mbstowcs, mbstowcs_s

Defined in header <stdlib.h>
size_t mbstowcs( wchar_t          *dst, const char          *src, size_t len)
(until C99)
size_t mbstowcs( wchar_t *restrict dst, const char *restrict src, size_t len)
(since C99)
errno_t mbstowcs_s(size_t *restrict retval, wchar_t *restrict dst,
                  rsize_t dstsz, const char *restrict src, rsize_t len);
(2) (since C11)
1) Converts a multibyte character string from the array whose first element is pointed to by src to its wide character representation. Converted characters are stored in the successive elements of the array pointed to by dst. No more than len wide characters are written to the destination array.
Each character is converted as if by a call to mbtowc, except that the mbtowc conversion state is unaffected. The conversion stops if:
* The multibyte null character was converted and stored.
* An invalid (in the current C locale) multibyte character was encountered.
* The next wide character to be stored would exceed len.
If src and dst overlap, the behavior is undefined
2) Same as (1), except that
* conversion is as-if by mbrtowc, not mbtowc
* the function returns its result as an out-parameter retval
* if no null character was written to dst after len wide characters were written, then L'\0' is stored in dst[len], which means len+1 total wide characters are written
* if dst is a null pointer, the number of wide characters that would be produced is stored in *retval
* the function clobbers the destination array from the terminating null and until dstsz
* If src and dst overlap, the behavior is unspecified.
* the following errors are detected at runtime and call the currently installed constraint handler function:
  • retval or src is a null pointer
  • dstsz or len is greater than RSIZE_MAX/sizeof(wchar_t) (unless dst is null)
  • dstsz is not zero (unless dst is null)
  • There is no null character in the first dstsz multibyte characters in the src array and len is greater than dstsz (unless dst is null)
As with all bounds-checked functions, mbstowcs_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdlib.h>.


In most implementations, mbstowcs updates a global static object of type mbstate_t as it processes through the string, and cannot be called simultaneously by two threads, mbsrtowcs should be used in such cases.

POSIX specifies a common extension: if dst is a null pointer, this function returns the number of wide characters that would be written to dst, if converted. Similar behavior is standard for mbstowcs_s and for mbsrtowcs.


dst - pointer to wide character array where the wide string will be stored
src - pointer to the first element of a null-terminated multibyte string to convert
len - number of wide characters available in the array pointed to by dst
dstsz - max number of wide characters that will be written (size of the dst array)
retval - pointer to a size_t object where the result will be stored

Return value

1) On success, returns the number of wide characters, excluding the terminating L'\0', written to the destination array. On conversion error (if invalid multibyte character was encountered), returns (size_t)-1.
2) zero on success (in which case the number of wide characters excluding terminating zero that were, or would be written to dst, is stored in *retval), non-zero on error. In case of a runtime constraint violation, stores (size_t)-1 in *retval (unless retval is null) and sets dst[0] to L'\0' (unless dst is null or dstmax is zero or greater than RSIZE_MAX)


#include <stdio.h>
#include <locale.h>
#include <stdlib.h>
#include <wchar.h>
int main(void)
    setlocale(LC_ALL, "en_US.utf8");
    const char* mbstr = u8"z\u00df\u6c34\U0001F34C"; // or u8"zß水🍌"
    wchar_t wstr[5];
    mbstowcs(wstr, mbstr, 5);
    wprintf(L"MB string: %s\n", mbstr);
    wprintf(L"Wide string: %ls\n", wstr);


MB string: zß水🍌
Wide string: zß水🍌


  • C11 standard (ISO/IEC 9899:2011):
    • The mbstowcs function (p: 359)
    • K. The mbstowcs_s function (p: 611-612)
  • C99 standard (ISO/IEC 9899:1999):
    • The mbstowcs function (p: 323)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The mbstowcs function

See also

converts a narrow multibyte character string to wide string, given state
converts a wide string to narrow multibyte character string
C++ documentation for mbstowcs

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