本文分析基于Linux Kernel 1.2.13
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更多请查看专栏，地址http://blog.csdn.net/column/details/linux-kernel-net.html
作者：闫明
注：标题中的”（上）“，”（下）“表示分析过程基于数据包的传递方向：”（上）“表示分析是从底层向上分析、”（下）“表示分析是从上向下分析。
下面是发送数据的流程：
应用层发送数据包的入口函数是BSD socket层的sock_write()函数，在分析该函数之前，先分析下socket的创建，系统调用sys_socket()对应的BSD socket层函数为sock_socket()
sock_socket()函数
/* *Perform the socket system call. we locate the appropriate
*family, then create a fresh socket.
*/
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) //查找对应的协议族
{
if (pops[i] == NULL) continue;
if (pops[i]->family == family)
break;
}
if (i == NPROTO) //查找未果，返回错误
{
return -EINVAL;
}
ops = pops[i];//指针指向该协议族的原型操作函数集
/*
*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())) //获取一个socket，已经完成了socket部分初始化设置
{
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) //调用INET层函数，inet_create()函数，创建inet层的socket，即sock结构
{
sock_release(sock);
return(i);
}
if ((fd = get_fd(SOCK_INODE(sock))) < 0) //根据sock结构中的inode，分配文件描述符
{
sock_release(sock);
return(-EINVAL);
}
return(fd);
}
该函数的大体功能：
1、分配socket,sock结构，用于BSD和INET层的socket
2、分配inode和file结构，用于文件操作
3、返回文件操作描述符，用于应用程序的使用
其中初始化分配一个socket的方法如下：
/* *Allocate a socket. */struct socket *sock_alloc(void){struct inode * inode;struct socket * sock;inode = get_empty_inode();//获一个空的文件结点if (!inode)return NULL;//文件结点相应字段赋值inode->i_mode = S_IFSOCK;inode->i_sock = 1;inode->i_uid = current->uid;inode->i_gid = current->gid;sock = &inode->u.socket_i;//给sicket结构指针赋值，可以看到inode和socket一一对应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;}
执行完，然后调用INET层的inet_create()函数
/* *Create an inet socket. * *FIXME: Gcc would generate much better code if we set the parameters *up in in-memory structure order. Gcc68K even more so *///创建inet socket，即sock结构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);//分配空间if (sk == NULL) return(-ENOBUFS);sk->num = 0;sk->reuse = 0;switch(sock->type) {case SOCK_STREAM:case SOCK_SEQPACKET:.................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;//原型指针指向UDP的原型定义break; case SOCK_RAW:.........................break;case SOCK_PACKET:..........................break;default:kfree_s((void *)sk, sizeof(*sk));return(-ESOCKTNOSUPPORT);}sk->socket = sock;//可以看出sock和socket的对应关系.................sk->type = sock->type;sk->stamp.tv_sec=0;sk->protocol = protocol;sk->wmem_alloc = 0;sk->rmem_alloc = 0;sk->sndbuf = SK_WMEM_MAX;sk->rcvbuf = SK_RMEM_MAX;......................................//sock的初始化/* this is how many unacked bytes we will accept for this socket. */sk->max_unacked = 2048; /* needs to be at most 2 full packets. *//* how many packets we should send before forcing an ack. if this is set to zero it is the same as sk->delay_acks = 0 */sk->max_ack_backlog = 0;sk->inuse = 0;sk->delay_acks = 0;skb_queue_head_init(&sk->write_queue);skb_queue_head_init(&sk->receive_queue);sk->mtu = 576;sk->prot = prot;sk->sleep = sock->wait;sk->daddr = 0;//远端地址sk->saddr = 0 /* ip_my_addr() */;//本地地址sk->err = 0;sk->next = NULL;sk->pair = NULL;sk->send_tail = NULL;//发送链表尾sk->send_head = NULL;//发送链表头..............................skb_queue_head_init(&sk->back_log);//初始化双链表..................................sk->ip_tos=0;sk->ip_ttl=64;................................... if (sk->num) //本地端口号不空{/* * It assumes that any protocol which allows * the user to assign a number at socket * creation time automatically * shares. */put_sock(sk->num, sk);//根据端口号将sock结构加入sock表中sk->dummy_th.source = ntohs(sk->num);}if (sk->prot->init) //UDP的初始化函数为空{err = sk->prot->init(sk);if (err != 0) {destroy_sock(sk);return(err);}}return(0);}
返回文件描述的操作符
/* *Obtains the first available file descriptor and sets it up for use. *///根据文件inode指针创建文件结构，并返回文件操作的操作符，用于应用程序的使用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();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;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);}
下面开始正式看发送数据的最顶层函数--sock_write()函数
/* *Write data to a socket. We verify that the user area ubuf..ubuf+size-1 is *readable by the user process. */static int sock_write(struct inode *inode, struct file *file, char *ubuf, int size){struct socket *sock;int err;if (!(sock = socki_lookup(inode))) //返回inode结构的对应的socket结构{printk("NET: sock_write: can't find socket for inode!\n");return(-EBADF);}if (sock->flags & SO_ACCEPTCON) return(-EINVAL);if(size<0)return -EINVAL;if(size==0)return 0;if ((err=verify_area(VERIFY_READ,ubuf,size))<0) return err;return(sock->ops->write(sock, ubuf, size,(file->f_flags & O_NONBLOCK)));//调用inet_write()函数}
inet_write()函数
static int inet_write(struct socket *sock, char *ubuf, int size, int noblock){return inet_send(sock,ubuf,size,noblock,0);}
inet_send()函数
static int inet_send(struct socket *sock, void *ubuf, int size, int noblock, unsigned flags){struct sock *sk = (struct sock *) sock->data;//从socket结构中取出sock指针if (sk->shutdown & SEND_SHUTDOWN) {send_sig(SIGPIPE, current, 1);return(-EPIPE);}if(sk->err)return inet_error(sk);/* We may need to bind the socket. */if(inet_autobind(sk)!=0)//自动分配本地端口号，并将sk根据端口号加入sock表中return(-EAGAIN);return(sk->prot->write(sk, (unsigned char *) ubuf, size, noblock, flags));//调用udp_write()函数}