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。
上篇分析了应用层经过BSD socket层到INET socket层的函数调用关系和数据的处理流程，INET层会调用具体的传输层协议，还是以UDP协议为例
udp_write()函数
static int udp_write(struct sock *sk, unsigned char *buff, int len, int noblock,
unsigned flags)
{
return(udp_sendto(sk, buff, len, noblock, flags, NULL, 0));
} |
在分析udp_sendto()函数之前，先了解一下sockaddr_in结构，这是标准的网络接口地址结构的定义
|
struct sockaddr_in {
short int sin_family; /* Address family 地址族 */
unsigned short int sin_port; /* Port number 端口号 */
struct in_addr sin_addr; /* Internet address 网络地址 */
/* Pad to size of `struct sockaddr'. */
unsigned char __pad[__SOCK_SIZE__ - sizeof(short int) -
sizeof(unsigned short int) - sizeof(struct in_addr)];
};
#define sin_zero __pad /* for BSD UNIX comp. -FvK */ |
udp_sentdto()函数
|
static int udp_sendto(struct sock *sk, unsigned char *from, int len, int noblock,
unsigned flags, struct sockaddr_in *usin, int addr_len)
{
struct sockaddr_in sin;
int tmp;
/*
* Check the flags. We support no flags for UDP sending
*/
if (flags&~MSG_DONTROUTE)
return(-EINVAL);
/*
* Get and verify the address.
*/
if (usin) //如果usin不是空
{
if (addr_len < sizeof(sin))
return(-EINVAL);
memcpy(&sin,usin,sizeof(sin));
if (sin.sin_family && sin.sin_family != AF_INET)
return(-EINVAL);
if (sin.sin_port == 0)
return(-EINVAL);
}
else //usin为空
{
if (sk->state != TCP_ESTABLISHED)
return(-EINVAL);
sin.sin_family = AF_INET;//协议族
sin.sin_port = sk->dummy_th.dest;//目的端口
sin.sin_addr.s_addr = sk->daddr;//目的地址
}
/*
* BSD socket semantics. You must set SO_BROADCAST to permit
* broadcasting of data.
*/
if(sin.sin_addr.s_addr==INADDR_ANY)//目的地址是全0地址，对应当前主机
sin.sin_addr.s_addr=ip_my_addr();//将目的地址设为当前主机的网络地址
if(!sk->broadcast && ip_chk_addr(sin.sin_addr.s_addr)==IS_BROADCAST)
return -EACCES; /* Must turn broadcast on first */
sk->inuse = 1;
/* Send the packet. */
tmp = udp_send(sk, &sin, from, len, flags);//调用udp_send()真正的发送数据
/* The datagram has been sent off. Release the socket. */
release_sock(sk);
return(tmp);
} |
