mbstowcs — convert a multibyte string to a wide-character string


#include <stdlib.h>
size_t mbstowcs( wchar_t *dest,
  const char *src,
  size_t n);


If dest is not a NULL pointer, the mbstowcs() function converts the multibyte string src to a wide-character string starting at dest. At most n wide characters are written to dest. The conversion starts in the initial state. The conversion can stop for three reasons:

  1. An invalid multibyte sequence has been encountered. In this case (size_t) −1 is returned.

  2. n non-L'\0' wide characters have been stored at dest. In this case the number of wide characters written to dest is returned, but the shift state at this point is lost.

  3. The multibyte string has been completely converted, including the terminating null wide character ('\0'). In this case the number of wide characters written to dest, excluding the terminating null wide character, is returned.

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

If dest is NULL, n 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 order to avoid the case 2 above, the programmer should make sure n is greater or equal to mbstowcs(NULL,src,0)+1.


The mbstowcs() function returns 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.




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

The function mbsrtowcs(3) provides a better interface to the same functionality.


mbsrtowcs(3), wcstombs(3)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) Bruno Haible <>

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References consulted:
  GNU glibc-2 source code and manual
  Dinkumware C library reference
  OpenGroup's Single UNIX specification
  ISO/IEC 9899:1999