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 of system 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_OFF sk->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 sock sk->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？
