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Technical Reference: Base Operating System and Extensions, Volume 1

mbsrtowcs Subroutine


Convert a character string to a wide-character string (restartable).


Standard Library (libc.a)


#include <wchar.h>

size_t mbsrtowcs ((wchar_t * dst, const char ** src, size_t len, mbstate_t * ps) ;


The mbsrtowcs function converts a sequence of characters, beginning in the conversion state described by the object pointed to by ps, from the array indirectly pointed to by src into a sequence of corresponding wide-characters. If dst is not a null pointer, the converted characters are stored into the array pointed to by dst. Conversion continues up to and including a terminating null character, which is also stored. Conversion stops early in either of the following cases:

Each conversion takes place as if by a call to the mbrtowc function.

If dst is not a null pointer, the pointer object pointed to by src is assigned either a null pointer (if conversion stopped due to reaching a terminating null character) or the address just past the last character converted (if any). If conversion stopped due to reaching a terminating null character, and if dst is not a null pointer, the resulting state described is the initial conversion state.

If ps is a null pointer, the mbsrtowcs function uses its own internal mbstate_t object, which is initialised 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 mbsrtowcs.

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

Return Values

If the input conversion encounters a sequence of bytes that do not form a valid character, an encoding error occurs. In this case, the mbsrtowcs function stores the value of the macro EILSEQ in errno and returns (size_t)-1); the conversion state is undefined. Otherwise, it returns the number of characters successfully converted, not including the terminating null (if any).

Error Codes

The mbsrtowcs function may fail if:

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

Implementation Specifics

This subroutine is part of Base Operating System (BOS) subroutine.

Related Information

The mbsinit (mbsinit Subroutine) subroutine, mbrtowc (mbrtowc Subroutine) subroutine.

The wchar.h file.

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