How kernels recognize sin_zero (sockaddr_in structure) pushed

问题

According to https://stackoverflow.com/questions/28277079/need-i-push-sin-zero-sockaddr-in, it's my election reserve the space of sin_zero in the stack (sub esp, 8) or not. If I push, ok, I've pushed 16 bytes (sizeof(sockaddr_in)), but if I don't reserve the 8 bytes of sin_zero how kernels know?

struct sockaddr_in {
    short            sin_family;   // e.g. AF_INET, AF_INET6
    unsigned short   sin_port;     // e.g. htons(3490)
    struct in_addr   sin_addr;     // see struct in_addr, below
    char             sin_zero[8];  // zero this if you want to
}; 

来源：https://stackoverflow.com/questions/28280581/how-kernels-recognize-sin-zero-sockaddr-in-structure-pushed