Messages in this thread | | | Date | Mon, 26 Aug 2019 06:22:29 +0000 | From | Eric Wong <> | Subject | Re: Why the edge-triggered mode doesn't work for epoll file descriptor? |
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Heiher <r@hev.cc> wrote: > Hello, > > I've added a pipe file descriptor (fd1) to an epoll (fd3) with > EPOLLOUT in edge-triggered mode, and then added the fd3 to another > epoll (fd4) with EPOLLIN in edge-triggered too. > > Next, waiting for fd4 without timeout. When fd1 to be writable, i > think epoll_wait(fd4, ...) only return once, because all file > descriptors are added in edge-triggered mode. > > But, the actual result is returns many and many times until do once > eopll_wait(fd3, ...).
It looks like you can trigger a wakeup loop with printf writing to the terminal (not a pipe), and that write to the terminal triggering the EPOLLOUT wakeup over and over again.
I don't know TTY stuff at all, but I assume it's intended for terminals.
You refer to "pipe file descriptor (fd1)", but I can't reproduce the error when running your code piped to "tee" and using strace to check epoll_wait returns.
"strace ./foo | tee /dev/null" only shows one epoll_wait returning.
> e.events = EPOLLIN | EPOLLET; > e.data.u64 = 1; > if (epoll_ctl (efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0) > return -3; > > e.events = EPOLLOUT | EPOLLET; > e.data.u64 = 2; > if (epoll_ctl (efd[1], EPOLL_CTL_ADD, 1, &e) < 0) > return -4;
Since epfd[1] is waiting for stdout...
> for (;;) { > struct epoll_event events[16]; > int nfds; > > nfds = epoll_wait (efd[0], events, 16, -1); > printf ("nfds: %d\n", nfds);
Try outputting your message to stderr instead of stdout:
fprintf(stderr, "nfds: %d\n", nfds);
And then run your program so stdout and stderr point to different files:
./foo | tee /dev/null
(so stdout becomes a pipe, and stderr remains your terminal)
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