Linux内核源代码情景分析-基于socket的进程间通信

    一、利用插口实现进程间通信的流程示意图如下：

    

                                有连接插口通信流程示意图

    二、client_server_local_sock，利用Unix域协议进行通信的客户端程序和服务端程序。

    我们以一个实际的例子来讲解socket进程间通信。

    先看server端：

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>

int main()
{
	 int server_sockfd, client_sockfd;
	 int server_len, client_len;
	 struct sockaddr_un server_address;
	 struct sockaddr_un client_address;

	 unlink("/tmp/server_socket");//将可能已经存在的同名文件先删除
	 server_sockfd = socket(AF_UNIX, SOCK_STREAM,0);
	 if( server_sockfd < 0)
	 {
		  perror("server");
		  exit(1);
	 }

	 server_address.sun_family = AF_UNIX;
	 strcpy(server_address.sun_path, "/tmp/server_socket");
	 server_len =sizeof(server_address);
	 int result = 0;
	 result = bind(server_sockfd, (struct sockaddr *)&server_address, server_len);
	 if (result < 0)
	 {
		  perror("bind");
		  exit(1);
	 }

	 listen(server_sockfd, 5);
	 while(1)
	 {
		  char ch;
		  printf("server waiting\n");

		  client_len =sizeof(client_address);
		  //client_sockfd = accept(server_sockfd, (struct sockaddr *)&client_address, &client_len);
		  client_sockfd = accept(server_sockfd, NULL, NULL);//返回的是newsock对应的文件号
		 if (client_sockfd < 0)
		 {
			  perror("accept");
			  exit(1);
		 }


		  read(client_sockfd, &ch, 1);//client_sockfd是newsock对应的文件号
		  ch++;
		  write(client_sockfd, &ch, 1);
		  close(client_sockfd);
	 }
}

    再看client端：

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>

int main()
{
	 int sockfd;
	 int len;
	 struct sockaddr_un address;
	 int result;
	 char ch = 'A';

	 sockfd =socket(AF_UNIX, SOCK_STREAM, 0);
	 if (sockfd < 0 )
	 {
		  perror("socket");
		  exit(1);
	 }

	 address.sun_family = AF_UNIX;
	 strcpy(address.sun_path, "/tmp/server_socket");
	 len = sizeof(address);

	 result = connect(sockfd, (struct sockaddr *)&address, len);

	 if(result < 0)
	 {
		  perror("oops: client");
		  exit(1);
	 }

	 write(sockfd, &ch,1);
	 read(sockfd, &ch, 1);
	 printf("char from server = %c\n", ch);
	 close(sockfd);
	 exit(0);
}

    首先执行./server，得到的结果是：

jltxgcy@jltxgcy-desktop:~/Desktop/client_server_local_sock-master/server$ ./server
server waiting

    然后执行./client，得到的结果是：

jltxgcy@jltxgcy-desktop:~/Desktop/client_server_local_sock-master/client$ ./client
char from server = B

    此时server端又一次输出server waiting，继续等待来自client端的请求。

    三、我们先来看server端，socket(AF_UNIX, SOCK_STREAM,0)，如上图所示为创建插口。其系统调用为：

asmlinkage long sys_socket(int family, int type, int protocol)//family为AF_UNIX,type为SOCK_STREAM，protocol为0
{
	int retval;
	struct socket *sock;

	retval = sock_create(family, type, protocol, &sock);
	if (retval < 0)
		goto out;

	retval = sock_map_fd(sock);
	if (retval < 0)
		goto out_release;

out:
	/* It may be already another descriptor 8) Not kernel problem. */
	return retval;

out_release:
	sock_release(sock);
	return retval;
}

    前面说过，插口对于用户程序而言就是特殊的已打开文件。内核中为插口定义了一种特殊的文件类型，形成一种特殊的文件系统sockfs，如下:

static struct vfsmount *sock_mnt;
static DECLARE_FSTYPE(sock_fs_type, "sockfs", sockfs_read_super,
	FS_NOMOUNT|FS_SINGLE);

    在系统初始化时，要通过kern_mount()安装这个文件系统。安装时有个作为连接件的vfsmount数据结构，这个结构的地址就保存在一个全局的指针sock_mnt中。具体过程可参考 Linux内核源代码情景分析-特殊文件系统/proc。

    在sys_socket中，我们首先调用sock_create，建立一个socket数据结构，然后调用sock_map_fd将其”映射“到一个已打开文件中。sock_create如下：

int sock_create(int family, int type, int protocol, struct socket **res)
{
	int i;
	struct socket *sock;

	/*
	 *	Check protocol is in range
	 */
	if(family<0 || family>=NPROTO)
		return -EAFNOSUPPORT;

	/* Compatibility.

