NAME

wctomb — convert a wide-character code to a character

SYNOPSIS

#include <stdlib.h>

int wctomb(char *s, wchar_t wchar);


DESCRIPTION

^[CX] Except for requirements relating to data races, the functionality described on this reference page is aligned with the ISO C standard. Any other conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard for all wctomb() functionality except in relation to data races.

The wctomb() function shall determine the number of bytes needed to represent the character corresponding to the wide-character code whose value is wchar (including any change in the shift state). It shall store the character representation (possibly multiple bytes and any special bytes to change shift state) in the array object pointed to by s (if s is not a null pointer). At most {MB_CUR_MAX} bytes shall be stored. If wchar is 0, a null byte shall be stored, preceded by any shift sequence needed to restore the initial shift state, and wctomb() shall be left in the initial shift state.

The behavior of this function is affected by the LC_CTYPE category of the current locale. For a state-dependent encoding, this function shall be placed into its initial state by a call for which its character pointer argument, s, is a null pointer. Subsequent calls with s as other than a null pointer shall cause the internal state of the function to be altered as necessary. A call with s as a null pointer shall cause this function to return a non-zero value if encodings have state dependency, and 0 otherwise. Changing the LC_CTYPE category causes the shift state of this function to be unspecified.

The wctomb() function ^[CX]  need not be thread-safe; however, it  shall avoid data races with all other functions.

The implementation shall behave as if no function defined in this volume of POSIX.1-2024 calls wctomb().

RETURN VALUE

If s is a null pointer, wctomb() shall return a non-zero or 0 value, if character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, wctomb() shall return -1 if the value of wchar does not correspond to a valid character, or return the number of bytes that constitute the character corresponding to the value of wchar.

In no case shall the value returned be greater than the value of the {MB_CUR_MAX} macro.

ERRORS

The wctomb() function shall fail if:

[EILSEQ]

^[CX] An invalid wide-character code is detected.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

When the ISO C standard introduced threads in C11, it required wctomb() to avoid data races (with itself as well as with other functions), whereas POSIX.1-2008 did not require it to be thread-safe, and in many implementations it did not avoid data races with itself and still does not. The ISO C committee intend to change the requirements in a future version of the ISO C standard, but since POSIX.1 currently refers to C17 it is necessary for it not to defer to the ISO C standard regarding data races in order to continue to allow this function not to avoid data races with itself.

FUTURE DIRECTIONS

It is expected that a change in a future version of the ISO C standard will allow a future version of this standard to remove the data race exception from the statement that it defers to the ISO C standard.

SEE ALSO

mblen, mbtowc, mbstowcs, wcstombs

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 4. Derived from the ANSI C standard.

Issue 6

Extensions beyond the ISO C standard are marked.

A note indicating that this function need not be reentrant is added to the DESCRIPTION.

Issue 7

Austin Group Interpretations 1003.1-2001 #156 and #170 are applied.

Issue 8

Austin Group Defects 708 and 1302 are applied, aligning this function with the ISO/IEC 9899:2018 standard, except in relation to data races.

Austin Group Defect 1572 is applied, removing CX shading from some text derived from the ISO C standard.