[Bug 240015] kevent example to check event.flags is wrong.

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240015

            Bug ID: 240015
           Summary: kevent example to check event.flags is wrong.
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: Manual Pages
          Assignee: bugs at FreeBSD.org
          Reporter: osho at pcc-software.org
                CC: doc at FreeBSD.org

The example code used for kqueue (2) is a bit wrong.  Checking event.flags is
not needed.

What is expected?
Remove event.flags check like this:
@@ -655,8 +655,6 @@
     ret = kevent(kq, &event, 1, NULL, 0, NULL);
     if (ret == -1)
        err(EXIT_FAILURE, "kevent register");
-    if (event.flags & EV_ERROR)
-       errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));

     for (;;) {
        /* Sleep until something happens. */

Why it is expected?
If I understand the manual correctly, if |nevents| argument is 0, -1 would
be returned for error, and it won't modify |changelist| argument.
In the above example, event.
>     The kevent() system call returns the number of events placed in the
>     eventlist, up to the value given by nevents.  If an error occurs while
>     processing an element of the changelist and there is enough room in the
>     eventlist, then the event will be placed in the eventlist with EV_ERROR
>     set in flags and the system error in data.  Otherwise, -1 will be
>     returned, and errno will be set to indicate the error condition.  If the
>     time limit expires, then kevent() returns 0.

However, the above code expect the kernel to update |event|.  Also, a caller
should not usually expect the callee updates const value.

I believe the following code explains my understanding is correct:
https://svnweb.freebsd.org/base/head/sys/kern/kern_event.c?revision=350421&view=markup#l1235
i.e. if |nevents| is 0, registration failure should be notified via return
value instead of |changelist| parameter.

Note that this issue was not originally found by myself, but suggested by the
person who is not familiar with filing a FreeBSD issue.  I confirmed the
suggestion correct by investigating the kernel before filing the issue.
Also, note that NetBSD and OpenBSD use different example code and not affected
by this issue.

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