Re: Why the edge-triggered mode doesn't work for epoll file descriptor?

From: Eric Wong
Date: Mon Aug 26 2019 - 02:22:31 EST

Heiher <r@xxxxxx> 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.

> = 1;
> if (epoll_ctl (efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
> return -3;
> = 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)