From cppreference.com
Defined in header
size_tmbstowcs(wchar_t*dst,constchar*src,size_tlen) (1)(until C99)size_tmbstowcs(wchar_t*restrictdst,constchar*restrictsrc,size_tlen)(since C99)errno_tmbstowcs_s(size_t*restrictretval,wchar_t*restrictdst,rsize_tdstsz,constchar*restrictsrc,rsize_tlen); (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
, 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
, not
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
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
.Notes
In most implementations, mbstowcs updates a global static object of type
as it processes through the string, and cannot be called simultaneously by two threads,
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
.
Parameters
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)
Example
Run this code
#include<locale.h>#include<stdio.h>#include<stdlib.h>#include<wchar.h>intmain(void){setlocale(LC_ALL,"en_US.utf8");constchar*mbstr=u8"z\u00df\u6c34\U0001F34C";// or u8"zß水🍌"wchar_twstr[5];mbstowcs(wstr,mbstr,5);wprintf(L"MB string: %s\n",mbstr);wprintf(L"Wide string: %ls\n",wstr);}Output:
MB string: zß水🍌 Wide string: zß水🍌 References
C23 standard (ISO/IEC 9899:2024):
7.22.8.1 The mbstowcs function (p: TBD)
K.3.6.5.1 The mbstowcs_s function (p: TBD)
C17 standard (ISO/IEC 9899:2018):
7.22.8.1 The mbstowcs function (p: TBD)
K.3.6.5.1 The mbstowcs_s function (p: TBD)
C11 standard (ISO/IEC 9899:2011):
7.22.8.1 The mbstowcs function (p: 359)
K.3.6.5.1 The mbstowcs_s function (p: 611-612)
C99 standard (ISO/IEC 9899:1999):
7.20.8.1 The mbstowcs function (p: 323)
C89/C90 standard (ISO/IEC 9899:1990):
4.10.8.1 The mbstowcs function
See also