IO复用之poll函数
介绍
poll的机制与select类似，与select在本质上没有多大差别，管理多个描述符也是进行轮询，根据描述符的状态进行处理，但是poll没有最大文件描述符数量的限制。poll和select同样存在一个缺点就是，包含大量文件描述符的数组被整体复制于用户态和内核的地址空间之间，而不论这些文件描述符是否就绪，它的开销随着文件描述符数量的增加而线性增大。
函数说明
#include <poll.h>
int poll ( struct pollfd *fds, unsigned int nfds, int timeout);
从函数原型可以看到poll是用pollfd类型的数组来存储要监控的描述符，因此没有数量的限制。
pollfd结构体定义如下：
struct pollfd {
int fd; /* 文件描述符 */
short events; /* 等待的事件 */
short revents; /* 实际发生了的事件 */
} ;
每一个pollfd结构体指定了一个被监视的文件描述符，可以传递多个结构体，指示poll()监视多个文件描述符。每个结构体的events域是监视该文件描述符的事件掩码，由用户来设置这个域。revents域是文件描述符的操作结果事件掩码，内核在调用返回时设置这个域。events域中请求的任何事件都可能在revents域中返回。
返回值：成功时，poll()返回结构体中revents域不为0的文件描述符个数；如果在超时前没有任何事件发生，poll()返回0；失败时，poll()返回-1
示例代码
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/poll.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <errno.h>
#define SERVER_PORT 12345
#define TRUE 1
#define FALSE 0
main (int argc, char *argv[])
{
int len, rc, on = 1;
int listen_sd = -1, new_sd = -1;
int desc_ready, end_server = FALSE, compress_array = FALSE;
int close_conn;
char buffer[80];
struct sockaddr_in addr;
int timeout;
struct pollfd fds[200];
int nfds = 1, current_size = 0, i, j;
/*************************************************************/
/* Create an AF_INET stream socket to receive incoming */
/* connections on */
/*************************************************************/
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
if (listen_sd < 0)
{
perror("socket() failed");
exit(-1);
}
/*************************************************************/
/* Allow socket descriptor to be reuseable */
/*************************************************************/
rc = setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR,
(char *)&on, sizeof(on));
if (rc < 0)
{
perror("setsockopt() failed");
close(listen_sd);
exit(-1);
}
/*************************************************************/
/* Set socket to be nonblocking. All of the sockets for */
/* the incoming connections will also be nonblocking since */
/* they will inherit that state from the listening socket. */
/*************************************************************/
rc = ioctl(listen_sd, FIONBIO, (char *)&on);
if (rc < 0)
{
perror("ioctl() failed");
close(listen_sd);
exit(-1);
}
/*************************************************************/
/* Bind the socket */
/*************************************************************/
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(SERVER_PORT);
rc = bind(listen_sd,
(struct sockaddr *)&addr, sizeof(addr));
if (rc < 0)
{
perror("bind() failed");
close(listen_sd);
exit(-1);
}
/*************************************************************/
/* Set the listen back log */
/*************************************************************/
rc = listen(listen_sd, 32);
if (rc < 0)
{
perror("listen() failed");
close(listen_sd);
exit(-1);
}
/*************************************************************/
/* Initialize the pollfd structure */
/*************************************************************/
memset(fds, 0 , sizeof(fds));
/*************************************************************/
/* Set up the initial listening socket */
/*************************************************************/
fds[0].fd = listen_sd;
fds[0].events = POLLIN;
/*************************************************************/
/* Initialize the timeout to 3 minutes. If no */
/* activity after 3 minutes this program will end. */
/* timeout value is based on milliseconds. */
/*************************************************************/
timeout = (3 * 60 * 1000);
/*************************************************************/
/* Loop waiting for incoming connects or for incoming data */
/* on any of the connected sockets. */
/*************************************************************/
do
{
/***********************************************************/
/* Call poll() and wait 3 minutes for it to complete. */
/***********************************************************/
printf("Waiting on poll()...\n");
rc = poll(fds, nfds, timeout);
/***********************************************************/
/* Check to see if the poll call failed. */
/***********************************************************/
if (rc < 0)
{
perror(" poll() failed");
break;
}
/***********************************************************/
/* Check to see if the 3 minute time out expired. */
/***********************************************************/
if (rc == 0)
{
printf(" poll() timed out. End program.\n");
break;
}
/***********************************************************/
/* One or more descriptors are readable. Need to */
/* determine which ones they are. */
/***********************************************************/
current_size = nfds;
for (i = 0; i < current_size; i++)
{
/*********************************************************/
/* Loop through to find the descriptors that returned */
