【socket】三次握手是在listen中完成，accept只从完成连接的队列中拿出一个连接

• 看了网上的一些博客，说法不一，查阅了相关英文文档，也只了解了大概。所以我就自己设计实验来看下，服务端三次握手到底是在什么时候完成的。
• 首先客户端三次握手毫无疑问是在connect中完成的。
• 服务端到底是在listen中完成还是在accept中完成我们可以设计一个实验，就是说，在服务端只写listen，不写accept。
• 这样一来如果accept参与了三次握手，那么在没有accept的服务端程序中，客户端的connect不可能返回。
• 如果accept没有参与三次握手，那么客户端的connect就可以返回，测试代码如下：

//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; 
}

• 还存在一种可能，那就是accept参与了三次握手，并且只参与了第三次握手。也就是接收客户端的ack。但是通过查阅相关资料，发现accept只会从listen的连接完成的队列中取头部连接出来，进行处理：

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.

参考资料：

• 外网资料（需要翻墙）
• 优秀博客