	   This uglymoron is moved from INET layer to here to avoid
	   deadlock in module load.
	 */
	if (family == PF_INET && type == SOCK_PACKET) {//我们只关心Unix域，也就是family为AF_UNIX
		static int warned; 
		if (!warned) {
			warned = 1;
			printk(KERN_INFO "%s uses obsolete (PF_INET,SOCK_PACKET)\n", current->comm);
		}
		family = PF_PACKET;
	}
		
#if defined(CONFIG_KMOD) && defined(CONFIG_NET)//不关心条件编译
	/* Attempt to load a protocol module if the find failed. 
	 * 
	 * 12/09/1996 Marcin: But! this makes REALLY only sense, if the user 
	 * requested real, full-featured networking support upon configuration.
	 * Otherwise module support will break!
	 */
	if (net_families[family]==NULL)
	{
		char module_name[30];
		sprintf(module_name,"net-pf-%d",family);
		request_module(module_name);
	}
#endif

	net_family_read_lock();
	if (net_families[family] == NULL) {//系统初始化或安装模块时，要将指向相应网域的这个数据结构的指针填入一个数组net_families[]中，否则就说明系统尚不支持给定的网域，Unix域的net_proto_family数据结构为unix_family_ops
		i = -EAFNOSUPPORT;
		goto out;
	}

/*
 *	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())) //分配一个socket数据结构并进行一些初始化
	{
		printk(KERN_WARNING "socket: no more sockets\n");
		i = -ENFILE;		/* Not exactly a match, but its the
					   closest posix thing */
		goto out;
	}

	sock->type  = type;//AF_UNIX

	if ((i = net_families[family]->create(sock, protocol)) < 0) //创建插口sock结构
	{
		sock_release(sock);
		goto out;
	}

	*res = sock;

out:
	net_family_read_unlock();
	return i;//返回fd
}

    unix_family_ops，如下：

struct net_proto_family unix_family_ops = {
	PF_UNIX,
	unix_create
};

    继续阅读代码，sock_alloc，分配一个socket数据结构并进行一些初始化，代码如下：

struct socket *sock_alloc(void)
{
	struct inode * inode;
	struct socket * sock;

	inode = get_empty_inode();
	if (!inode)
		return NULL;

	inode->i_sb = sock_mnt->mnt_sb;
	sock = socki_lookup(inode);

	inode->i_mode = S_IFSOCK|S_IRWXUGO;//i_mode设置成S_IFSOCK
	inode->i_sock = 1;//i_sock设置成1
	inode->i_uid = current->fsuid;
	inode->i_gid = current->fsgid;

	sock->inode = inode;
	init_waitqueue_head(&sock->wait);
	sock->fasync_list = NULL;
	sock->state = SS_UNCONNECTED;//刚分配socket时的状态
	sock->flags = 0;
	sock->ops = NULL;
	sock->sk = NULL;
	sock->file = NULL;

	sockets_in_use[smp_processor_id()].counter++;
	return sock;
}

    由如下代码可见，取得一个inode结构是取得一个socket结构的必要条件。不仅如此，socket结构其实只是inode结构中的一部分。

extern __inline__ struct socket *socki_lookup(struct inode *inode)
{
	return &inode->u.socket_i;
}

    其中，socket数据结构如下：

struct socket
{
	socket_state		state;

	unsigned long		flags;
	struct proto_ops	*ops;
	struct inode		*inode;
	struct fasync_struct	*fasync_list;	/* Asynchronous wake up list	*/
	struct file		*file;		/* File back pointer for gc	*/
	struct sock		*sk;
	wait_queue_head_t	wait;

	short			type;
	unsigned char		passcred;
};

    返回到sock_create，继续执行net_families[family]->create，创建插口sock结构，我们刚才已经知道了这个create指向unix_create，代码如下：

static int unix_create(struct socket *sock, int protocol)
{
	if (protocol && protocol != PF_UNIX)
		return -EPROTONOSUPPORT;

	sock->state = SS_UNCONNECTED;

	switch (sock->type) {
	case SOCK_STREAM://有连接的socket通信
		sock->ops = &unix_stream_ops;//执行了unix_stream_ops
		break;
		/*
		 *	Believe it or not BSD has AF_UNIX, SOCK_RAW though
		 *	nothing uses it.
		 */
	case SOCK_RAW:
		sock->type=SOCK_DGRAM;
	case SOCK_DGRAM:
		sock->ops = &unix_dgram_ops;
		break;
	default:
		return -ESOCKTNOSUPPORT;
	}

