NAME

mbtowc — convert a character to a wide-character code

SYNOPSIS

#include <stdlib.h>

int mbtowc(wchar_t *restrict pwc, const char *restrict s, size_t n);


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 mbtowc() functionality except in relation to data races.

If s is not a null pointer, mbtowc() shall determine the number of bytes that constitute the character pointed to by s. It shall then determine the wide-character code for the value of type wchar_t that corresponds to that character. (The value of the wide-character code corresponding to the null byte is 0.) If the character is valid and pwc is not a null pointer, mbtowc() shall store the wide-character code in the object pointed to by pwc.

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 at program startup and can be returned to that 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. If the implementation employs special bytes to change the shift state, these bytes shall not produce separate wide-character codes, but shall be grouped with an adjacent character. Changing the LC_CTYPE category causes the shift state of this function to be unspecified. At most n bytes of the array pointed to by s shall be examined.

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

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

RETURN VALUE

If s is a null pointer, mbtowc() 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, mbtowc() shall either return 0 (if s points to the null byte), or return the number of bytes that constitute the converted character (if the next n or fewer bytes form a valid character), or return -1 ^[CX]  and shall set errno to indicate the error (if they do not form a valid character).

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

ERRORS

The mbtowc() function shall fail if:

[EILSEQ]

^[CX] An invalid character sequence is detected. In the POSIX locale an [EILSEQ] error cannot occur since all byte values are valid characters.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

When the ISO C standard introduced threads in C11, it required mbtowc() 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, mbstowcs, wctomb, wcstombs

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 4. Aligned with the ISO C standard.

Issue 6

The mbtowc() prototype is updated for alignment with the ISO/IEC 9899:1999 standard.

Extensions beyond the ISO C standard are marked.

Issue 7

Austin Group Interpretation 1003.1-2001 #170 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0372 [109] and XSH/TC1-2008/0373 [195] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0209 [663,674] is 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.