Select receives both normal data and out of band data

This is a server program , Use select Receive common data at the same time （ Surveillance is readable ） and Out of band data （ Monitor exceptions ）.

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

int main(int argc, char* argv[])
{
    
	if (argc <= 2)
	{
    
		return 1;
	}
	const char* ip = argv[1];
	int port = atoi(argv[2]);

	struct sockaddr_in address;
	bzero(&address, sizeof(address));
	address.sin_family = AF_INET;
	address.sin_port = htons(port);
	inet_pton(AF_INET, ip, &address.sin_addr);

	int listenfd = socket(PF_INET, SOCK_STREAM, 0);
	assert(listenfd >= 0);

	int ret = bind(listenfd, (struct sockaddr*)&address, sizeof(address));
	assert(ret != -1);

	ret = listen(listenfd, 5);
	assert(ret != -1);

	struct sockaddr_in client_address;
	socklen_t client_addrlength = sizeof(client_address);
	int connfd = accept(listenfd, (struct sockaddr*)&client_address, &client_addrlength);
	if (connfd < 0)
	{
    
		printf("errno is: %d\n", errno);
		close(listenfd);
	}

	char buf[1024];
	fd_set read_fds;	/*  One for testing   Readable state   File descriptor set  */
	fd_set exception_fds;	/*  One for testing   Abnormal state of   File descriptor set  */
	FD_ZERO(&read_fds);
	FD_ZERO(&exception_fds);

	while (1)
	{
    
		memset(buf, '\0', sizeof(buf));
		/*  Every time you call  select  We have to start again before  read_fds  and  exception_fds  Set file descriptor in  connfd, Because the set of file descriptors will be modified by the kernel after the event  */
		FD_SET(connfd, &read_fds);
		FD_SET(connfd, &exception_fds);
		ret = select(connfd + 1, &read_fds, NULL, &exception_fds, NULL);
		if (ret < 0)
		{
    
			printf("selection failure\n");
			break;
		}

		/*  For readable Events , Use ordinary  recv  Function to read data  */
		if (FD_ISSET(connfd, &read_fds))
		{
    
			ret = recv(connfd, buf, sizeof(buf) - 1, 0);
			if (ret <= 0)
			{
    
				break;
			}
			printf("get %d bytes of normal data: %s\n", ret, buf);
		}
		/*  For exceptional events , Use band  MSG_OOB  logo  recv  Function to read out of band data  */
		else if (FD_ISSET(connfd, &exception_fds))
		{
    
			ret = recv(connfd, buf, sizeof(buf) - 1, MSG_OOB);
			if (ret <= 0)
			{
    
				break;
			}
			printf("get %d bytes of oob data: %s\n", ret, buf);
		}
	}
	close(connfd);
	close(listenfd);
	return 0;
}