I can see why it is useful to cast sockaddr to sockaddr_in, but I don't understand how this is possible. From what I've read, they're the same size and sockaddr_in is added with sin_zero to make it the same size. I would like to know how the compiler knows where to get the information from sockaddr_in if it is layed out differently to sockaddr.
我可以看出为什么将sockaddr转换为sockaddr_in是有用的，但我不明白这是怎么回事。根据我的阅读，它们的大小相同，sockaddr_in与sin_zero一起添加，使其大小相同。我想知道编译器如何知道从sockaddr_in获取信息的位置，如果它与sockaddr的布局不同。
1 个解决方案
#1
16
It is possible because you normally cast pointers, not the structures themselves. You do what in natural language means "please treat this pointer to a socket structure as a pointer to an internet socket structure instead". Compiler has no problems to re-interpret the pointer.
这是可能的，因为你通常会投射指针，而不是结构本身。你用自然语言做的意思是“请把这个指针作为指向互联网套接字结构的指针替换为套接字结构”。编译器重新解释指针没有问题。
Here is more detailed description taken up from comments:
以下是评论中更详细的描述：
A sockaddr is 16 bytes in size - the first two bytes are the sa_family, and the remaining 14 bytes are the sa_data which is arbitrary data. A sockaddr_in is also 16 bytes in size - the first 2 bytes are the sin_family (always AF_INET), the next 2 bytes are the sin_port, the next 4 bytes are the sin_addr (IP address), and the last 8 bytes are the sin_zero which is unused in IPv4 and provided only to ensure 16 bytes. This way, you can look at sockaddr.sa_family first, and if it is AF_INET then interpret the entire sockaddr as a sockaddr_in.
sockaddr的大小为16个字节 - 前两个字节是sa_family，剩下的14个字节是sa_data，它是任意数据。 sockaddr_in的大小也是16个字节 - 前2个字节是sin_family（总是AF_INET），接下来的2个字节是sin_port，接下来的4个字节是sin_addr（IP地址），最后8个字节是sin_zero在IPv4中未使用，仅提供16个字节。这样，您可以先查看sockaddr.sa_family，如果是AF_INET，则将整个sockaddr解释为sockaddr_in。
A sockaddr_in is not stored inside of sockaddr.sa_data field. The entire sockaddr is the entire sockaddr_in (when sockaddr.sa_family is AF_INET, that is). If you take a sockaddr* pointer and cast it to a sockaddr_in* pointer, then:
sockaddr_in不存储在sockaddr.sa_data字段中。整个sockaddr是整个sockaddr_in（当sockaddr.sa_family是AF_INET时，就是这样）。如果你带一个sockaddr *指针并将其转换为sockaddr_in *指针，那么：
-
sockaddr.sa_familyissockaddr_in.sin_family - sockaddr.sa_family是sockaddr_in.sin_family
- bytes 0-1 of
sockaddr.sa_dataaresockaddr_in.sin_port - sockaddr.sa_data的字节0-1是sockaddr_in.sin_port
- bytes 2-5 are
sockaddr_in.sin_addr - 字节2-5是sockaddr_in.sin_addr
- bytes 6-13 are
sockaddr_in.sin_zero. - 字节6-13是sockaddr_in.sin_zero。
#1
16
It is possible because you normally cast pointers, not the structures themselves. You do what in natural language means "please treat this pointer to a socket structure as a pointer to an internet socket structure instead". Compiler has no problems to re-interpret the pointer.
这是可能的，因为你通常会投射指针，而不是结构本身。你用自然语言做的意思是“请把这个指针作为指向互联网套接字结构的指针替换为套接字结构”。编译器重新解释指针没有问题。
Here is more detailed description taken up from comments:
以下是评论中更详细的描述：
A sockaddr is 16 bytes in size - the first two bytes are the sa_family, and the remaining 14 bytes are the sa_data which is arbitrary data. A sockaddr_in is also 16 bytes in size - the first 2 bytes are the sin_family (always AF_INET), the next 2 bytes are the sin_port, the next 4 bytes are the sin_addr (IP address), and the last 8 bytes are the sin_zero which is unused in IPv4 and provided only to ensure 16 bytes. This way, you can look at sockaddr.sa_family first, and if it is AF_INET then interpret the entire sockaddr as a sockaddr_in.
sockaddr的大小为16个字节 - 前两个字节是sa_family，剩下的14个字节是sa_data，它是任意数据。 sockaddr_in的大小也是16个字节 - 前2个字节是sin_family（总是AF_INET），接下来的2个字节是sin_port，接下来的4个字节是sin_addr（IP地址），最后8个字节是sin_zero在IPv4中未使用，仅提供16个字节。这样，您可以先查看sockaddr.sa_family，如果是AF_INET，则将整个sockaddr解释为sockaddr_in。
A sockaddr_in is not stored inside of sockaddr.sa_data field. The entire sockaddr is the entire sockaddr_in (when sockaddr.sa_family is AF_INET, that is). If you take a sockaddr* pointer and cast it to a sockaddr_in* pointer, then:
sockaddr_in不存储在sockaddr.sa_data字段中。整个sockaddr是整个sockaddr_in（当sockaddr.sa_family是AF_INET时，就是这样）。如果你带一个sockaddr *指针并将其转换为sockaddr_in *指针，那么：
-
sockaddr.sa_familyissockaddr_in.sin_family - sockaddr.sa_family是sockaddr_in.sin_family
- bytes 0-1 of
sockaddr.sa_dataaresockaddr_in.sin_port - sockaddr.sa_data的字节0-1是sockaddr_in.sin_port
- bytes 2-5 are
sockaddr_in.sin_addr - 字节2-5是sockaddr_in.sin_addr
- bytes 6-13 are
sockaddr_in.sin_zero. - 字节6-13是sockaddr_in.sin_zero。