string/wcstombs.c

#include <windows.h>

size_t wcstombs(char *mbstr, const wchar_t *wcstr, size_t count) {
	if(!mbstr) return (size_t)(WideCharToMultiByte(CP_ACP, 0, wcstr, -1, NULL, 0, NULL, NULL) - 1);
	if(!count) return 0;

	const wchar_t *t = wcstr;
	while(*t && t - wcstr < count) t++;
	SetLastError(0);
	int r = WideCharToMultiByte(CP_ACP, 0, wcstr, ((size_t)(t - wcstr) < count) ? -1 : count, mbstr, count, NULL, NULL);
	if(r) return mbstr[r - 1] ? r : r - 1;
	if(GetLastError()) return (size_t)-1;		// Error
	return 0;
}