The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


wcrtomb - convert a wide-character code to a character (restartable)


#include <wchar.h>

size_t wcrtomb(char *restrict
s, wchar_t wc, mbstate_t *restrict ps);


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard. [Option End]

If s is a null pointer, the wcrtomb() function shall be equivalent to the call:

wcrtomb(buf, L'\0', ps)

where buf is an internal buffer.

If s is not a null pointer, the wcrtomb() function shall determine the number of bytes needed to represent the character that corresponds to the wide character given by wc (including any shift sequences), and store the resulting bytes in the array whose first element is pointed to by s. At most {MB_CUR_MAX} bytes are stored. If wc is a null wide character, a null byte shall be stored, preceded by any shift sequence needed to restore the initial shift state. The resulting state described shall be the initial conversion state.

If ps is a null pointer, the wcrtomb() function shall use its own internal mbstate_t object, which is initialized at program start-up to the initial conversion state. Otherwise, the mbstate_t object pointed to by ps shall be used to completely describe the current conversion state of the associated character sequence. The implementation shall behave as if no function defined in this volume of POSIX.1-2017 calls wcrtomb().

[CX] [Option Start] The wcrtomb() function need not be thread-safe if called with a NULL ps argument. [Option End]

The behavior of this function shall be affected by the LC_CTYPE category of the current locale.

The wcrtomb() function shall not change the setting of errno if successful.


The wcrtomb() function shall return the number of bytes stored in the array object (including any shift sequences). When wc is not a valid wide character, an encoding error shall occur. In this case, the function shall store the value of the macro [EILSEQ] in errno and shall return (size_t)-1; the conversion state shall be undefined.


The wcrtomb() function shall fail if:

An invalid wide-character code is detected.

The wcrtomb() function may fail if:

[CX] [Option Start] ps points to an object that contains an invalid conversion state. [Option End]

The following sections are informative.










mbsinit, wcsrtombs

XBD <wchar.h>


First released in Issue 5. Included for alignment with ISO/IEC 9899:1990/Amendment 1:1995 (E).

Issue 6

In the DESCRIPTION, a note on using this function in a threaded application is added.

Extensions beyond the ISO C standard are marked.

The normative text is updated to avoid use of the term "must" for application requirements.

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

Issue 7

Austin Group Interpretation 1003.1-2001 #148 is applied, clarifying that the wcrtomb() function need not be thread-safe if called with a NULL ps argument.

Austin Group Interpretation 1003.1-2001 #170 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0714 [88] and XSH/TC1-2008/0715 [105] are applied.

End of informative text.


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