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wctomb, wctomb_s

Defined in header `<stdlib.h>`
int wctomb( char *s, wchar_t wc );	(1)
errno_t wctomb_s( int restrict status, char restrict s, rsize_t ssz, wchar_t wc );	(2)	(since C11)

1) Converts a wide character wc to multibyte encoding and stores it (including any shift sequences) in the char array whose first element is pointed to by s. No more than MB_CUR_MAX characters are stored. The conversion is affected by the current locale's LC_CTYPE category.

If wc is the null character, the null byte is written to s, preceded by any shift sequences necessary to restore the initial shift state.

If s is a null pointer, this function resets the global conversion state and determines whether shift sequences are used.

2) Same as (1), except that the result is returned in the out-parameter status and the following errors are detected at runtime and call the currently installed constraint handler function:

ssz is less than the number of bytes that would be written (unless s is null)
ssz is greater than RSIZE_MAX (unless s is null)
s is a null pointer but ssz is not zero

As with all bounds-checked functions, wctomb_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 <stdlib.h>.

Notes

Each call to wctomb updates the internal global conversion state (a static object of type mbstate_t, known only to this function). If the multibyte encoding uses shift states, this function is not reentrant. In any case, multiple threads should not call wctomb without synchronization: wcrtomb or wctomb_s may be used instead.

Unlike most bounds-checked functions, wctomb_s does not null-terminate its output, because it is designed to be used in loops that process strings character-by-character.

Parameters

s	-	pointer to the character array for output
wc	-	wide character to convert
ssz	-	maximum number of bytes to write to `s` (size of the array `s`)
status	-	pointer to an out-parameter where the result (length of the multibyte sequence or the shift sequence status) will be stored

Return value

1) If s is not a null pointer, returns the number of bytes that are contained in the multibyte representation of wc or -1 if wc is not a valid character.

If s is a null pointer, resets its internal conversion state to represent the initial shift state and returns 0 if the current multibyte encoding is not state-dependent (does not use shift sequences) or a non-zero value if the current multibyte encoding is state-dependent (uses shift sequences).

2) zero on success, in which case the multibyte representation of wc is stored in s and its length is stored in *status, or, if s is null, the shift sequence status is stored in status). Non-zero on encoding error or runtime constraint violation, in which case (size_t)-1 is stored in *status. The value stored in *status never exceeds MB_CUR_MAX

Example

#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
 
void demo(wchar_t wc)
{
    const char* dep = wctomb(NULL, wc) ? "Yes" : "No";
    printf("State-dependent encoding? %s.\n", dep);
 
    char mb[MB_CUR_MAX];
    int len = wctomb(mb, wc);
    printf("wide char '%lc' -> multibyte char [", wc);
    for (int idx = 0; idx < len; ++idx)
        printf("%s%#2x", idx ? " " : "", (unsigned char)mb[idx]);
    printf("]\n");
}
 
int main(void)
{
    setlocale(LC_ALL, "en_US.utf8");
    printf("MB_CUR_MAX = %zu\n", MB_CUR_MAX);
    demo(L'A');
    demo(L'\u00df');
    demo(L'\U0001d10b');
}

Possible output:

MB_CUR_MAX = 6
State-dependent encoding? No.
wide char 'A' -> multibyte char [0x41]
State-dependent encoding? No.
wide char 'ß' -> multibyte char [0xc3 0x9f]
State-dependent encoding? No.
wide char '𝄋' -> multibyte char [0xf0 0x9d 0x84 0x8b]

References

C17 standard (ISO/IEC 9899:2018):

7.22.7.3 The wctomb function (p: 261)
K.3.6.4.1 The wctomb_s function (p: 443)

C11 standard (ISO/IEC 9899:2011):

7.22.7.3 The wctomb function (p: 358-359)
K.3.6.4.1 The wctomb_s function (p: 610-611)

C99 standard (ISO/IEC 9899:1999):

7.20.7.3 The wctomb function (p: 322-323)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.7.3 The wctomb function