Convert char to wchar_t using standard library?

I have a function that expects a wchar_t array as a parameter.I don't know of a standard library function to make a conversion from char to wchar_t so I wrote a quick dirty function, but I want a reliable solution free from bugs and undefined behaviors. Does the standard library have a function that makes this conversion ?

My code:

wchar_t *ctow(const char *buf, wchar_t *output)
{
    const char ANSI_arr[]    =  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789`~!@#$%^&*()-_=+[]{}\\|;:'\",<.>/? \t\n\r\f";
    const wchar_t WIDE_arr[] = L"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789`~!@#$%^&*()-_=+[]{}\\|;:'\",<.>/? \t\n\r\f";

    size_t n = 0, len = strlen(ANSI_arr);

    while (*buf) {
        for (size_t x = 0; x < len; x++) {
            if (*buf == ANSI_arr[x]) {
                output[n++] = WIDE_arr[x];
                break;
            }
        }
        buf++;
    }
    output[n] = L'\0';
    return output;
}

It does in the header wchar.h. It is called btowc:

The btowc function returns WEOF if c has the value EOF or if (unsigned char)c does not constitute a valid single-byte character in the initial shift state. Otherwise, it returns the wide character representation of that character.