Linux kernel Analysis

Kernel: 2.6.34

TCP is the most widely used transport layer protocol, which provides the connection-oriented and reliable byte-throttling service, but it is precisely because of these characteristics that TCP is more complex than UDP, and it is divided into two parts [creation and use]. This article mainly includes the creation of TCP and the process of shaking hands three times.

When programming, you typically create a TCP Socket with the following statement:

Sockets (Af_inet, SOCK_DGRAM, ipproto_tcp)

Starting from this analysis, call interface [NET/SOCKET.C]: syscall_define3 (socket)

Two-step key operations are performed: Sock_create () and SOCK_MAP_FD ()

retval = sock_create (family, type, protocol, &sock);     
if (retval < 0)
 goto out;
retval = SOCK_MAP_FD (sock, Flags & (O_cloexec | O_nonblock));
if (retval < 0)
 Goto out_release;

Sock_create () is used to create a SOCKET,SOCK_MAP_FD () map to a file descriptor, so that the socket can be accessed through FD, focusing on the creation process of the sock_create ().

Sock_create ()-> __sock_create ()

From the __sock_create () code, you see that the creation consists of two steps: Sock_alloc () and Pf->create (). Sock_alloc () allocates the sock memory space and initializes the Inode;pf->create () to initialize SK.

Sock = 

Sock_alloc ();
Sock->type = type;
..... PF = rcu_dereference (net_families[family]);
..... Pf->create (NET, sock, Protocol, Kern);

Sock_alloc ()

Allocates space, allocates nodes (including sockets) through New_inode (), then obtains sock through Socket_i macros, in fact the inode and Sock are distributed together in New_inode (), and the structure is called Sock_alloc.

Inode = New_inode (SOCK_MNT->MNT_SB);     
Sock = Socket_i (inode);

Sets the parameters of the Inode and returns sock.

inode-

>i_mode = S_ifsock | S_irwxugo;     
Inode->i_uid = Current_fsuid ();     
Inode->i_gid = Current_fsgid ();     
Return sock;

Continue to look down on the specific creation process: New_inode (), after the allocation, will set the I_ino and I_state values.

struct Inode *new_inode (struct super_block *sb)     
{     
 ...     
 Inode = Alloc_inode (SB);     
 if (inode) {     
  spin_lock (&inode_lock);     
  __inode_add_to_lists (SB, NULL, inode);     
  Inode->i_ino = ++last_ino;     
  inode->i_state = 0;     
  Spin_unlock (&inode_lock);     
 }     
 return inode;     
}

The Alloc_inode ()-> Sb->s_op->alloc_inode (), SB is SOCK_MNT->MNT_SB, so Alloc_inode () points to the SOCKFS action function Sock_ Alloc_inode.

static const struct 

super_operations sockfs_ops = {     
 . Alloc_inode = Sock_alloc_inode,     
 . Destroy_inode =sock_ Destroy_inode,     
 . Statfs = Simple_statfs,     
};