Date: Thu, 27 Dec 2018 18:47:14 -0700 From: Alan Somers <asomers@freebsd.org> To: Willem Jan Withagen <wjw@digiware.nl> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Using kqueue with aio_read/write Message-ID: <CAOtMX2iOy4Uf%2B9%2BuYhbX-wXJ68E57CjTW0aLPsa3dH__n4oP_w@mail.gmail.com> In-Reply-To: <8753521a-4555-ec2a-5efc-dee2660b4d9b@digiware.nl> References: <8753521a-4555-ec2a-5efc-dee2660b4d9b@digiware.nl>
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On Thu, Dec 27, 2018 at 6:15 PM Willem Jan Withagen <wjw@digiware.nl> wrote:
>
> Hi,
>
> Im trying to understand why I cannot get so code to work.
> This is the smallest extract I can make to show my problem.
>
> I would expect the kevent() call to return every timeo tick.
> Even if I tell it NOT to time-out I get these spurts of errors
>
> Since there is nothing to trigger the AIO-event, I would expect kqueue
> to hold indefinitly.
>
> But it does not generate anything other than errors
> And instead it repeatedly complains that there is a permission error:
> get_events_kevent: EV_Error(1) kevent(): Operation not permitted
>
> But I'm not getting where that would the case...
>
> Surely a pilot error, but I do overlook it al the time.
> So suggestions are welcome.
>
> Thanx,
> --WjW
>
> #include <aio.h>
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/stat.h>
> #include <sys/event.h>
> #include <unistd.h>
>
> #define BUFFER_SIZE 512
> #define MAX_EVENTS 32
>
> #define FILENAME "/tmp/aio_test"
> char filename[256];
> int fd;
> int done = 0;
>
> void get_events_kevent(int fd, int kq)
> {
> printf("get_events function fd = %d, kq = %d\n", fd, kq);
> int i = 0, errcnt = 0, err, ret, reterr, rev;
> int search = 1;
>
> int timeout_ms = 10;
> struct timespec timeo = {
> timeout_ms / 1000,
> (timeout_ms % 1000) * 1000 * 1000
> };
> struct kevent filter[16];
> struct kevent changed[16];
>
> EV_SET(&filter[0], fd, EVFILT_AIO,
> EV_ADD,
> 0, 0, 0 );
This is the first problem. There's no need to explicitly set
EVFILT_AIO on the kqueue. It gets set by the aio_read(2) or similar
syscall. And this invocation wouldn't be correct anyway, because for
AIO the ident field refers to the address of the struct aiocb, not the
file descriptor. If the only events you care about are AIO, then you
can pass NULL as the filter argument to kevent. I suspect this is the
cause of your problem. The kernel probably thinks you're trying to
register for an aiocb that's outside of your address space or
something like that.
> while (!done) {
> printf("+");
> rev = kevent(kq, filter, 1, changed, 16, 0); //&timeo);
> if (rev < 0) {
> perror("kevent error");
> } else if (rev == 0) {
> printf("T");
> } else {
> printf("rev(%d)\n", rev);
> if (changed[0].flags == EV_ERROR) {
> errno = changed[0].data;
> printf( "%s: EV_Error(%d) kevent(): %s\n", __func__, errno,
> strerror(errno));
> memset(&changed[0], 0, sizeof(struct kevent));
> } else {
> err = aio_error((struct aiocb*)changed[0].udata);
No need to call aio_error(2) after kevent(2) returns. You can go
straight to aio_return. aio_error shouldn't hurt, but it isn't
necessary.
> ret = aio_return((struct aiocb*)changed[0].udata);
> if (ret < 0 )
> reterr = errno;
> if (err != 0) {
> printf( "%s: slot: %d, Error(%d) at aio_error():
> %s\n", __func__, i, err, strerror (err));
> errcnt++;
> if (errcnt > 50) {
> exit(3);
> }
> }
> }
> }
> }
> }
>
> int main()
> {
> (void) strcpy(filename, FILENAME);
> unlink(filename);
> fd = open(filename, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
> if (fd == -1) {
> printf( "%s: Error at open(): %s\n", __func__, strerror(errno));
> exit(1);
> }
>
> int kq = kqueue();
> if (fd == -1) {
> printf( "%s: Error at kqueue(): %s\n", __func__, strerror(errno));
> exit(1);
> }
>
> get_events_kevent( fd, kq);
>
> close(kq);
> close(fd);
> return 0;
> }
>
>
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