problem description:

casts struct struct type data into unsigned long

The reason why

cannot be converted is that the structural Type (including union) is not a Scalar Type; Conversion is only possible between quantitative types. Quantitative types include arithmetic types and pointer types, and arithmetic types include integer types and floating point types.

even if it’s


struct in_addr type:

struct in_addr  {     in_addr_t s_addr; }; in_addr_t 一般为 32位的unsigned int，其字节顺序为网络顺序（network byte ordered)，即该无符号整数采用大端字节序。 这里即使内存中是unsigned int 类型的，也不能实际使用成unsigned int，因为整体类型不一样，c语言禁止强制转换，强制转换是'低级类型'，转'高级类型'； 但是可以使用指针强制将这段内存解释成unsigned int类型的数据。 即unsigned int a1 = *(unsigned int *)&((struct in_addr) a2);