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man: make clearer that sd_bus_get_timeout() returns an absolute time-out

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https://lists.freedesktop.org/archives/systemd-devel/2023-January/048714.html
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Lennart Poettering 2023-01-09 12:58:09 +01:00
parent a3310012e8
commit f977356a82
1 changed files with 15 additions and 17 deletions
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32
man/sd_bus_get_fd.xml
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@ -66,23 +66,21 @@
<constant>POLLIN</constant>, <constant>POLLOUT</constant>, … events, or negative on error.
</para>
<para><function>sd_bus_get_timeout()</function> returns the timeout in µs to pass to
<function>poll()</function> or a similar call when waiting for events on the specified bus
connection. The returned timeout may be zero, in which case a subsequent I/O polling call
should be invoked in non-blocking mode. The returned timeout may be
<constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely,
without any applied timeout. Note that the returned timeout should be considered only a
maximum sleeping time. It is permissible (and even expected) that shorter timeouts are used by
the calling program, in case other event sources are polled in the same event loop. Note that
the returned time-value is absolute, based of <constant>CLOCK_MONOTONIC</constant> and specified
in microseconds. When converting this value in order to pass it as third argument to
<function>poll()</function> (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
<citerefentry project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
instead of plain <function>poll()</function>, which understands timeouts with nano-second
granularity).</para>
<para><function>sd_bus_get_timeout()</function> returns the <emphasis>absolute</emphasis> time-out in µs,
from which the relative time-out to pass to <function>poll()</function> (or a similar call) can be
derived, when waiting for events on the specified bus connection. The returned timeout may be zero, in
which case a subsequent I/O polling call should be invoked in non-blocking mode. The returned timeout may
be <constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely, without any
applied timeout. Note that the returned timeout should be considered only a maximum sleeping time. It is
permissible (and even expected) that shorter timeouts are used by the calling program, in case other
event sources are polled in the same event loop. Note that the returned time-value is absolute, based of
<constant>CLOCK_MONOTONIC</constant> and specified in microseconds. When converting this value in order
to pass it as third argument to <function>poll()</function> (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 <citerefentry
project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>2</manvolnum></citerefentry> instead
of plain <function>poll()</function>, which understands timeouts with nano-second granularity).</para>
<para>These three functions are useful to hook up a bus connection object with an external or
manual event loop involving <function>poll()</function> or a similar I/O polling call. Before