select use flow chart

Note: socket process is not written

Lines of code to be added in network programming and their significance

1. Define select related variables

	int listenfd,connfd,i;
	/*Socket descriptor / link descriptor (input phase) / variable*/	

	int nread,maxfd,maxi,sockfd;
	/*Ready number / maximum link descriptor / used to record the current maximum used subscript of the link descriptor array, reducing useless traversal/
		Link descriptor (the difference is that it is used for I/O multiplexing traversal)*/
	
	int client[FD_SETSIZE];
	/*Store the entered link descriptor*/
	
	fd_set allest,rset;
	/*File set descriptor: all sets / readable sets*/

2.select the related data initialization of related variables

	maxfd = listenfd; 
	/*Initialize the maximum link descriptor,*/
	maxi = -1;
	for (i = 0; i < FD_SETSIZE; i++) client[i] = -1;
	FD_ZERO(&allest);
	/*Empty allset descriptor set*/
	FD_SET(listenfd, &allest);
	 /*Add listening descriptor to allset*/

3.select to monitor readable events

	rset = allest;
	nread = select(maxfd+1, &rset, NULL, NULL, NULL);
		/*Call select to monitor readable events*/

4. Put new link descriptors into array and descriptor set

	if (FD_ISSET(listenfd, &rset))
	/*Determine whether there is a change in the server socket (whether there is a new link)*/
	{
		cliaddr_lin = sizeof(cliaddr);
		connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &cliaddr_lin);
		for (i = 0; i < FD_SETSIZE; i++)
		{
			if (client[i] < 0)
			{
				client[i] = connfd;
/				/*New link descriptor in array*/
				break;
			}
		}
		FD_SET(connfd, &allest);
		 /* Join connfd from customer to descriptor set */
		if (connfd > maxfd) maxfd = connfd;
		/*New link descriptor*/
		if (i > maxi) maxi = i;
		if (--nread == 0) continue;
	}

5. Batch read link descriptor

for (i = 0; i <= maxi; i++)
{
	if ((sockfd = client[i]) < 0) continue;
	if (FD_ISSET(sockfd, &rset))
	{
		if ((n = read (sockfd, buf, MAXLIN)) == 0)
		{
			close(sockfd);
			FD_CLR(sockfd, &allest);
			client[i] = -1;
		}else
		{
			if (n < 0) perror("read:");
			if (n > 0) write(sockfd, buf, n);
		}
	if (--nread == 0) break;
	}
}

routine

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#define LINK_NUMBER 20
#define SERV_PORT 8000
#define MAXLIN 100

int main(int argc, char const *argv[])
{
	int listenfd,connfd,i;
	int nread,maxfd,maxi,sockfd;
	int client[FD_SETSIZE];
	ssize_t n;
	fd_set allest,rset;
	socklen_t cliaddr_lin;
	struct sockaddr_in servaddr,cliaddr;
	char buf[MAXLIN],str[INET_ADDRSTRLEN];

	listenfd = socket(AF_INET,SOCK_STREAM,0);

	bzero(&servaddr, sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
	servaddr.sin_port = htons(SERV_PORT);

	bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));

	listen(listenfd,LINK_NUMBER);

	maxfd = listenfd;
	maxi = -1;
	for (i = 0; i < FD_SETSIZE; i++) client[i] = -1;
	FD_ZERO(&allest);
	FD_SET(listenfd, &allest);

	while(1)
	{
		rset = allest;
		nread = select(maxfd+1, &rset, NULL, NULL, NULL);
		if (nread < 0)
		{
			perror("select error:");
			break;
		}

		if (FD_ISSET(listenfd, &rset))
		{
			cliaddr_lin = sizeof(cliaddr);
			connfd = accept(listenfd, (struct sockaddr *)&cliaddr, &cliaddr_lin);

			printf("received from %s at PORT %d\n",
			       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
			       ntohs(cliaddr.sin_port));

			for (i = 0; i < FD_SETSIZE; i++)
			{
				if (client[i] < 0)
				{
					client[i] = connfd;
					break;
				}
			}
			if (i == FD_SETSIZE)
			{
				perror("too many clients");
				exit(1);
			}
			FD_SET(connfd, &allest);
			if (connfd > maxfd) maxfd = connfd;
			if (i > maxi) maxi = i;
			if (--nread == 0) continue;	
		}

		for (i = 0; i <= maxi; i++)
		{
			if ((sockfd = client[i]) < 0) continue;

			if (FD_ISSET(sockfd, &rset))
			{
				if ((n = read (sockfd, buf, MAXLIN)) == 0)
				{
					close(sockfd);
					FD_CLR(sockfd, &allest);
					client[i] = -1;
				}else
				{
					if (n < 0)
					{
						perror("read:");
					}
					if (n > 0)
					{
						write(sockfd, buf, n);
					}
	
				}
			
				if (--nread == 0) break;
			}
		}
		/*socket*/
		/*while(1)
		{
			if((n = read(connfd,buf,MAXLIN)) <= 0) 
			{
				printf("err" );
				break;
			}

			printf("received from %s at PORT %d::%s\n",
			       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
			       ntohs(cliaddr.sin_port),buf);

			write(connfd,buf,n);
		}
		close(connfd);*/
	}

	return 0;
}

References and blogs

Linux chat room project – ChatRome (select Implementation)
Detailed explanation of select function of Linux I/O reuse
The second edition of unix network programming