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

Question

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?

Answer 1

Polling it for output readiness doesn't mean you will get notified when some output occurs: it means that you'll get notified when there is output buffer space available so you can output (but you should still check the return value of your output function. The buffer state may have changed between polling and outputting; always check return values).