下面通过学习linux 1.2.13源码进一步理解socket通信机制。对该版本源码的学习主要参考《Linux内核网络栈源代码情景分析》（曹桂平 编著）。
要理解socket的本质，就要理解当调用socket函数时，该函数到底创建了什么？返回了什么？
int socket(int family, int type, int protocol);
socket 函数为用户层函数，该函数对应的内核函数为sock_socket(socket.c文件），源码如下：
static int sock_socket(int family, int type, int protocol)
{int i, fd;struct socket *sock;struct proto_ops *ops;/* Locate the correct protocol family. */for (i = 0; i < NPROTO; ++i) {if (pops[i] == NULL) continue;if (pops[i]->family == family) break;}if (i == NPROTO) {return -EINVAL;}ops = pops[i];/** Check that this is a type that we know how to manipulate and* the protocol makes sense here. The family can still reject the* protocol later.*/if ((type != SOCK_STREAM && type != SOCK_DGRAM &&type != SOCK_SEQPACKET && type != SOCK_RAW &&type != SOCK_PACKET) || protocol < 0)return(-EINVAL);/** Allocate the socket and allow the family to set things up. if* the protocol is 0, the family is instructed to select an appropriate* default.*/if (!(sock = sock_alloc())) {printk("NET: sock_socket: no more sockets\n");return(-ENOSR); /* Was: EAGAIN, but we are out ofsystem resources! */}sock->type = type;sock->ops = ops;if ((i = sock->ops->create(sock, protocol)) < 0) {sock_release(sock);return(i);}if ((fd = get_fd(SOCK_INODE(sock))) < 0) {sock_release(sock);return(-EINVAL);}return(fd);
}
sock_socket 函数完成如下工作：
（1）分配socket、sock结构，这两个结构在网络栈的不同层次表示一个套接字连接。
（2）分配inode、file结构用于普通文件操作。
（3）分配一个文件描述符并返回给应用程序作为以后的操作句柄。
sock_alloc 函数用于分配一个inode节点，并返回该节点的socket指针
struct socket *sock_alloc(void)
{struct inode * inode;struct socket * sock;inode = get_empty_inode();if (!inode)return NULL;inode->i_mode = S_IFSOCK;inode->i_sock = 1;inode->i_uid = current->uid;inode->i_gid = current->gid;sock = &inode->u.socket_i;sock->state = SS_UNCONNECTED;sock->flags = 0;sock->ops = NULL;sock->data = NULL;sock->conn = NULL;sock->iconn = NULL;sock->next = NULL;sock->wait = &inode->i_wait;sock->inode = inode; /* "backlink": we could use pointer arithmetic instead */sock->fasync_list = NULL;sockets_in_use++;return sock;
}
inode的定义如下
/* include/fs.h */
struct inode {dev_t i_dev;unsigned long i_ino;umode_t i_mode;nlink_t i_nlink;uid_t i_uid;gid_t i_gid;dev_t i_rdev;off_t i_size;time_t i_atime;time_t i_mtime;time_t i_ctime;unsigned long i_blksize;unsigned long i_blocks;unsigned long i_version;struct semaphore i_sem;struct inode_operations * i_op;struct super_block * i_sb;struct wait_queue * i_wait;struct file_lock * i_flock;struct vm_area_struct * i_mmap;struct inode * i_next, * i_prev;struct inode * i_hash_next, * i_hash_prev;struct inode * i_bound_to, * i_bound_by;struct inode * i_mount;unsigned short i_count;unsigned short i_wcount;unsigned short i_flags;unsigned char i_lock;unsigned char i_dirt;unsigned char i_pipe;unsigned char i_sock;unsigned char i_seek;unsigned char i_update;union {struct pipe_inode_info pipe_i;struct minix_inode_info minix_i;struct ext_inode_info ext_i;struct ext2_inode_info ext2_i;struct hpfs_inode_info hpfs_i;struct msdos_inode_info msdos_i;struct umsdos_inode_info umsdos_i;struct iso_inode_info isofs_i;struct nfs_inode_info nfs_i;struct xiafs_inode_info xiafs_i;struct sysv_inode_info sysv_i;struct socket socket_i;void * generic_ip;} u;
