File descriptors in the Linux kernel (Part 2) --sockets and file descriptors-EEWORLD

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Kernel version：2.6.14

CPU architecture：ARM920T

Author：ce123(http://blog.csdn.net/ce123)

Sockets are closely related to the file system. We can operate sockets through the file system's open, read, write, and close functions. The following is a simple example.

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/****************************************************************************/
/*Introduction: TCPServer Example */
/****************************************************************************/
#include
#include
#include
#include
#include
#include
#include
#include
int main(int argc, char *argv[])
{
int sockfd,new_fd;
struct sockaddr_in server_addr;
struct sockaddr_in client_addr;
int sin_size,portnumber;
const char hello[]="Hello\n";
if(argc!=2)
{
fprintf(stderr,"Usage:%s portnumber\a\n",argv[0]);
exit(1);
}
if((portnumber=atoi(argv[1]))<0)
{
fprintf(stderr,"Usage:%s portnumber\a\n",argv[0]);
exit(1);
}
/* The server starts to create a socket descriptor */
if((sockfd=socket(AF_INET,SOCK_STREAM,0))==-1)
{
fprintf(stderr,"Socket error:%s\n\a",strerror(errno));
exit(1);
}
/* Server side fills in sockaddr structure */
bzero(&server_addr,sizeof(struct sockaddr_in));
server_addr.sin_family=AF_INET;
server_addr.sin_addr.s_addr=htonl(INADDR_ANY);
server_addr.sin_port=htons(portnumber);
/* bind sockfd descriptor */
if(bind(sockfd,(struct sockaddr *)(&server_addr),sizeof(struct sockaddr))==
-1)
{
fprintf(stderr,"Bind error:%s\n\a",strerror(errno));
exit(1);
}
/* Listen to sockfd descriptor */
if(listen(sockfd,5)==-1)
{
fprintf(stderr,"Listen error:%s\n\a",strerror(errno));
exit(1);
}
while(1)
{
/* The server blocks until the client program establishes a connection */
sin_size=sizeof(struct sockaddr_in);
if((new_fd=accept(sockfd,(struct sockaddr *)(&client_addr),&sin_size))==-1)
{
fprintf(stderr,"Accept error:%s\n\a",strerror(errno));
exit(1);
}
fprintf(stderr,"Server get connection from %s\n",
inet_ntoa(client_addr.sin_addr));
if(write(new_fd,hello,strlen(hello))==-1)
{
fprintf(stderr,"Write Error:%s\n",strerror(errno));
exit(1);
}
/* This communication has ended */
close(new_fd);
/* loop next */
}
close(sockfd);
exit(0);
}

The following figure illustrates how socket and fd are connected.

Let's analyze it in detail. sys_socket is the main entry point for socket related functions.

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net/socket.c
/*
* System call vectors.
*
* Argument checking cleaned up. Saved 20% in size.
* This function doesn't need to set the kernel lock because
* it is set by the callees.
*/
asmlinkage long sys_socketcall(int call, unsigned long __user *args)
{
unsigned long a[6];
unsigned long a0,a1;
int err;
if(call<1||call>SYS_RECVMSG)
return -SINGLE SELECT;
/* copy_from_user should be SMP safe. */
if (copy_from_user(a, args, nargs[call]))
return -EFAULT;
err = audit_socketcall(nargs[call]/sizeof(unsigned long), a);
if (err)
return err;
a0=a[0];
a1=a[1];
switch(call)
{
case SYS_SOCKET:
err = sys_socket(a0,a1,a[2]);
break;
case SYS_BIND:
err = sys_bind(a0,(struct sockaddr __user *)a1, a[2]);
break;
case SYS_CONNECT:
err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]);
break;
case SYS_LISTEN:
err = sys_listen(a0,a1);
break;
case SYS_ACCEPT:
err = sys_accept(a0,(struct sockaddr __user *)a1, (int __user *)a[2]);
break;
case SYS_GETSOCKNAME:
err = sys_getsockname(a0,(struct sockaddr __user *)a1, (int __user *)a[2]);
break;
case SYS_GETPEERNAME:
err = sys_getpeername(a0, (struct sockaddr __user *)a1, (int __user *)a[2]);
break;
case SYS_SOCKETPAIR:
err = sys_socketpair(a0,a1, a[2], (int __user *)a[3]);
break;
case SYS_SEND:
err = sys_send(a0, (void __user *)a1, a[2], a[3]);
break;
case SYS_SENDTO:
err = sys_sendto(a0,(void __user *)a1, a[2], a[3],
(struct sockaddr __user *)a[4], a[5]);
break;
case SYS_RECV:
err = sys_recv(a0, (void __user *)a1, a[2], a[3]);
break;
case SYS_RECVFROM:
err = sys_recvfrom(a0, (void __user *)a1, a[2], a[3],
(struct sockaddr __user *)a[4], (int __user *)a[5]);
break;
case SYS_SHUTDOWN:
err = sys_shutdown(a0,a1);
break;
case SYS_SETSOCKOPT:
err = sys_setsockopt(a0, a1, a[2], (char __user *)a[3], a[4]);
break;
case SYS_GETSOCKOPT:
err = sys_getsockopt(a0, a1, a[2], (char __user *)a[3], (int __user *)a[4]);
break;
case SYS_SENDMSG:
err = sys_sendmsg(a0, (struct msghdr __user *) a1, a[2]);
break;
case SYS_RECVMSG:
err = sys_recvmsg(a0, (struct msghdr __user *) a1, a[2]);
break;
default:
err = -ONE SELECT;
break;
}
return err;
} /* It may be already another descriptor 8) Not kernel problem. */
return retval;
out_release:
sock_release(sock);
return retval;
}

