TCP server communication flow

1. Server communication model

tcp The server
Answer the phone model ： Create socket （ Server phone ）, binding ip And port on socket （ Bind the fixed telephone number ）, Listen for new connections （ monitor ）, Accept the connection （ Answer the phone ）, Sending and receiving data （ Reading and writing is right socket File operations ）（ Talking and communicating ）, close （ Hang up ）

explain : There may be more than one host ip So we have to bind

explain ： The client does not need to bind , The system randomly assigns a port to him

Detailed process

explain ：
listen monitor Create a completed queue and an incomplete queue , The unfinished queue reaches the completed queue after three handshakes , Turn the server into passive receiving mode
accept Extract the connection from the completed queue , Get a new connected socket , Use this connected socket to communicate with the client , The original socket is only used to process new requests and receive new connections , Don't use the original communication .

Socket port and... Are connected ip And the original listening socket ip It is the same as the port
To serve many clients , More than one

effect ： Specialization , Improve work efficiency , Avoid confusion in a socket

Function description ：

bind function
function ： binding ip And port on the server socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
sockfd：
socket File descriptor , Corresponding to the target socket
addr:
Construct out IP Address plus slogan , Get server ipv4 Socket structure address
addrlen:
sizeof(addr) length ,ipv4 Size of structure
Return value ：
Successfully returns 0, Failure to return -1, Set up errno

listen function
function ： Listen for connections
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int listen(int sockfd, int backlog);
sockfd:
socket File descriptor ,socket Socket
backlog:
Queue up 3 Second handshake queue and just established 3 The number of links in the secondary handshake queue and
The maximum value of the sum of completed connections and incomplete connections （ General filling 128 Will be enough ）

explain ：
Check the system default backlog
cat /proc/sys/net/ipv4/tcp_max_syn_backlog

Return value
listen() Successfully returns 0, Failure to return -1.

accept function
function ： Extract a new connection from the completed connection queue
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
sockdf:
socket File descriptor , Socket
addr:
Out parameter , Return link client address information , contain IP Address and port number
Get client ipv4 Socket structure address
addrlen:
Address of socket structure size
Incoming and outgoing parameters （ value - result ）, Pass in sizeof(addr) size , Function returns the size of the received address structure
Return value ：
Successfully returned a new socket File descriptor , Create a new socket , For communicating with clients Don't use that old socket
Failure to return -1, Set up errno

explain ： If there is no new connection ,accept It will block