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


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


#include <wchar.h>

size_t mbrtowc(wchar_t *restrict
pwc, const char *restrict s,
n, 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 mbrtowc() function shall be equivalent to the call:

mbrtowc(NULL, "", 1, ps)

In this case, the values of the arguments pwc and n are ignored.

If s is not a null pointer, the mbrtowc() function shall inspect at most n bytes beginning at the byte pointed to by s to determine the number of bytes needed to complete the next character (including any shift sequences). If the function determines that the next character is completed, it shall determine the value of the corresponding wide character and then, if pwc is not a null pointer, shall store that value in the object pointed to by pwc. If the corresponding wide character is the null wide character, the resulting state described shall be the initial conversion state.

If ps is a null pointer, the mbrtowc() function shall use its own internal mbstate_t object, which shall be 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 mbrtowc().

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

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

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


The mbrtowc() function shall return the first of the following that applies:

If the next n or fewer bytes complete the character that corresponds to the null wide character (which is the value stored).
between 1 and n inclusive
If the next n or fewer bytes complete a valid character (which is the value stored); the value returned shall be the number of bytes that complete the character.
If the next n bytes contribute to an incomplete but potentially valid character, and all n bytes have been processed (no value is stored). When n has at least the value of the {MB_CUR_MAX} macro, this case can only occur if s points at a sequence of redundant shift sequences (for implementations with state-dependent encodings).
If an encoding error occurs, in which case the next n or fewer bytes do not contribute to a complete and valid character (no value is stored). In this case, [EILSEQ] shall be stored in errno and the conversion state is undefined.


The mbrtowc() function shall fail if:

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

The mbrtowc() 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, mbsrtowcs

XBD <wchar.h>


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

Issue 6

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

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

ISO/IEC 9899:1999 standard, Technical Corrigendum 1 is incorporated.

Issue 7

Austin Group Interpretation 1003.1-2001 #170 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0369 [109,105] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0204 [663,674] is applied.

End of informative text.


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