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mbsrtowcs(3) Library Functions Manual mbsrtowcs(3)


mbsrtowcs - convert a multibyte string to a wide-character string (restartable)


Standard C library (libc, -lc)


#include <wchar.h>
size_t mbsrtowcs(wchar_t dest[restrict .dsize],
                 const char **restrict src,
                 size_t dsize, mbstate_t *restrict ps);


If dest is not NULL, convert the multibyte string *src to a wide-character string starting at dest. At most dsize wide characters are written to dest. The shift state *ps is updated. The conversion is effectively performed by repeatedly calling mbrtowc(dest, *src, n, ps) where n is some positive number, as long as this call succeeds, and then incrementing dest by one and *src by the number of bytes consumed. The conversion can stop for three reasons:

An invalid multibyte sequence has been encountered. In this case, *src is left pointing to the invalid multibyte sequence, (size_t) -1 is returned, and errno is set to EILSEQ.
dsize non-L'\0' wide characters have been stored at dest. In this case, *src is left pointing to the next multibyte sequence to be converted, and the number of wide characters written to dest is returned.
The multibyte string has been completely converted, including the terminating null wide character ('\0'), which has the side effect of bringing back *ps to the initial state. In this case, *src is set to NULL, and the number of wide characters written to dest, excluding the terminating null wide character, is returned.

If dest is NULL, dsize is ignored, and the conversion proceeds as above, except that the converted wide characters are not written out to memory, and that no length limit exists.

In both of the above cases, if ps is NULL, a static anonymous state known only to the mbsrtowcs() function is used instead.

In order to avoid the case 2 above, the programmer should make sure dsize is greater than or equal to mbsrtowcs(NULL,src,0,ps)+1.

The programmer must ensure that there is room for at least dsize wide characters at dest.

This function is a restartable version of mbstowcs(3).


The number of wide characters that make up the converted part of the wide-character string, not including the terminating null wide character. If an invalid multibyte sequence was encountered, (size_t) -1 is returned, and errno set to EILSEQ.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
mbsrtowcs () Thread safety MT-Unsafe race:mbsrtowcs/!ps


C11, POSIX.1-2008.


POSIX.1-2001, C99.


The behavior of mbsrtowcs() depends on the LC_CTYPE category of the current locale.

Passing NULL as ps is not multithread safe.


iconv(3), mbrtowc(3), mbsinit(3), mbsnrtowcs(3), mbstowcs(3)

2024-05-02 Linux man-pages 6.8