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MBRTOWC(3) Library Functions Manual MBRTOWC(3)

mbrtowc — converts a multibyte character to a wide character (restartable)

#include <wchar.h>

size_t
mbrtowc(wchar_t * restrict wc, const char * restrict s, size_t n, mbstate_t * restrict mbs);

The () function examines at most n bytes of the multibyte character byte string pointed to by s, converts those bytes to a wide character, and stores the wide character in the wchar_t object pointed to by wc if wc is not NULL and s points to a valid character.

Conversion happens in accordance with the conversion state described by the mbstate_t object pointed to by mbs. The mbstate_t object must be initialized to zero before the application's first call to (). If the previous call to mbrtowc() did not return (size_t)-1, the mbstate_t object can safely be reused without reinitialization.

The behaviour of () is affected by the LC_CTYPE category of the current locale. If the locale is changed without reinitialization of the mbstate_t object pointed to by mbs, the behaviour of mbrtowc() is undefined.

Unlike mbtowc(3), () will accept an incomplete byte sequence pointed to by s which does not form a complete character but is potentially part of a valid character. In this case, mbrtowc() consumes all such bytes. The conversion state saved in the mbstate_t object pointed to by mbs will be used to restart the suspended conversion during the next call to mbrtowc().

In state-dependent encodings, s may point to a special sequence of bytes called a “shift sequence”. Shift sequences switch between character code sets available within an encoding scheme. One encoding scheme using shift sequences is ISO/IEC 2022-JP, which can switch e.g. from ASCII (which uses one byte per character) to JIS X 0208 (which uses two bytes per character). Shift sequence bytes correspond to no individual wide character, so () treats them as if they were part of the subsequent multibyte character. Therefore they do contribute to the number of bytes in the multibyte character.

Special cases in interpretation of arguments are as follows:

wc == NULL
The conversion from a multibyte character to a wide character is performed and the conversion state may be affected, but the resulting wide character is discarded.

This can be used to find out how many bytes are contained in the multibyte character pointed to by s.

s == NULL
() ignores wc and n, and behaves equivalent to 
mbrtowc(NULL, "", 1, mbs);

which attempts to use the mbstate_t object pointed to by mbs to start or continue conversion using the empty string as input, and discards the conversion result.

If conversion succeeds, this call always returns zero. Unlike mbtowc(3), the value returned does not indicate whether the current encoding of the locale is state-dependent, i.e. uses shift sequences.

mbs == NULL
() uses its own internal state object to keep the conversion state, instead of an mbstate_t object pointed to by mbs. This internal conversion state is initialized once at program startup. It is not safe to call mbrtowc() again with a NULL mbs argument if mbrtowc() returned (size_t)-1 because at this point the internal conversion state is undefined.

Calling any other functions in never changes the internal conversion state object of ().

0
The bytes pointed to by s form a terminating NUL character. If wc is not NULL, a NUL wide character has been stored in the wchar_t object pointed to by wc.
positive
s points to a valid character, and the value returned is the number of bytes completing the character. If wc is not NULL, the corresponding wide character has been stored in the wchar_t object pointed to by wc.
(size_t)-1
s points to an illegal byte sequence which does not form a valid multibyte character in the current locale. mbrtowc() sets errno to EILSEQ. The conversion state object pointed to by mbs is left in an undefined state and must be reinitialized before being used again.

Because applications using mbrtowc() are shielded from the specifics of the multibyte character encoding scheme, it is impossible to repair byte sequences containing encoding errors. Such byte sequences must be treated as invalid and potentially malicious input. Applications must stop processing the byte string pointed to by s and either discard any wide characters already converted, or cope with truncated input.

(size_t)-2
s points to an incomplete byte sequence of length n which has been consumed and contains part of a valid multibyte character. The character may be completed by calling mbrtowc() again with s pointing to one or more subsequent bytes of the multibyte character and mbs pointing to the conversion state object used during conversion of the incomplete byte sequence.

The mbrtowc() function may cause an error in the following cases:

[]
s points to an invalid multibyte character.
[]
mbs points to an invalid or uninitialized mbstate_t object.

mbrlen(3), mbtowc(3), setlocale(3)

The mbrtowc() function conforms to ISO/IEC 9899/AMD1:1995 (“ISO C90, Amendment 1”). The restrict qualifier is added at ISO/IEC 9899:1999 (“ISO C99”).

mbrtowc() is not suitable for programs that care about internals of the character encoding scheme used by the byte string pointed to by s.

It is possible that mbrtowc() fails because of locale configuration errors. An “invalid” character sequence may simply be encoded in a different encoding than that of the current locale.

The special cases for s == NULL and mbs == NULL do not make any sense. Instead of passing NULL for mbs, mbtowc(3) can be used.

Earlier versions of this man page implied that calling mbrtowc() with a NULL s argument would always set mbs to the initial conversion state. But this is true only if the previous call to mbrtowc() using mbs did not return (size_t)-1 or (size_t)-2. It is recommended to zero the mbstate_t object instead.

February 8, 2016 OpenBSD-6.4