Socket programming (medium)

socket Programming （ in ）

1. The network accesses the public network ip Or private network ip

• When the client and server are on the same machine , Visit the public network ip Or private network ip Fine
• Clients and servers need to cross the network （ When not on the same machine ）, Access the server public network ip

2.tcp The programming process

• Server side ： Create socket , Binding address information , monitor , Get a new connection , Sending and receiving data , Close the connection
• client ： Create socket , Do not bind address information （ Can be bound ）, A connection , Sending and receiving data , Close the connection
• Listening meaning ： monitor tcp Client new link , Establish a new connection with the client （ Be careful ： here tcp The connection is established in the kernel ）
• Get the meaning of the new connection ： Get the socket descriptor of the new connection , every last tcp The connection produces a socket descriptor
• The meaning of initiating a connection ： Send... To the server txp Connect

3. Correlation function

monitor ：

int listen(int sockfd,int backlog);

• sockfd: socket descriptor
• backlog:tcp Number of concurrent connections （ Size of completed connection ）
• Connection queue not completed ： Connections that are still in the process of connection establishment are placed in this queue （ It can be understood that the connection with three handshakes is in the queue ）
• Size of completed connection queue ： Connection established , Connections that can communicate normally are placed in this queue （ It can be understood that the connection after three handshakes is in the queue ）
• It can be modified by ：/proc/sys/net/ipv4/tcp_max_syn_backlog The value of it , Modify the size of the incomplete connection queue
• Return value ： Success for 0, Failure is -1

Blocking call ：

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

• sockfd： socket descriptor
• addr： Address information structure , A structure that describes client address information （ip、 port ）
• addrlen: Length of address information
• Return value ： Successfully returned the newly connected socket , Failure to return -1
• Be careful ： If there is no established connection in the completed queue , The block ; If you get a new connection , Just go back to
• Socket returned ： The socket of the new connection returned is for communication with the client , No monitoring function , It also contains the address information of the client
• summary ： The service side with socket Created socket descriptor , Is used to listen on sockets , Listen for new connections
  The service side with accept Create a new connection socket , Is to communicate with the client

Connect ：

int connect(int sockfd,const struct sockaddr *addr,socklen_t addrlen);

• sockfd: socket descriptor
• addr: Address information structure , Describe the server address information （ip、 port ）
• addrlen： Length of address information
• Return value ：0 For success ,-1 The failure
• Be careful ： This function can not only complete the connection function , If the client is not bound , At the same time, you can bind the address information of the client

send data ：

ssize_t send(int sockfd,const void *buf,size_t len,int flags);

• sockfd: socket descriptor （accept Received , Not listening on sockets ）
• buf： send out buf Description pointing to space
• len： Data length
• flags：0（ Blocking transmission ）
• Return value ： Successfully return the number of bytes sent , Failure to return -1

receive data ：

ssize_t resv(int sockfd,void *buf,size_t len,int flags);

• sockfd: socket descriptor
• buf: Store the received data in buf Designated space , Space needs to be developed by programmers in advance
• len： Number of bytes expected to be accepted
• flags:0（ Blocking reception ）
• Return value ： success , Number of bytes received ,0, Close the connection end to end ,-1, Receive error