我阅读了C编程中的poll，并在poll(2)手册页上构建了一个应用程序。
以下是示例：
#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 POLLWRBANDn",
i);
}
if (fds[i].revents = !0) {
/* Data may be written on device number i. */
printf("Data may be written on device number %d POLLOUTn",
i);
}
if (fds[i].revents = !0) {
/* A hangup has occurred on device number i. */
printf("A hangup has occurred on device number %dn", i);
}
}
}
}
return 0;
}
注意：dev0和dev1是正常文件。当我运行程序时，如果dev0和dev1中没有发生任何事件，就会显示消息。但我原以为，当一些写入文件的操作发生时，它才会显示消息。我错了吗？
轮询它是否准备好输出并不意味着在发生某些输出时会收到通知：这意味着当有可用的输出缓冲区空间时，你会收到通知以便输出(但你仍然应该检查输出函数的返回值。缓冲区状态可能在轮询和输出之间发生了变化；请始终检查返回值)。
最小FIFO命名管道示例
使用常规文件将看不到任何有趣的东西，因为这些文件总是立即给出POLLIN：如何选择()等待常规文件描述符(非套接字)？
使用poll最简单的方法是使用命名管道，如下所示。这应该为它们的主要应用程序：套接字和设备文件做好准备。
来源如下。用法：
sudo mknod poll0.tmp p
sudo mknod poll1.tmp p
sudo chmod 666 poll*.tmp
./poll.out
在另一个外壳上：
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
因此，请注意poll是如何在不循环的情况下等待读取的。
冷却器示例：
(while true; do date; sleep 1; done) > poll0.tmp &
(while true; do date; sleep 2; done) > poll1.tmp &
0每1秒写入一次，1每两秒钟写入一次。这显示了poll()如何同时处理两个输入，而不会使彼此停滞。
来源：
#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=%.*sn", i, n, n, buf);
}
if (revents & POLLHUP) {
printf("POLLHUP i=%dn", 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;
}
}
}
}
编译使用：
gcc -o poll.out -std=c99 poll.c
在Ubuntu 14.04中测试。
GitHub上游。
线路：
close(pfds[i].fd);
pfds[i].fd *= -1;
是必需的，否则您将永远获得POLLHUP，另请参阅：如何使用poll C函数在Linux中监视命名管道？
更有趣的是，创建一个实现pollfops的Linux内核模块：如何在内核模块代码中添加轮询函数？
我会给你一个如何更正的提示。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 POLLWRBANDn", i);
}
/* check for write readiness */
if (fds[i].revents & POLLOUT) {
printf("Data may be written on device number %d POLLOUTn", i);
}
/* check for hang-up */
if (fds[i].revents & POLLHUP) {
printf("A hangup has occurred on device number %dn", i);
}