wcsrtombs, wcsrtombs_s

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> .

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 in 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 in * retval (unless retval is null) and sets dst [ 0 ] to ' \0 ' (unless dst is null or dstmax is zero or greater than RSIZE_MAX )

#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"zß水🍌"
}

Multibyte string: zß水🍌
Length, including '\0': 11