C套接字sockaddr和sockaddr_storage背后的推理-ReasoningbehindCsocketssockaddrandsockaddr_storage

40  

Functions that expect a pointer to struct sockaddr probably typecast the pointer you send them to sockaddr when you send them a pointer to struct sockaddr_storage. In that way, they access it as if it was a struct sockaddr.

函数期望一个指向struct sockaddr的指针，当你向sockaddr发送一个指向struct sockaddr_storage的指针时，它可能会将你发送给sockaddr的指针进行排版。通过这种方式，他们访问它就好像它是一个结构体sockaddr。

struct sockaddr_storage is designed to fit in both a struct sockaddr_in and struct sockaddr_in6

struct sockaddr_storage被设计成适合于struct sockaddr_in和struct sockaddr_in6。

You don't create your own struct sockaddr, you usually create a struct sockaddr_in or a struct sockaddr_in6 depending on what IP version you're using. In order to avoid trying to know what IP version you will be using, you can use a struct sockaddr_storage which can hold either. This will in turn be typecasted to struct sockaddr by the connect(), bind(), etc functions and accessed that way.

您不会创建自己的struct sockaddr，您通常会创建一个struct sockaddr_in或struct sockaddr_in6，这取决于您使用的IP版本。为了避免尝试知道将要使用的IP版本，您可以使用一个struct sockaddr_storage，它可以保存其中任何一个。然后通过connect()、bind()等函数将其类型设置为struct sockaddr，并通过这种方式进行访问。

You can see all of these structs below (the padding is implementation specific, for alignment purposes):

您可以在下面看到所有这些结构(为对齐目的，填充是特定于实现的):

struct sockaddr {
   unsigned short    sa_family;    // address family, AF_xxx
   char              sa_data[14];  // 14 bytes of protocol address
};


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
};


struct sockaddr_in6 {
    u_int16_t       sin6_family;   // address family, AF_INET6
    u_int16_t       sin6_port;     // port number, Network Byte Order
    u_int32_t       sin6_flowinfo; // IPv6 flow information
    struct in6_addr sin6_addr;     // IPv6 address
    u_int32_t       sin6_scope_id; // Scope ID
};

struct sockaddr_storage {
    sa_family_t  ss_family;     // address family

    // all this is padding, implementation specific, ignore it:
    char      __ss_pad1[_SS_PAD1SIZE];
    int64_t   __ss_align;
    char      __ss_pad2[_SS_PAD2SIZE];
};

So as you can see, if the function expects an IPv4 address, it will just read the first 4 bytes (because it assumes the struct is of type struct sockaddr. Otherwise it will read the full 16 bytes for IPv6).

如您所见，如果函数需要一个IPv4地址，它将只读取前4个字节(因为它假设结构体是struct sockaddr类型。否则它将读取IPv6的全部16字节)。

7  

In C++ classes with at least one virtual function are given a TAG. That tag allows you to dynamic_cast<>() to any of the classes your class derives from and vice versa. The TAG is what allows dynamic_cast<>() to work. More or less, this can be a number or a string...

在c++类中，具有至少一个虚函数的类被赋予一个标记。该标记允许您将dynamic_cast<>()转换为类派生的任何类，反之亦然。这个标记允许dynamic_cast<>()工作。或多或少，这可以是一个数字或一个字符串…

In C we are limited to structures. However, structures can also be assigned a TAG. In fact, if you look at all the structures that theprole posted in his answer, you will notice that they all start with 2 bytes (an unsigned short) which represents what we call the family of the address. This defines exactly what the structure is and thus its size, fields, etc.

在C语言中，我们仅限于结构。但是，也可以为结构分配一个标记。事实上，如果你看一下theprole在他的回答中发布的所有结构，你会发现它们都以2个字节(一个无符号的短)开头，这表示我们所说的地址的族。它准确地定义了结构是什么，因此它的大小、字段等等。

Therefore you can do something like this:

因此你可以这样做:

int bind(int fd, struct sockaddr *in, socklen_t len)
{
  switch(in->sa_family)
  {
  case AF_INET:
    if(len