I'm not cygwin user, but cygwin seems not to support multibyte function.
Following program outputs 5 on VC6 as expected, but 10 on cygwin. 
Hmm...

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

int main(int argc, char* argv[])
{
	const char s[] = "あいうえお";
	size_t len;
	wchar_t *buf;

	setlocale(LC_ALL, "");

	len = strlen(s); /* 10 */

	buf = (wchar_t*)malloc((len+1)*sizeof(wchar_t));

	len = mbstowcs(buf, s, len+1);

	printf("--------> %d\n", len); /* should be 5 */

	return 0;
}