我在C编程中阅读有关民意调查的内容，并在poll(2) man page上构建了一个应用程序。
这是一个例子：
#include<stdio.h>
#include <stropts.h>
#include <poll.h>
#include <fcntl.h>
int main() {
struct pollfd fds[2];
int timeout_msecs = -1;
int ret;
int i;
/* Open STREAMS device. */
fds[0].fd = open("/home/jeshwanth/mywork/poll/dev0", O_RDONLY);
fds[1].fd = open("/home/jeshwanth/mywork/poll/dev1", O_RDONLY);
fds[0].events = POLLOUT | POLLWRBAND;
fds[1].events = POLLOUT | POLLWRBAND;
while (1) {
ret = poll(fds, 2, timeout_msecs);
if (ret > 0) {
/* An event on one of the fds has occurred. */
for (i = 0; i < 2; i++) {
if (fds[i].revents != 0) {
/* Priority data may be written on device number i. */
printf(
"Priority Data may be written on device number %d POLLWRBAND\n",
i);
}
if (fds[i].revents = !0) {
/* Data may be written on device number i. */
printf("Data may be written on device number %d POLLOUT\n",
i);
}
if (fds[i].revents = !0) {
/* A hangup has occurred on device number i. */
printf("A hangup has occurred on device number %d\n", i);
}
}
}
}
return 0;
}
注意：dev0和dev1是普通文件。当我运行程序时，如果dev0和dev1中没有发生任何事件，则会显示该消息。但是我期待当一些写入文件时，只有它才会显示消息。我错了吗？
轮询输出就绪并不意味着当出现输出时会收到通知：这意味着当有输出缓冲区空间可以输出时会收到通知（但是你仍然应该检查输出函数的返回值） 。轮询和输出之间的缓冲区状态可能已经改变;总是检查返回值）。
最小FIFO命名管道示例
你将无法看到任何有趣的常规文件，因为那些总是立即给POLLIN：How can select() wait on regular file descriptors (non-sockets)?
使用poll最简单的方法是使用命名管道，如下所示。这应该为您的主要应用程序做好准备：套接字和设备文件。
来源如下。用法：
sudo mknod poll0.tmp p
sudo mknod poll1.tmp p
sudo chmod 666 poll*.tmp
./poll.out
在另一个shell上：
printf a > poll0.tmp
printf b > poll1.tmp
输出：
loop
POLLIN i=0 n=1 buf=a
loop
POLLHUP i=0
loop
POLLIN i=1 n=1 buf=b
POLLHUP i=1
loop
请注意poll如何在没有循环的情况下等待读取。
冷却器的例子：
(while true; do date; sleep 1; done) > poll0.tmp &
(while true; do date; sleep 2; done) > poll1.tmp &
0每秒写一次，1每两秒写一次，这表明poll()同时处理两个输入，而不会相互拖延。
资源：
#define _XOPEN_SOURCE 700
#include <fcntl.h> /* creat, O_CREAT */
#include <poll.h> /* poll */
#include <stdio.h> /* printf, puts, snprintf */
#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS */
#include <unistd.h> /* read */
int main(void) {
enum { N = 2 };
char buf[1024], path[1024];
int fd, i, n;
short revents;
struct pollfd pfds[N];
for (i = 0; i < N; ++i) {
snprintf(path, sizeof(path), "poll%d.tmp", i);
/* O_NONBLOCK is required or else the open blocks
* until the other side of the pipe opens. */
fd = open(path, O_RDONLY | O_NONBLOCK);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
pfds[i].fd = fd;
/* Only events in this mask will be listened to.
* However, there are also some events that are unmaskable,
* notably POLLHUP when pipe closes! */
pfds[i].events = POLLIN;
}
while (1) {
puts("loop");
i = poll(pfds, N, -1);
if (i == -1) {
perror("poll");
exit(EXIT_FAILURE);
}
for (i = 0; i < N; ++i) {
revents = pfds[i].revents;
if (revents & POLLIN) {
n = read(pfds[i].fd, buf, sizeof(buf));
printf("POLLIN i=%d n=%d buf=%.*s\n", i, n, n, buf);
}
if (revents & POLLHUP) {
printf("POLLHUP i=%d\n", i);
/* This happens when the other side closed.
* This event is only cleared when we close the reader. */
/* poll won't set POLLHUP anymore once all fds are closed.
* Any futher polls on this will give the POLLNVAL event instead. */
close(pfds[i].fd);
/* negative fds are ignored. So if we negate an FD,
* we can both turn if off for a while, and turn it on
* later on by re-nagating it. */
pfds[i].fd *= -1;
}
}
}
}
编译：
gcc -o poll.out -std=c99 poll.c
在Ubuntu 14.04中测试过。
线条：
close(pfds[i].fd);
pfds[i].fd *= -1;
是必需的，否则你永远得到POLLHUP，参见：How to use the poll C function to watch named pipes in Linux?
为了更有趣，创建一个实现poll fops的Linux内核模块：How to add poll function to the kernel module code?
我会告诉你如何纠正它。 revents被解释为几个位标志。
/* check for priority write readiness */
if (fds[i].revents & POLLWRBAND) {
printf("Priority Data may be written on device number %d POLLWRBAND\n", i);
}
/* check for write readiness */
if (fds[i].revents & POLLOUT) {
printf("Data may be written on device number %d POLLOUT\n", i);
}
/* check for hang-up */
if (fds[i].revents & POLLHUP) {
printf("A hangup has occurred on device number %d\n", i);
}