< c‎ | string‎ | multibyte
Revision as of 17:52, 6 February 2014 by (Talk)

Defined in header <stdlib.h>
size_t wcstombs( char          *dst, const wchar_t          *src, size_t len );
(until C99)
size_t wcstombs( char *restrict dst, const wchar_t *restrict src, size_t len );
(since C99)

Converts a sequence of wide characters from the array whose first element is pointed to by src to its narrow multibyte representation that begins in the initial shift state. Converted characters are stored in the successive elements of the char array pointed to by dst. No more than len bytes are written to the destination array.

Each character is converted as if by a call to wctomb, except that the wctomb's conversion state is unaffected. The conversion stops if:

  • The null character was converted and stored.
  • A wchar_t was found that does not correspond to a valid character in the current C locale.
  • The next multibyte character to be stored would exceed len.



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

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


dst - pointer to narrow character array where the multibyte character will be stored
src - pointer to the first element of a null-terminated wide string to convert
len - number of bytes available in the array pointed to by dst

Return value

On success, returns the number of bytes (including any shift sequences, but excluding the terminating '\0') written to the character array whose first element is pointed to by dst.

On conversion error (if invalid wide character was encountered), returns (size_t)-1.


#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
int main () 
    // 4 wide characters
    const wchar_t src[] = L"z\u00df\u6c34\U0001f34c";
    // they occupy 10 bytes in UTF-8
    char dst[11];
    setlocale(LC_ALL, "en_US.utf8");
    printf("wide-character string: '%ls'\n",src);
    for (size_t ndx=0; ndx < sizeof src/sizeof src[0]; ++ndx)
        printf("   src[%2zu] = %#8x\n", ndx, src[ndx]);
    int rtn_val = wcstombs(dst, src, sizeof dst);
    printf("rtn_val = %d\n", rtn_val);
    if (rtn_val > 0)
        printf("multibyte string:  '%s'\n",dst);
    for (size_t ndx=0; ndx<sizeof dst; ++ndx)
        printf("   dst[%2zu] = %#2x\n", ndx, (unsigned char)dst[ndx]);


wide-character string: 'zß水🍌'
   src[ 0] =     0x7a
   src[ 1] =     0xdf
   src[ 2] =   0x6c34
   src[ 3] =  0x1f34c
   src[ 4] =        0
rtn_val = 10
multibyte string:  'zß水🍌'
   dst[ 0] = 0x7a
   dst[ 1] = 0xc3
   dst[ 2] = 0x9f
   dst[ 3] = 0xe6
   dst[ 4] = 0xb0
   dst[ 5] = 0xb4
   dst[ 6] = 0xf0
   dst[ 7] = 0x9f
   dst[ 8] = 0x8d
   dst[ 9] = 0x8c
   dst[10] =  0

See also

converts a wide string to narrow multibyte character string, given state
(function) [edit]
converts a narrow multibyte character string to wide string
(function) [edit]
C++ documentation for wcstombs