poll C Linux函数如何工作？(Why is this message not only displayed when a file is written to (using the poll C Linux function)?)

 我在阅读C编程中的民意调查，并在poll（2）手册页中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，则显示该消息。 但我期待一些写入文件发生时，只有它应该显示消息。 我错了吗？ 

I was reading about poll in C programming and built an application given on the poll(2) man page.
 
Here is the example:
 
#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;
}
 
Note: dev0 and dev1 are normal files. When I run the program, if no event occurred in dev0 and dev1, the message is displayed. But I was expecting when some write into the file happens, only then should it display the message. Am I wrong?

原文：https://stackoverflow.com/questions/12444679