In the network transport protocol, TCP provides a reliable, complex, connection-oriented data stream (SOCK_STREAM) service that establishes connections through a three-stage handshake. TCP has a “retransmission confirmation” mechanism, that is, the receiving end will send a positive confirmation signal after receiving the data. If the sending end receives the positive confirmation signal, it will continue to send other data, if not, it will re-send.

In contrast, UDP is a connectionless, unreliable datagram (SOCK_DGRAM) transport service. Instead of establishing a connection using the UDP socket interface, the server can communicate (the recvfrom function and sendto function) after calling socket() to generate a socket and calling bind() to bind the port. After the client generates a socket with socket(), it can send and receive data to the server address.

Special note here: TCP USES a stream socket (SOCK_STREAM), UDP USES a datagram socket (SOCK_DGRAM)

UDP socket programming example:

The server code is as follows:

#include<sys/types.h>
#include<sys/socket.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<netdb.h>
#include<stdarg.h>
#include<string.h>

#define SERVER_PORT 8000
#define BUFFER_SIZE 1024
#define FILE_NAME_MAX_SIZE 512

int main()
{

 struct sockaddr_in server_addr;
 bzero(&server_addr, sizeof(server_addr));
 server_addr.sin_family = AF_INET;
 server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
 server_addr.sin_port = htons(SERVER_PORT);


 int server_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
 if(server_socket_fd == -1)
 {
  perror("Create Socket Failed:");
  exit(1);
 }


 if(-1 == (bind(server_socket_fd,(struct sockaddr*)&server_addr,sizeof(server_addr))))
 {
  perror("Server Bind Failed:");
  exit(1);
 }


 while(1)
 {

  struct sockaddr_in client_addr;
  socklen_t client_addr_length = sizeof(client_addr);


  char buffer[BUFFER_SIZE];
  bzero(buffer, BUFFER_SIZE);
  if(recvfrom(server_socket_fd, buffer, BUFFER_SIZE,0,(struct sockaddr*)&client_addr, &client_addr_length) == -1)
  {
   perror("Receive Data Failed:");
   exit(1);
  }


  char file_name[FILE_NAME_MAX_SIZE+1];
  bzero(file_name,FILE_NAME_MAX_SIZE+1);
  strncpy(file_name, buffer, strlen(buffer)>FILE_NAME_MAX_SIZE?FILE_NAME_MAX_SIZE:strlen(buffer));
  printf("%sn", file_name);
 }
 close(server_socket_fd);
 return 0;
}

The client code is as follows:

#include<sys/types.h>
#include<sys/socket.h>
#include<unistd.h>
#include<netinet/in.h>
#include<arpa/inet.h>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<netdb.h>
#include<stdarg.h>
#include<string.h>

#define SERVER_PORT 8000
#define BUFFER_SIZE 1024
#define FILE_NAME_MAX_SIZE 512

int main()
{

 struct sockaddr_in server_addr;
 bzero(&server_addr, sizeof(server_addr));
 server_addr.sin_family = AF_INET;
 server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
 server_addr.sin_port = htons(SERVER_PORT);


 int client_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
 if(client_socket_fd < 0)
 {
  perror("Create Socket Failed:");
  exit(1);
 }


 char file_name[FILE_NAME_MAX_SIZE+1];
 bzero(file_name, FILE_NAME_MAX_SIZE+1);
 printf("Please Input File Name On Server:t");
 scanf("%s", file_name);

 char buffer[BUFFER_SIZE];
 bzero(buffer, BUFFER_SIZE);
 strncpy(buffer, file_name, strlen(file_name)>BUFFER_SIZE?BUFFER_SIZE:strlen(file_name));


 if(sendto(client_socket_fd, buffer, BUFFER_SIZE,0,(struct sockaddr*)&server_addr,sizeof(server_addr)) < 0)
 {
  perror("Send File Name Failed:");
  exit(1);
 }

 close(client_socket_fd);
 return 0;
}

Readers can refer to the previous post: (link: #) and note the comparison between UDP and TCP workflows. To deepen the understanding of the principles of the program.