[socket] the three handshakes are completed in listen, and accept only takes out one connection from the queue that completes the connection

• I read some blogs on the Internet , There are different opinions , I checked the relevant English documents , I only know about . So I'll design my own experiment to see , When did the server complete the three handshakes .
• First of all, the three handshakes of the client are undoubtedly in connect Done in .
• The server is listen Is it finished in accept We can design an experiment , That is to say , Write only on the server listen, Don't write accept.
• So if accept Participated in three handshakes , Then there is no accept In the server program of , Client's connect It's impossible to return .
• If accept Did not participate in three handshakes , So the client's connect I can go back , The test code is as follows ：

//client.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 

void error(const char *msg)
{
    
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[])
{
    
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];
    if (argc < 3) {
    
       fprintf(stderr,"usage %s hostname port\n", argv[0]);
       exit(0);
    }
    portno = atoi(argv[2]);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    server = gethostbyname(argv[1]);
    if (server == NULL) {
    
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
    }
    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr, 
         (char *)&serv_addr.sin_addr.s_addr,
         server->h_length);
    serv_addr.sin_port = htons(portno);
    if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
        error("ERROR connecting");
    printf("Please enter the message: ");
    bzero(buffer,256);
    fgets(buffer,255,stdin);
    n = write(sockfd,buffer,strlen(buffer));
    if (n < 0) 
         error("ERROR writing to socket");
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
    if (n < 0) 
         error("ERROR reading from socket");
    printf("%s\n",buffer);
    close(sockfd);
    return 0;
}

//server.c
/* A simple server in the internet domain using TCP The port number is passed as an argument */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

void error(const char *msg)
{
    
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
    
     int sockfd, newsockfd, portno;
     socklen_t clilen;
     char buffer[256];
     struct sockaddr_in serv_addr, cli_addr;
     int n;
     if (argc < 2) {
    
         fprintf(stderr,"ERROR, no port provided\n");
         exit(1);
     }
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0) 
        error("ERROR opening socket");
     bzero((char *) &serv_addr, sizeof(serv_addr));
     portno = atoi(argv[1]);
     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons(portno);
     if (bind(sockfd, (struct sockaddr *) &serv_addr,
              sizeof(serv_addr)) < 0) 
              error("ERROR on binding");
     listen(sockfd,5);
     /*clilen = sizeof(cli_addr); newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); if (newsockfd < 0) error("ERROR on accept"); bzero(buffer,256); n = read(newsockfd,buffer,255); if (n < 0) error("ERROR reading from socket"); printf("Here is the message: %s\n",buffer); n = write(newsockfd,"I got your message",18); if (n < 0) error("ERROR writing to socket"); close(newsockfd); close(sockfd);*/
     int i =1;
     while(1)
     {
    
        i++;
     }
     printf("%d",i);
     return 0; 
}

• There is also a possibility , That's it accept Participated in three handshakes , And only participated in the third handshake . That is, the receiving client ack. But by looking up relevant information , Find out accept Only from the listen Take the header from the queue after the connection is completed , To deal with ：

The completed connection queue is almost always empty because when an entry is 
placed on this queue, the server’s call to accept returns, and the server takes the 
completed connection off the queue.

Reference material ：

• Extranet information （ Need to climb over the wall ）
• Excellent blog