	return unix_create1(sock) ? 0 : -ENOMEM;
}

    unix_stream_ops，代码如下：

struct proto_ops unix_stream_ops = {
	family:		PF_UNIX,
	
	release:	unix_release,
	bind:		unix_bind,
	connect:	unix_stream_connect,
	socketpair:	unix_socketpair,
	accept:		unix_accept,
	getname:	unix_getname,
	poll:		unix_poll,
	ioctl:		unix_ioctl,
	listen:		unix_listen,
	shutdown:	unix_shutdown,
	setsockopt:	sock_no_setsockopt,
	getsockopt:	sock_no_getsockopt,
	sendmsg:	unix_stream_sendmsg,
	recvmsg:	unix_stream_recvmsg,
	mmap:		sock_no_mmap,
};

    然后调用unix_create1，进一步创建sock结构，代码如下：

static struct sock * unix_create1(struct socket *sock)
{
	struct sock *sk;

	if (atomic_read(&unix_nr_socks) >= 2*files_stat.max_files)
		return NULL;

	MOD_INC_USE_COUNT;
	sk = sk_alloc(PF_UNIX, GFP_KERNEL, 1);//分配了一个插口sock结构
	if (!sk) {
		MOD_DEC_USE_COUNT;
		return NULL;
	}

	atomic_inc(&unix_nr_socks);

	sock_init_data(sock,sk);

	sk->write_space		=	unix_write_space;//指向了unix_write_space

	sk->max_ack_backlog = sysctl_unix_max_dgram_qlen;
	sk->destruct = unix_sock_destructor;
	sk->protinfo.af_unix.dentry=NULL;
	sk->protinfo.af_unix.mnt=NULL;
	sk->protinfo.af_unix.lock = RW_LOCK_UNLOCKED;
	atomic_set(&sk->protinfo.af_unix.inflight, 0);
	init_MUTEX(&sk->protinfo.af_unix.readsem);/* single task reading lock */
	init_waitqueue_head(&sk->protinfo.af_unix.peer_wait);
	sk->protinfo.af_unix.list=NULL;
	unix_insert_socket(&unix_sockets_unbound, sk);//一个sock数据结构插入到一个unix_socket结构队列

	return sk;
}

    每个插口都有个socket数据结构，同时又有一个sock数据结构，后者可以说是对前者的一种补充，两者间是一对一的对应关系。在socket结构中有个指针sk执向其对应的sock结构，而在sock结构中则有个指针socket指向其对应的socket结构，所以两者可以说是同一个东西的两个侧面。分配了sock结构以后就是它的初始化，sock_init_data()的代码如下：

void sock_init_data(struct socket *sock, struct sock *sk)
{
	skb_queue_head_init(&sk->receive_queue);//重要，sk_buff_head数据结构
	skb_queue_head_init(&sk->write_queue);//重要，sk_buff_head数据结构
	skb_queue_head_init(&sk->error_queue);

	init_timer(&sk->timer);
	
	sk->allocation	=	GFP_KERNEL;
	sk->rcvbuf	=	sysctl_rmem_default;//64KB
	sk->sndbuf	=	sysctl_wmem_default;
	sk->state 	= 	TCP_CLOSE;//插口的状态刚开始为TCP_CLOSE
	sk->zapped	=	1;
	sk->socket	=	sock;//互相指向

	if(sock)
	{
		sk->type	=	sock->type;
		sk->sleep	=	&sock->wait;
		sock->sk	=	sk;//互相指向
	} else
		sk->sleep	=	NULL;

	sk->dst_lock		=	RW_LOCK_UNLOCKED;
	sk->callback_lock	=	RW_LOCK_UNLOCKED;

	sk->state_change	=	sock_def_wakeup;
	sk->data_ready		=	sock_def_readable;//重要
	sk->write_space		=	sock_def_write_space;
	sk->error_report	=	sock_def_error_report;
	sk->destruct            =       sock_def_destruct;

	sk->peercred.pid 	=	0;
	sk->peercred.uid	=	-1;
	sk->peercred.gid	=	-1;
	sk->rcvlowat		=	1;
	sk->rcvtimeo		=	MAX_SCHEDULE_TIMEOUT;
	sk->sndtimeo		=	MAX_SCHEDULE_TIMEOUT;

	atomic_set(&sk->refcnt, 1);
}

    在sock结构中有几个双向链队列，