Messages in this thread | | | Date | Thu, 06 Sep 2007 00:50:33 +0200 | From | "Michael Kerrisk" <> | Subject | Re: [PATCH] Revised timerfd() interface |
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Hi Davide,
> > > > > > As I think about this more, I see more problems with > > > > > > your argument. timerfd needs the ability to get and > > > > > > get-while-setting just as much as the earlier APIs. > > > > > > Consider a library that creates a timerfd file descriptor that > > > > > > is handed off to an application: that library may want > > > > > > to modify the timer settings without having to create a > > > > > > new file descriptor (the app mey not be able to be told about > > > > > > the new fd). Your argument just doesn't hold, AFAICS. > > > > > > > > > > Such hypotethical library, in case it really wanted to offer such > > > > > functionality, could simply return an handle instead of the raw > > > > > fd, and take care of all that stuff in userspace. > > > > > > > > Did I miss something? Is it not the case that as soon as the > > > > library returns a handle, rather than an fd, then the whole > > > > advantage of timerfd() (being able to select/poll/epoll on > > > > the timer as well as other fds) is lost? > > > > > > Why? The handle would simply be a little struct where the timerfd fd > > > is > > > stored, and a XXX_getfd() would return it. > > > So my point is, I doubt such functionalities are really needed, and I > > > also argue that the kernel is the best place for such wrapper code > > > to go. > > > > So what happens if one thread (via the library) wants modify > > a timer's settings at the same timer as another thread is > > select()ing on it? The first thread can't do this by creating > > a new timerfd timer, since it wants to affect the select() > > in the other thread? > > It can be done w/out any problems. The select thread will be notified > whenever the new timer setting expires.
We are going in circles here. I think you are missing my point. Consider the following
[[ Thread A: calls library function which creates a timerfd file descriptor.
Thread B: calls select() on the timerfd file descriptor.
Thread A: calls library function which wants to: a) modify timer settings, and retrieve copy of current timer settings, and later b) restore old timer settings. ]]
This seems a quite reasonable use-case to me, and the existing interface simply can't support it.
Cheers,
Michael -- Michael Kerrisk maintainer of Linux man pages Sections 2, 3, 4, 5, and 7
Want to help with man page maintenance? Grab the latest tarball at http://www.kernel.org/pub/linux/docs/manpages , read the HOWTOHELP file and grep the source files for 'FIXME'.
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