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mbstowcs, _mbstowcs_l

Converts a sequence of multibyte characters to a corresponding sequence of wide characters. More secure versions of these functions are available; see mbstowcs_s, _mbstowcs_s_l.


size_t mbstowcs(
   wchar_t *wcstr,
   const char *mbstr,
   size_t count 
);
size_t _mbstowcs_l(
   wchar_t *wcstr,
   const char *mbstr,
   size_t count,
   _locale_t locale
);
template <size_t size>
size_t mbstowcs(
   wchar_t (&wcstr)[size],
   const char *mbstr,
   size_t count 
); // C++ only
template <size_t size>
size_t _mbstowcs_l(
   wchar_t (&wcstr)[size],
   const char *mbstr,
   size_t count,
   _locale_t locale
); // C++ only

[out] wcstr

The address of a sequence of wide characters.

[in] mbstr

The address of a sequence of null terminated multibyte characters.

[in] count

The maximum number of multibyte characters to convert.

[in] locale

The locale to use.

If mbstowcs successfully converts the source string, it returns the number of converted multibyte characters. If the wcstr argument is NULL, the function returns the required size (in wide characters) of the destination string. If mbstowcs encounters an invalid multibyte character, it returns –1. If the return value is count, the wide-character string is not null-terminated.

Security note Security Note:

Ensure that wcstr and mbstr do not overlap, and that count correctly reflects the number of multibyte characters to convert.

The mbstowcs function converts up to a maximum number of count multibyte characters pointed to by mbstr to a string of corresponding wide characters that are determined by the current locale. It stores the resulting wide-character string at the address represented by wcstr. The result is similar to a series of calls to mbtowc. If mbstowcs encounters the single-byte null character ('\0') either before or when count occurs, it converts the null character to a wide-character null character (L'\0') and stops. Thus the wide-character string at wcstr is null-terminated only if a null character is encountered during conversion. If the sequences pointed to by wcstr and mbstr overlap, the behavior is undefined.

If the wcstr argument is NULL, mbstowcs returns the number of wide characters that would result from conversion, not including a null terminator. The source string must be null-terminated for the correct value to be returned. If you need the resulting wide character string to be null-terminated, add one to the returned value.

If the mbstr argument is NULL, or if count is > INT_MAX, the invalid parameter handler is invoked, as described in Parameter Validation . If execution is allowed to continue, errno is set to EINVAL and the function returns -1.

mbstowcs uses the current locale for any locale-dependent behavior; _mbstowcs_l is identical except that it uses the locale passed in instead. For more information, see Locale.

In C++, these functions have template overloads that invoke the newer, secure counterparts of these functions. For more information, see Secure Template Overloads.

Routine

Required header

mbstowcs

<stdlib.h>

_mbstowcs_l

<stdlib.h>

For additional compatibility information, see Compatibility in the Introduction.

// crt_mbstowcs.c
// compile with: /W3
// illustrates the behavior of the mbstowcs function

#include <stdlib.h>
#include <stdio.h>
#include <locale.h>

int main( void )
{
    size_t size;
    int nChar = 2; // number of characters to convert
    int requiredSize;

    unsigned char    *pmbnull  = NULL;
    unsigned char    *pmbhello = NULL;
    char* localeInfo;
    
    wchar_t *pwchello = L"\x3042\x3043"; // 2 Hiragana characters
    wchar_t *pwc;

    /* Enable the Japanese locale and codepage */
    localeInfo = setlocale(LC_ALL, "Japanese_Japan.932");
    printf("Locale information set to %s\n", localeInfo);
    
    printf( "Convert to multibyte string:\n" );

    requiredSize = wcstombs( NULL, pwchello, 0); // C4996
    // Note: wcstombs is deprecated; consider using wcstombs_s
    printf("  Required Size: %d\n", requiredSize);

    /* Add one to leave room for the null terminator. */
    pmbhello = (unsigned char *)malloc( requiredSize + 1);
    if (! pmbhello)
    {
        printf("Memory allocation failure.\n");
        return 1;
    }
    size = wcstombs( pmbhello, pwchello, requiredSize + 1); // C4996
    // Note: wcstombs is deprecated; consider using wcstombs_s
    if (size == (size_t) (-1))
    {
        printf("Couldn't convert string. Code page 932 may"
                " not be available.\n");
        return 1;
    }
    printf( "  Number of bytes written to multibyte string: %u\n",
            (unsigned int) size );
    printf( "  Hex values of the " );
    printf( " multibyte characters: %#.2x %#.2x %#.2x %#.2x\n",
            pmbhello[0], pmbhello[1], pmbhello[2], pmbhello[3] );
    printf( "  Codepage 932 uses 0x81 to 0x9f as lead bytes.\n\n");

    printf( "Convert back to wide-character string:\n" );

    /* Assume we don't know the length of the multibyte string.
     Get the required size in characters, and allocate enough space. */

    requiredSize = mbstowcs(NULL, pmbhello, 0); // C4996
    /* Add one to leave room for the NULL terminator */
    pwc = (wchar_t *)malloc( (requiredSize + 1) * sizeof( wchar_t ));
    if (! pwc)
    {
        printf("Memory allocation failure.\n");
        return 1;
    }
    size = mbstowcs( pwc, pmbhello, requiredSize + 1); // C4996
    if (size == (size_t) (-1))
    {
       printf("Couldn't convert string--invalid multibyte character.\n");
    }
    printf( "  Characters converted: %u\n", (unsigned int)size );
    printf( "  Hex value of first 2" );
    printf( " wide characters: %#.4x %#.4x\n\n", pwc[0], pwc[1] );
    free(pwc);
    free(pmbhello);
}
Locale information set to Japanese_Japan.932
Convert to multibyte string:
  Required Size: 4
  Number of bytes written to multibyte string: 4
  Hex values of the  multibyte characters: 0x82 0xa0 0x82 0xa1
  Codepage 932 uses 0x81 to 0x9f as lead bytes.

Convert back to wide-character string:
  Characters converted: 2
  Hex value of first 2 wide characters: 0x3042 0x3043

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.

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