wcstombs, wcstombs_s

Defined in header `<stdlib.h>`
	(1)
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)
errno_t wcstombs_s( size_t restrict retval, char restrict dst, rsize_t dstsz, const wchar_t *restrict src, rsize_t len );		(2)	(since C11)

1) 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 L'\0' was converted and stored. The bytes stored in this case are the unshift sequence (if necessary) followed by '\0',

* 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.

If src and dst overlap, the behavior is unspecified.

2) Same as (1), except that

* conversion is as-if by wcrtomb, not wctomb

* the function returns its result as an out-parameter retval

* if the conversion stops without writing a null character, the function will store '\0' in the next byte in dst, which may be dst[len] or dst[dstsz], whichever comes first (meaning up to len+1/dstsz+1 total bytes may be written). In this case, there may be no unshift sequence written before the terminating null.

* if dst is a null pointer, the number of bytes 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 (unless dst is null)
dstsz is not zero (unless dst is null)
len is greater than dstsz and the conversion does not encounter null or encoding error in the src array by the time dstsz is reached (unless dst is null)

As with all bounds-checked functions, wcstombs_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>.

Notes

In most implementations, wcstombs updates a global static object of type mbstate_t as it processes through the string, and cannot be called simultaneously by two threads, wcsrtombs or wcstombs_s 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 and wcstombs_s.

Parameters

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
dstsz	-	max number of bytes 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 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.

2) Returns zero on success (in which case the number of bytes 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 '\0' (unless dst is null or dstmax is zero or greater than RSIZE_MAX)

Example

Run this code

#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
 
int main(void)
{
    // 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]);
}

Output:

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

References

C11 standard (ISO/IEC 9899:2011):

7.22.8.2 The wcstombs function (p: 360)

K.3.6.5.2 The wcstombs_s function (p: 612-614)

C99 standard (ISO/IEC 9899:1999):

7.20.8.2 The wcstombs function (p: 324)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.8.2 The wcstombs function

Compiler support
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Technical Specifications
Symbol index

wcsrtombswcsrtombs_s (C95)(C11)	converts a wide string to narrow multibyte character string, given state (function)
mbstowcsmbstowcs_s (C11)	converts a narrow multibyte character string to wide string (function)
C++ documentation for wcstombs