Re: [systemd-devel] sd_bus_get_timeout returns absolute time despite what's documented
Hey Lennart, Yes, I was referring to man pages I read a few years ago when I used sd_bus_get_timeout() for the first time. And, before posting this mail, I also checked on my current Linux development machine, which has systemd v247. So I was not that far :-) The fixed wording in v250 is correct, understandable and explicit to me. All good now. Thanks a lot. Stanislav. --- Original Message --- On Monday, January 9th, 2023 at 1:02 PM, Lennart Poettering wrote: > On Mo, 09.01.23 12:53, Lennart Poettering (lenn...@poettering.net) wrote: > > > https://www.freedesktop.org/software/systemd/man/sd_bus_get_fd.html#Description > > > > Note that the returned time-value is absolute, based of > > CLOCK_MONOTONIC and specified in microseconds. When converting > > this value in order to pass it as third argument to poll() (which > > expects relative milliseconds), care should be taken to convert to > > a relative time and use a division that rounds up to ensure the > > I/O polling operation doesn't sleep for shorter than necessary, > > which might result in unintended busy looping (alternatively, use > > ppoll(2) instead of plain poll(), which understands timeouts with > > nano-second granularity). > > > > That's pretty explicit already, no? > > > This was fixed in 2021 btw, 25060a570c106cf5a14a3268bb0d38d9feb7fdab > i.e. systemd 250. Upgrade! > > > (I mean, you have half a point, the first sentence of the explanation > > might people think this was a relative timeout, but we all read the > > full documentation, no, before actually using this API, no? ;-)) > > > > Anyway, will prep a fix that rewords the first sentence to make this > > clearer right away. > > > https://github.com/systemd/systemd/pull/25985 > > Lennart > > -- > Lennart Poettering, Berlin
Re: [systemd-devel] sd_bus_get_timeout returns absolute time despite what's documented
On Mo, 09.01.23 12:53, Lennart Poettering (lenn...@poettering.net) wrote: > https://www.freedesktop.org/software/systemd/man/sd_bus_get_fd.html#Description > > Note that the returned time-value is absolute, based of > CLOCK_MONOTONIC and specified in microseconds. When converting > this value in order to pass it as third argument to poll() (which > expects relative milliseconds), care should be taken to convert to > a relative time and use a division that rounds up to ensure the > I/O polling operation doesn't sleep for shorter than necessary, > which might result in unintended busy looping (alternatively, use > ppoll(2) instead of plain poll(), which understands timeouts with > nano-second granularity). > > That's pretty explicit already, no? This was fixed in 2021 btw, 25060a570c106cf5a14a3268bb0d38d9feb7fdab i.e. systemd 250. Upgrade! > (I mean, you have half a point, the first sentence of the explanation > might people think this was a relative timeout, but we all read the > full documentation, no, before actually using this API, no? ;-)) > > Anyway, will prep a fix that rewords the first sentence to make this > clearer right away. https://github.com/systemd/systemd/pull/25985 Lennart -- Lennart Poettering, Berlin
Re: [systemd-devel] sd_bus_get_timeout returns absolute time despite what's documented
On Sa, 07.01.23 10:35, Stanislav Angelovič (stanislav.angelo...@protonmail.com) wrote: > Hi everyone, > > According to sd_bus_get_timeout(3) man page, sd_bus_get_timeout() > returns "timeout in us to pass to poll()" and it also states that > "the returned time-value is relative". > > However, in reality, this function returns an absolute time point > (a-ka relative to the monotonic clock epoch). Hence, the returned > value cannot be simply converted to milliseconds and given to > poll(). It must first be subtracted from the current monotonic clock > time. > > I followed the documentation and had a bug in my code for some time > :) Shall the documentation be updated to mention the necessity of > conversion from absolute to relative? Or does sd_bus_get_timeout() > have a bug and shall this function do the subtraction for the user > and return relative time? Hmm, the man page says this currently: https://www.freedesktop.org/software/systemd/man/sd_bus_get_fd.html#Description Note that the returned time-value is absolute, based of CLOCK_MONOTONIC and specified in microseconds. When converting this value in order to pass it as third argument to poll() (which expects relative milliseconds), care should be taken to convert to a relative time and use a division that rounds up to ensure the I/O polling operation doesn't sleep for shorter than necessary, which might result in unintended busy looping (alternatively, use ppoll(2) instead of plain poll(), which understands timeouts with nano-second granularity). That's pretty explicit already, no? (I mean, you have half a point, the first sentence of the explanation might people think this was a relative timeout, but we all read the full documentation, no, before actually using this API, no? ;-)) Anyway, will prep a fix that rewords the first sentence to make this clearer right away. Lennart -- Lennart Poettering, Berlin
[systemd-devel] sd_bus_get_timeout returns absolute time despite what's documented
Hi everyone, According to sd_bus_get_timeout(3) man page, sd_bus_get_timeout() returns "timeout in us to pass to poll()" and it also states that "the returned time-value is relative". However, in reality, this function returns an absolute time point (a-ka relative to the monotonic clock epoch). Hence, the returned value cannot be simply converted to milliseconds and given to poll(). It must first be subtracted from the current monotonic clock time. I followed the documentation and had a bug in my code for some time :) Shall the documentation be updated to mention the necessity of conversion from absolute to relative? Or does sd_bus_get_timeout() have a bug and shall this function do the subtraction for the user and return relative time? Thanks, kind regards, Stanislav.
