char vs signed char vs unsigned char

gistfile1.txt

void foo(unsigned char c)
{
	printf("void test( unsigned char c ): %d\n", c);
}
void foo(signed char c)
{
	printf("void test( signed char c ): %d\n", c );
}

void test()
{
  char _c = '_';
  signed char sc = 's';
  unsigned char uc = 'u';
  foo(_c); // error C2668: 'test' : 오버로드된 함수에 대한 호출이 모호합니다.
}