};
inode 结构是文件系统的一个结构体，该结构体中的成员变量u指明了该inode结构具体的文件类型，当inode是用于socket通信时，u的值就为socket_i。sock_alloc 的作用就是创建inode结构体，然后返回socket_i的地址。至于具体如何分配inode涉及到文件系统方面的知识，这里暂不讨论。
当协议族为AF_INET时，ops->create 将调用inet_create(struct socket*sock, int protocol)函数。该函数将创建一个sock结构体并使得socket的data指针指向该sock结构体。
static int inet_create(struct socket *sock, int protocol)
{struct sock *sk;struct proto *prot;int err;sk = (struct sock *) kmalloc(sizeof(*sk), GFP_KERNEL);if (sk == NULL) return(-ENOBUFS);sk->num = 0;sk->reuse = 0;switch(sock->type) {case SOCK_STREAM:case SOCK_SEQPACKET:if (protocol && protocol != IPPROTO_TCP) {kfree_s((void *)sk, sizeof(*sk));return(-EPROTONOSUPPORT);}protocol = IPPROTO_TCP;sk->no_check = TCP_NO_CHECK;prot = &tcp_prot;break;case SOCK_DGRAM:if (protocol && protocol != IPPROTO_UDP) {kfree_s((void *)sk, sizeof(*sk));return(-EPROTONOSUPPORT);}protocol = IPPROTO_UDP;sk->no_check = UDP_NO_CHECK;prot=&udp_prot;break;case SOCK_RAW:if (!suser()) {kfree_s((void *)sk, sizeof(*sk));return(-EPERM);}if (!protocol) {kfree_s((void *)sk, sizeof(*sk));return(-EPROTONOSUPPORT);}prot = &raw_prot;sk->reuse = 1;sk->no_check = 0; /** Doesn't matter no checksum is* performed anyway.*/sk->num = protocol;break;case SOCK_PACKET:if (!suser()) {kfree_s((void *)sk, sizeof(*sk));return(-EPERM);}if (!protocol) {kfree_s((void *)sk, sizeof(*sk));return(-EPROTONOSUPPORT);}prot = &packet_prot;sk->reuse = 1;sk->no_check = 0; /* Doesn't matter no checksum is* performed anyway.*/sk->num = protocol;break;default:kfree_s((void *)sk, sizeof(*sk));return(-ESOCKTNOSUPPORT);}sk->socket = sock;
#ifdef CONFIG_TCP_NAGLE_OFFsk->nonagle = 1;
#else sk->nonagle = 0;
#endif sk->type = sock->type;sk->stamp.tv_sec=0;sk->protocol = protocol;...... sk->timer.function = &net_timer;skb_queue_head_init(&sk->back_log);sk->blog = 0;sock->data =(void *) sk; //socket 指向 socksk->dummy_th.doff = sizeof(sk->dummy_th)/4;......if (sk->prot->init) {err = sk->prot->init(sk);if (err != 0) {destroy_sock(sk);return(err);}}return(0);
}
最后调用get_fd 返回一个文件描述符给上层应用。
/* socket.c */
static int get_fd(struct inode *inode)
{int fd;struct file *file;/** Find a file descriptor suitable for return to the user. */file = get_empty_filp(); // 获取一个闲置的file结构if (!file) return(-1);for (fd = 0; fd < NR_OPEN; ++fd)if (!current->files->fd[fd]) break;if (fd == NR_OPEN) {file->f_count = 0;return(-1);}FD_CLR(fd, ¤t->files->close_on_exec);current->files->fd[fd] = file; file->f_op = &socket_file_ops; // socket 文件操作file->f_mode = 3;file->f_flags = O_RDWR;file->f_count = 1;file->f_inode = inode;if (inode) inode->i_count++;file->f_pos = 0;return(fd);
}
get_fd 用于为网络套接字分配一个文件描述符，分配描述符的同时需要一个file结构，每个file结构都需要一个inode结构对应。内核维护一个file结构数据，get_empty_filp 函数即通过检查该数组，获取一个闲置的成员。f_op 字段的赋值实现了网络操作的普通文件接口。如果调用write、read函数进行操作就会调用相应的sock_read 和 sock_write 函数。
如何根据文件描述如fd找到相应的sock？