/* POLLIN and determine whether it's the listening */
/* or the active connection. */
/*********************************************************/
if(fds[i].revents == 0)
continue;
/*********************************************************/
/* If revents is not POLLIN, it's an unexpected result, */
/* log and end the server. */
/*********************************************************/
if(fds[i].revents != POLLIN)
{
printf(" Error! revents = %d\n", fds[i].revents);
end_server = TRUE;
break;
}
if (fds[i].fd == listen_sd)
{
/*******************************************************/
/* Listening descriptor is readable. */
/*******************************************************/
printf(" Listening socket is readable\n");
/*******************************************************/
/* Accept all incoming connections that are */
/* queued up on the listening socket before we */
/* loop back and call poll again. */
/*******************************************************/
do
{
/*****************************************************/
/* Accept each incoming connection. If */
/* accept fails with EWOULDBLOCK, then we */
/* have accepted all of them. Any other */
/* failure on accept will cause us to end the */
/* server. */
/*****************************************************/
new_sd = accept(listen_sd, NULL, NULL);
if (new_sd < 0)
{
if (errno != EWOULDBLOCK)
{
perror(" accept() failed");
end_server = TRUE;
}
break;
}
/*****************************************************/
/* Add the new incoming connection to the */
/* pollfd structure */
/*****************************************************/
printf(" New incoming connection - %d\n", new_sd);
fds[nfds].fd = new_sd;
fds[nfds].events = POLLIN;
nfds++;
/*****************************************************/
/* Loop back up and accept another incoming */
/* connection */
/*****************************************************/
} while (new_sd != -1);
}
/*********************************************************/
/* This is not the listening socket, therefore an */
/* existing connection must be readable */
/*********************************************************/
else
{
printf(" Descriptor %d is readable\n", fds[i].fd);
close_conn = FALSE;
/*******************************************************/
/* Receive all incoming data on this socket */
/* before we loop back and call poll again. */
/*******************************************************/
do
{
/*****************************************************/
/* Receive data on this connection until the */
/* recv fails with EWOULDBLOCK. If any other */
/* failure occurs, we will close the */
/* connection. */
/*****************************************************/
rc = recv(fds[i].fd, buffer, sizeof(buffer), 0);
if (rc < 0)
{
if (errno != EWOULDBLOCK)
{
perror(" recv() failed");
close_conn = TRUE;
}
break;
}
/*****************************************************/
/* Check to see if the connection has been */
/* closed by the client */
/*****************************************************/
if (rc == 0)
{
printf(" Connection closed\n");
close_conn = TRUE;
break;
}
/*****************************************************/
/* Data was received */
/*****************************************************/
len = rc;
printf(" %d bytes received\n", len);
/*****************************************************/
/* Echo the data back to the client */
/*****************************************************/
rc = send(fds[i].fd, buffer, len, 0);
if (rc < 0)
{
perror(" send() failed");
close_conn = TRUE;
break;
}
} while(TRUE);
/*******************************************************/
/* If the close_conn flag was turned on, we need */
/* to clean up this active connection. This */
/* clean up process includes removing the */
/* descriptor. */
/*******************************************************/
if (close_conn)
{
close(fds[i].fd);
fds[i].fd = -1;
compress_array = TRUE;
}
} /* End of existing connection is readable */
} /* End of loop through pollable descriptors */
/***********************************************************/
/* If the compress_array flag was turned on, we need */
/* to squeeze together the array and decrement the number */
/* of file descriptors. We do not need to move back the */
/* events and revents fields because the events will always*/
/* be POLLIN in this case, and revents is output. */
/***********************************************************/
if (compress_array)
{
compress_array = FALSE;
for (i = 0; i < nfds; i++)
{
if (fds[i].fd == -1)
{
for(j = i; j < nfds; j++)
{
fds[j].fd = fds[j+1].fd;
}
nfds--;
}
}
}
} while (end_server == FALSE); /* End of serving running. */
/*************************************************************/
/* Clean up all of the sockets that are open */
/*************************************************************/
for (i = 0; i < nfds; i++)
{
if(fds[i].fd >= 0)
close(fds[i].fd);
}
}