When an application creates a socket using socket(), sys_socket is executed, which is defined as follows

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asmlinkage long sys_socket(int family, int type, int protocol)
{
int retval;
struct socket *sock;
retval = sock_create(family, type, protocol, &sock);//创建socket
if (retval < 0)
goto out;
retval = sock_map_fd(sock); //Assign an unused file descriptor fd and associate the socket with fd
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;
}

The definition of the structure socket is as follows (include\linux\net.h):

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struct socket {
socket_state state;
unsigned long flags;
struct proto_ops *ops;
struct fasync_struct *fasync_list;
struct file *file; //Establish a connection with the file descriptor through this
struct sock *sk;
wait_queue_head_t wait;
short type;
};

Next, let's take a look at the sock_map_fd function

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int sock_map_fd(struct socket *sock)
{
int fd;
struct qstr this;
char name[32];
/*
* Find a file descriptor suitable for return to the user.
*/
fd = get_unused_fd(); //Assign an unused fd
if (fd >= 0) {
struct file *file = get_empty_filp();
if (!file) {
put_unused_fd(fd);
fd = -ENFILE;
goto out;
}
this.len = sprintf(name, "[%lu]", SOCK_INODE(sock)->i_ino);
this.name = name;
this.hash = SOCK_INODE(sock)->i_ino;
file->f_dentry = d_alloc(sock_mnt->mnt_sb->s_root, &this);
if (!file->f_dentry) {
put_filp(file);
put_unused_fd(fd);
fd = -ENOMEM;
goto out;
}
file->f_dentry->d_op = &sockfs_dentry_operations;
d_add(file->f_dentry, SOCK_INODE(sock));
file->f_vfsmnt = mntget(sock_mnt);
file->f_mapping = file->f_dentry->d_inode->i_mapping;
sock->file = file; // establish connection
file->f_op = SOCK_INODE(sock)->i_fop = &socket_file_ops; //socket operation function. When using the file system's IO function, the socket's IO function is actually used.
file->f_mode = FMODE_READ | FMODE_WRITE;
file->f_flags = O_RDWR;
file->f_pos = 0;
file->private_data = sock;
fd_install(fd, file);
}
out:
return fd;
}
static struct file_operations socket_file_ops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.aio_read = sock_aio_read,
.aio_write = sock_aio_write,
.poll = sock_poll,
.unlocked_ioctl = sock_ioctl,
.mmap = sock_mmap,
.open = sock_no_open, /* special open code to disallow open via /proc */
.release = sock_close,
.fasync = sock_fasync,
.readv = sock_readv,
.writev = sock_writev,
.sendpage = sock_sendpage
};

Reference address：File descriptors in the Linux kernel (Part 2) --sockets and file descriptors

Previous article：File descriptors in the Linux kernel (Part 3) -- recycling of fd
Next article：File descriptors in the Linux kernel (I) -- Basic knowledge introduction

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