while( ('\0' = *a) && ('\0' = *b) )

while( ('\0' = *a) && ('\0' = *b) )
while( ('\0' = *a) && ('\0' = *b) )

while( ('\0' = *a) && ('\0' = *b) )
*a不等于 ‘\0’ 而且 *b 也不等于'\0'
则执行 while循环
通常 :a b是字符串指针 当 a b指向 字符串(a="sdjkfh",b="urhjk")时,该语句的意思是
当字符*a *b 都不为空时执行 while循环