BSD socket

The BSD socket is a framework to the different families of socket that Linux supports.
The BSD socket concept is very similar to the VFS (virtual file system) layer.
This way different protocol families are supported by Linux, and their services are accessable to the user using a common socket interface.

VFS

VFS is a framework that provides a common interface to various different file systems/pipe/devices/sockets to the user without user knowing how things are organized inside the kernel.

socket()

用户调用socket函数时会调用内核层的sys_socket函数.sys_socket函数通过socket函数传递来的protocol,family
,type三个参数来创建对应的(TCP/UDP/RAW等)协议栈。

socket(family, type, protocol)函数在内核中执行流程：

socket()->sys_socketcall->sys_socket()->sock_create()->__sock_create()->net_families[?]->create()

asmlinkage long sys_socket(int family, int type, int protocol)
{
    int retval;
    struct socket *sock;
    /*
     * sock_create会调用__sock_create函数
     * 若family为PF_INET,就会调用inet_create函数来初始化socket结构体
     */
    retval = sock_create(family, type, protocol, &sock);
    if (retval < 0)
        goto out;

    /*
     * 把struct socket sock放入到VFS中，返回fd-套接字
     * 图Figure_3.2
     */
    retval = sock_map_fd(sock);
    if (retval < 0)
        goto out_release;
}

net_families:保存所有不同类型的协议族的全局链表

比如PF_INET协议族，inet_family_ops通过sock_regitser函数，把地址复制给net_families[PF_INET]

static struct net_proto_family inet_family_ops = {
    .family = PF_INET,
    .create = inet_create,
    .owner    = THIS_MODULE,
};


int sock_register(struct net_proto_family *ops)
{
    if (net_families[ops->family] == NULL) {
        net_families[ops->family]=ops;
        err = 0;
    }
}   

__sock_create函数

创建套接字时使用协议族参数作为偏移量，从net_families数组中获得协议族指针，进而调用该协议族的创建函数。

若family为PF_INET,就会调用inet_create函数来初始化socket结构体

static int __sock_create(int family, int type, int protocol, struct socket **res, int kern)
{
    /* 若family等于PF_INET, net_families[PF_INET]->create指向inet_create函数 */
    if ((err = net_families[family]->create(sock, protocol)) < 0) {
        sock->ops = NULL;
        goto out_module_put;
    }

}

socket返回一个fd(file descirptor),fd通过VFS可以找到相对应的socket信息。