2020/2/15学习Linux Socket时遇到的问题

1. C语言中int main（int argc, char *argv[])中这两个参数是什么意思？

argc：命令行参数的个数，默认值是1

argv：char类型的指针数组，用于保存命令行参数；argv[0]是带路径的程序名称，argv[1]是dos命令行中执行程序名后的第一个字符串，argv[2]是第二个字符串，以此类推.....

这两个参数的作用是：可以接收命令行中输入的参数【好方便啊！】

2. 设置服务器端Socket的地址

【如果只设定服务器机的一个ip地址】

这样设置：serveraddr是结构体sockaddr_in的实例；

而在sockaddr_in结构体中有一个成员sin_addr ，它是结构体in_addr的实例，

地址就保存在结构体sockaddr_in中的结构体sin_addr中的s_addr成员中——

serveraddr.sin_addr.s_addr = htons("192.168.0.10");

【如果要让服务器上的所有ip地址都得到连接请求】

需要一个特殊的地址

#define INADDR_ANY (uint32_t)0x00000000

serveraddr.sin_addr.s_addr = INADDR_ANY;

3. Socket TCP编程服务端

#include <netdb.h>
#include <sys/socket.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <signal.h>
#include <time.h>

//#define INADDR_ANY (uint32_t)0x00000000
int sockfd;

void sig_handler(int signo)
{
   if(signo == SIGINT){
       printf("server close\n");
       /*step 6 close socket*/
       close(sockfd);
       exit(1);//0 or 1 all can close
   }
}

/*output client's messages */
void out_addr(struct sockaddr_in *clientaddr)
{
   int port = ntohs(clientaddr->sin_port);//net to host
   char ip[16];
   memset(ip,0,sizeof(ip));
   inet_ntop(AF_INET, &clientaddr->sin_addr.s_addr, ip, sizeof(ip));//net to Dot decimal(same as 192.168.0.1)
   printf("client: %s(%d) connected\n", ip, port);
}

void do_service(int fd)
{
   long t = time(0);//write server's system time to client
   char *s = ctime(&t);
   size_t size = strlen(s) * sizeof(char);
   if(write(fd, s, size) != size){
       perror("write error");
   }
}

int main(int argc, char *argv[])
{
   if(argc < 2){
       printf("usage: %s #port\n", argv[0]);//print program's name
       exit(1);
}
   if(signal(SIGINT, sig_handler) == SIG_ERR){//SIGINT:ctrl+C can end ; signal function get SIG_ERR if error
       perror("signal sigint error"); //perror function is defined in <stdlib.h>,we can see" str: error message"
       exit(1);
   }

   /*step 1 creat socket
   *socket is a struct in kernel
   *AF_INET: IPV4
   *SOCK_STREAM: tcp protocol(udp: SOCK_DGRAM)
   **/
   sockfd = socket(AF_INET, SOCK_STREAM, 0);

   /*step 2 bind()
   *bound with socket and address(ip\port\intnet type)
*sockaddr_in is special net_struct for internet */
   struct sockaddr_in serveraddr;
   memset(&serveraddr, 0, sizeof(serveraddr)); //clear
   serveraddr.sin_family = AF_INET;      //IPV4(Host byte order is ok)
   serveraddr.sin_port = htons(atoi(argv[1]));//port from terminal(atoi:string to int)(htons:Host to Network byte order 16bit)
   serveraddr.sin_addr.s_addr = INADDR_ANY; //htons("192.168.0.10")
   if(bind(sockfd, (struct sockaddr*)&serveraddr, sizeof(serveraddr)) < 0){//cast as common address type : sockaddr
       perror("bind error");
       exit(1);
    }

   /*step 3 listen()
   *tell system to accept connecting request (in server port)
   *put connecting request to queue* (10 is the length of queue)
   */
   if(listen(sockfd, 10) < 0){
       perror("listen error");
       exit(1);
   }

   /*step 4 accept()
   *get a connection and return the new socket file descriptor(sockfd)
   *This sockfd(client's fd) is different from the sockfd in step 1(server's fd)
   */
   struct sockaddr_in clientaddr;
   socklen_t clientaddr_len = sizeof(clientaddr);//This is a pointer so we need to use & to get the message
   while(1){
       int fd = accept(sockfd,   (struct sockaddr*)&clientaddr, &clientaddr_len);
       if(fd < 0) {
            perror("accept error");
           continue;
       }