Select (Unix)

select是用于I/O多路转接的一个系统调用函数。

在C程序中，该系统调用在 sys/select.h 或 unistd.h 中声明，语法如下：
int select(int nfds, fd_set* readfds, fd_set* writefds, fd_set* errorfds, struct timeval* timeout);

为了维护fd_set类型的参数，会使用下面四个宏：FD_SET(), FD_CLR(), FD_ZERO() 和 FD_ISSET()。

返回值：
       这个函数将返回描述符集的个数， 如果超时返回为0，错误则返回-1。

参看：
select(2) 
poll(2)

select与epoll的区别

说明：以上无论epoll_create, fd_set都受限于系统中单个进程能够打开的文件句柄数。

示例 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

#include <sys/select.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

#define PORT "9421"

/* function prototypes */
void die(const char*);

int main(int argc, char **argv)
{
	int sockfd, new, maxfd, on = 1, nready, i;

	struct addrinfo *res0, *res, hints;

	char buffer[BUFSIZ];

	fd_set master, readfds;

	ssize_t nbytes;

	(void)memset(&hints, '\0', sizeof(struct addrinfo));

	hints.ai_family = AF_INET;
	hints.ai_socktype = SOCK_STREAM;
	hints.ai_protocol = IPPROTO_TCP;
	hints.ai_flags = AI_PASSIVE;

	if(-1 == (getaddrinfo(NULL, PORT, &hints, &res0)))
		die("getaddrinfo()");

	for(res = res0; res; res = res->ai_next)
	{
		if(-1 == (sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol)))
		{
			perror("socket()");
			continue;
		}

		if(-1 == (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (char*)&on, sizeof(int))))
		{
			perror("setsockopt()");
			continue;
		}

		if(-1 == (bind(sockfd, res->ai_addr, res->ai_addrlen)))
		{
			perror("bind");
			continue;
		}

		break;

	}

	if(-1 == sockfd)
		exit(EXIT_FAILURE);

	freeaddrinfo(res0);

	if(-1 == (listen(sockfd, 32)))
		die("listen()");

	if(-1 == (fcntl(sockfd, F_SETFD, O_NONBLOCK)))
		die("fcntl()");

	FD_ZERO(&master);
	FD_ZERO(&readfds);
	
	FD_SET(sockfd, &master);

	maxfd = sockfd;

	while(1)
	{
		memcpy(&readfds, &master, sizeof(master));

		(void)printf("running select()\n");

		if(-1 == (nready = select(maxfd+1, &readfds, NULL, NULL, NULL)))
			die("select()");

		(void)printf("Number of ready descriptor: %d\n", nready);

		for(i=0; i<=maxfd && nready>0; i++)
		{
			if(FD_ISSET(i, &readfds))
			{
				nready--;

				if(i == sockfd)
				{
					(void)printf("Trying to accept() new connection(s)\n");

					if(-1 == (new = accept(sockfd, NULL, NULL)))
					{
						if(EWOULDBLOCK != errno)
							die("accept()");

						break;
					}
					
					else
					{

						if(-1 == (fcntl(new, F_SETFD, O_NONBLOCK)))
							die("fcntl()");

						FD_SET(new, &master);

						if(maxfd < new)
							maxfd = new;
					}
				}

				else
				{
					(void)printf("recv() data from one of descriptors(s)\n");

					nbytes = recv(i, buffer, sizeof(buffer), 0);
					if(nbytes <= 0)
					{
						if(EWOULDBLOCK != errno)
							die("recv()");

						break;
					}

					buffer[nbytes] = '\0';
					printf("%s", buffer);
					
					(void)printf("%zi bytes received.\n", nbytes);

					close(i);
					FD_CLR(i, &master);

				}
			}

		}

	}

	return 0;
}

void die(const char *msg)
{
	perror(msg);
	exit(EXIT_FAILURE);
}

参见 
Berkeley套接字
轮询
epoll
kqueue
IOCP

外部链接 
 
 

C POSIX库
事件 (计算机)
系统调用