtomb (C11) mbrtoc32 (C11)	mblen wctombwctomb_s (C11) wcstombswcstombs_s (C11) wctob (C95) wcrtombwcrtomb_s (C95)(C11) wcsrtombswcsrtombs_s (C95)(C11) mbrlen (C95)
Types
mbstate_t (C95)

 

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

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);
    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)

See also

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)
C++ documentation for mbsrtowcs