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Showing content from https://en.cppreference.com/w/cpp/algorithm/../../cpp/../cpp/../c/string/multibyte/wcsrtombs.html below:

wcsrtombs, wcsrtombs_s - cppreference.com

(1) (since C95)
(until C99) size_t wcsrtombs( char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps ); (since C99) errno_t wcsrtombs_s( size_t *restrict retval, char *restrict dst, rsize_t dstsz, const wchar_t **restrict src, rsize_t len, mbstate_t *restrict ps ); (2) (since C11) 1)

Converts a sequence of wide characters from the array whose first element is pointed to by

*src

to its narrow multibyte representation that begins in the conversion state described by

*ps

. If

dst

is not null, converted characters are stored in the successive elements of the char array pointed to by

dst

. No more than

len

bytes are written to the destination array. Each character is converted as if by a call to

wcrtomb

. The conversion stops if:

The null character L'\0' was converted and stored. The bytes stored in this case are the unshift sequence (if necessary) followed by '\0', *src is set to null pointer value and *ps represents the initial shift state.
A wchar_t was found that does not correspond to a valid character in the current C locale. *src is set to point at the first unconverted wide character.
the next multibyte character to be stored would exceed len. *src is set to point at the first unconverted wide character. This condition is not checked if dst is a null pointer.

2)

Same as

(1)

, except that

the function returns its result as an out-parameter retval
if the conversion stops without writing a null character, the function will store '\0' in the next byte in dst, which may be dst[len] or dst[dstsz], whichever comes first (meaning up to len+1/dstsz+1 total bytes may be written). In this case, there may be no unshift sequence written before the terminating null.
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 (unless dst is null)
dstsz is not zero (unless dst is null)
len is greater than dstsz and the conversion does not encounter null or encoding error in the src array by the time dstsz is reached (unless dst is null)

As with all bounds-checked functions, wcsrtombs_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>.

[edit] Parameters dst - pointer to narrow character array where the multibyte characters will be stored src - pointer to pointer to the first element of a null-terminated wide string len - number of bytes available in the array pointed to by dst ps - pointer to the conversion state object dstsz - max number of bytes that will be written (size of the dst array) retval - pointer to a size_t object where the result will be stored [edit] Return value 1)

On success, returns the number of bytes (including any shift sequences, but excluding the terminating

'\0'

) written to the character array whose first element is pointed to by

dst

. If

dst

is a null pointer, returns the number of bytes that would have been written. On conversion error (if invalid wide character was encountered), returns

(size_t)-1

, stores

EILSEQ

errno

, and leaves

*ps

in unspecified state.

2)

Returns zero on success (in which case the number of bytes excluding terminating zero that were, or would be written to

dst

, is stored in

*retval

), non-zero on error. In case of a runtime constraint violation, stores

(size_t)-1

*retval

(unless

retval

is null) and sets

dst[0]

'\0'

(unless

dst

is null or

dstmax

is zero or greater than

RSIZE_MAX

)

[edit] Example

#include <stdio.h>
#include <locale.h>
#include <string.h>
#include <wchar.h>
 
void print_wide(const wchar_t* wstr)
{
    mbstate_t state;
    memset(&state, 0, sizeof state);
    size_t len = 1 + wcsrtombs(NULL, &wstr, 0, &state);
    char mbstr[len];
    wcsrtombs(mbstr, &wstr, len, &state);
    printf("Multibyte string: %s\n", mbstr);
    printf("Length, including '\\0': %zu\n", len);
}
 
int main(void)
{
    setlocale(LC_ALL, "en_US.utf8");
    print_wide(L"z\u00df\u6c34\U0001f34c"); // or L"zÃæ°´ð"
}

Output:

Multibyte string: zÃæ°´ð
Length, including '\0': 11

[edit] References

C17 standard (ISO/IEC 9899:2018):

7.29.6.4.2 The wcsrtombs function (p: 324-325)

K.3.9.3.2.2 The wcsrtombs_s function (p: 471-472)

C11 standard (ISO/IEC 9899:2011):

7.29.6.4.2 The wcsrtombs function (p: 446)

K.3.9.3.2.2 The wcsrtombs_s function (p: 649-651)

C99 standard (ISO/IEC 9899:1999):

7.24.6.4.2 The wcsrtombs function (p: 392)

[edit] See also

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