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Socket (socket): IP address + port number, which uniquely identifies a process in the network

In socket programming, both processes that establish a connection have a socket to identify a unique connection.

Network byte order: the network data flow is divided into big and small ends. The sending host usually sends out the data in the sending buffer in the order of memory address from low to high, and the receiving host stores the bytes received from the network in the receiving buffer in turn. It is also saved in the order of memory address from low to high. The address of the network data flow stipulates that the data sent first is the low address, and then the data sent is

Address.

According to the TCP/IP protocol, the network data flow should use large-end byte order, that is, low address and high bytes.

For example, if the port number is 1000 (0x3e8), address 0 is 0x03, address 1 is 0xe8, that is, first 0x03, then 0xe8. These 16 bits should also be low address memory 0x03 and high address memory 0xe8 in the buffer of the sending host. However, if the sending host is in small-end byte order, the 16-bit port number is interpreted as 0xe803, not 1000. Therefore, the sending host needs to convert the byte order before filling 1000 into the send buffer. Similarly, if the receiving host is in small-end byte order, the 16-bit source port number also needs to be converted. If the host is in large byte order, neither sending nor receiving needs to be converted. Similarly, 32-bit IP addresses should also consider network byte order and host byte order.

Server-side socket:

1. Create sockets: int socket (int domain,int type,int protocol) / / domain underlying protocol, transport type, TCP oriented byte stream

two。 Populate network information with struct socketaddr_in structures

3. Binding: populating socket information into the kernel

4. Set the socket status to listen and receive connection requests from the client

5. Wait for the customer request. When the request connection comes, accept the connection request and return a new socket (accept) corresponding to the connection.

6. New sockets communicate with clients

Accept function: accepts a connection on a socket.

Int accept (int sockfd,struct sockaddr*addr,socklen_t* addrlen)

The socket descriptor is the same in sockfd and listen (). Where the information that addr requires access is going to. Return value: if the execution is successful, the return value is 0, otherwise the error code is set and-1 is returned.

Bind function: bundles a local address with a set of interfaces.

Int bind (int sockfd, const struct sockaddr* addr, socklen_t len)

Suitable for unconnected Datagram or stream sockets, used before connect () or listen () calls. When a socket is created with socket (), it exists in a namespace (address family), but it is not named. The bind () function establishes a local bundle (host address / port number) for the socket by assigning a local name to an unnamed socket.

Server-side code:

1 # include 2 # include 3 # include 4 # include 5 # include 6 # include 7 # include 8 # include 9 int start (int _ port,char* ip) 10 {11 / / 1.create sock 12 int listen_sock=socket (AF_INET,SOCK_STREAM,0); 13 if (listen_sock < 0) {14 perror ("sock"); 15 exit (1); 16} 17 / / 2.fill information 18 struct sockaddr_in local 19 local.sin_family=AF_INET; 20 local.sin_port=htons (_ port); 21 local.sin_addr.s_addr=inet_addr (ip); 22 / / 3.bind 23 if (bind (listen_sock, (struct sockaddr*) & local,sizeof (local))

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