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Technical Reference: Base Operating System and Extensions, Volume 1

mbrtowc Subroutine


Convert a character to a wide-character code (restartable)


Standard Library (libc.a)


#include <wchar.h>

size_t mbrtowc (wchar_t * pwc, const char * s, size_t n, mbstate_t * ps) ;


If s is a null pointer, the mbrtowc function is 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 inspects 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 determines the value of the corresponding wide-character and then, if pwc is not a null pointer, stores that value in the object pointed to by pwc. If the corresponding wide-character is the null wide-character, the resulting state described is the initial conversion state.

If ps is a null pointer, the mbrtowc function uses its own internal mbstate_t object, which is initialized at program startup to the initial conversion state. Otherwise, the mbstate_t object pointed to by ps is used to completely describe the current conversion state of the associated character sequence. The implementation will behave as if no function defined in this specification calls mbrtowc.

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

Return Values

The mbrtowc function returns the first of the following that applies:

0 If the next n or fewer bytes complete the character that corresponds to the null wide-character (which is the value stored).
positive If the next n or fewer bytes complete a valid character (which is the value stored); the value returned is the number of bytes that complete the character.
(size_t)-2 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).
(size_t)-1 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 is stored in errno and the conversion state is undefined.

Error Codes

The mbrtowc function may fail if:

EINVAL ps points to an object that contains an invalid conversion state.
EILSEQ Invalid character sequence is detected.

Related Information

The mbsinit (mbsinit Subroutine) subroutine.

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