wctomb, wctomb_s

< c‎ | string‎ | multibyte
Defined in header <stdlib.h>
int wctomb( char *s, wchar_t wc );
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 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>.


[edit] 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.

[edit] 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

[edit] 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

[edit] 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]);
int main(void)
    setlocale(LC_ALL, "en_US.utf8");
    printf("MB_CUR_MAX = %zu\n", MB_CUR_MAX);

Possible output:

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]

[edit] References

  • C17 standard (ISO/IEC 9899:2018):
  • The wctomb function (p: 261)
  • K. The wctomb_s function (p: 443)
  • C11 standard (ISO/IEC 9899:2011):
  • The wctomb function (p: 358-359)
  • K. The wctomb_s function (p: 610-611)
  • C99 standard (ISO/IEC 9899:1999):
  • The wctomb function (p: 322-323)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • The wctomb function

[edit] See also

converts the next multibyte character to wide character
(function) [edit]
converts a wide character to its multibyte representation, given state
(function) [edit]