205 lines
6.3 KiB
HTML
205 lines
6.3 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SD_BUS_GET_FD</TITLE>
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</HEAD><BODY>
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<H1>SD_BUS_GET_FD</H1>
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Section: sd_bus_get_fd (3)<BR>Updated: <BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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sd_bus_get_fd, sd_bus_get_events, sd_bus_get_timeout - Get the file descriptor, I/O events and time-out to wait for from a message bus object
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<P>
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<B>
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</B><PRE>
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#include <<A HREF="file:///usr/include/systemd/sd-bus.h">systemd/sd-bus.h</A>>
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</PRE>
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<DL COMPACT>
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<DT id="1">
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<B>int sd_bus_get_fd(sd_bus *</B><I>bus</I><B>);</B>
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<DT id="2">
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<B>int sd_bus_get_events(sd_bus *</B><I>bus</I><B>);</B>
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<DT id="3">
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<B>int sd_bus_get_timeout(sd_bus *</B><I>bus</I><B>, uint64_t *</B><I>timeout_usec</I><B>);</B>
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<P>
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<B>sd_bus_get_fd()</B>
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<DD>returns the file descriptor used to communicate from a message bus object. This descriptor can be used with
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<B><A HREF="/cgi-bin/man/man2html?3+poll">poll</A></B>(3)
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or a similar function to wait for I/O events on the specified bus connection object. If the bus object was configured with the
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_set_fd">sd_bus_set_fd</A></B>(3)
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function, then the
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<I>input_fd</I>
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file descriptor used in that call is returned.
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<P>
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<B>sd_bus_get_events()</B>
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returns the I/O events to wait for, suitable for passing to
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<B>poll()</B>
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or a similar call. Returns a combination of
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<B>POLLIN</B>,
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<B>POLLOUT</B>, ... events, or negative on error.
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<P>
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<B>sd_bus_get_timeout()</B>
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returns the time-out in µs to pass to to
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<B>poll()</B>
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or a similar call when waiting for events on the specified bus connection. The returned time-out may be zero, in which case a subsequent I/O polling call should be invoked in non-blocking mode. The returned timeout may be
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<B>UINT64_MAX</B>
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in which case the I/O polling call may block indefinitely, without any applied time-out. Note that the returned time-out should be considered only a maximum sleeping time. It is permissible (and even expected) that shorter time-outs 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 relative and specified in microseconds. When converting this value in order to pass it as third argument to
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<B>poll()</B>
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(which expects milliseconds), care should be taken to 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
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<B><A HREF="/cgi-bin/man/man2html?3+ppoll">ppoll</A></B>(3)
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instead of plain
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<B>poll()</B>, which understands time-outs with nano-second granularity).
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<P>
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These three functions are useful to hook up a bus connection object with an external or manual event loop involving
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<B>poll()</B>
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or a similar I/O polling call. Before each invocation of the I/O polling call, all three functions should be invoked: the file descriptor returned by
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<B>sd_bus_get_fd()</B>
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should be polled for the events indicated by
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<B>sd_bus_get_events()</B>, and the I/O call should block for that up to the time-out returned by
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<B>sd_bus_get_timeout()</B>. After each I/O polling call the bus connection needs to process incoming or outgoing data, by invoking
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_process">sd_bus_process</A></B>(3).
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<P>
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Note that these function are only one of three supported ways to implement I/O event handling for bus connections. Alternatively use
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_attach_event">sd_bus_attach_event</A></B>(3)
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to attach a bus connection to an
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<B><A HREF="/cgi-bin/man/man2html?3+sd-event">sd-event</A></B>(3)
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event loop. Or use
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_wait">sd_bus_wait</A></B>(3)
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as a simple synchronous, blocking I/O waiting call.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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<P>
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<B>sd_bus_get_fd()</B>
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returns the file descriptor used for communication, or a negative
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<I>errno</I>-style error code on error.
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<P>
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<B>sd_bus_get_events()</B>
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returns the I/O event mask to use for I/O event watching, or a negative
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<I>errno</I>-style error code on error.
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<P>
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<B>sd_bus_get_timeout()</B>
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returns zero or positive on success, or a negative
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<I>errno</I>-style error code on error.
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<A NAME="lbAF"> </A>
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<H3>Errors</H3>
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<P>
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Returned errors may indicate the following problems:
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<P>
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<B>-EINVAL</B>
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<DL COMPACT><DT id="4"><DD>
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An invalid bus object was passed.
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</DL>
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<P>
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<B>-ECHILD</B>
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<DL COMPACT><DT id="5"><DD>
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The bus connection was allocated in a parent process and is being reused in a child process after
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<B>fork()</B>.
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</DL>
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<P>
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<B>-ENOTCONN</B>
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<DL COMPACT><DT id="6"><DD>
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The bus connection has been terminated.
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</DL>
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<P>
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<B>-EPERM</B>
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<DL COMPACT><DT id="7"><DD>
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Two distinct file descriptors were passed for input and output using
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<B>sd_bus_set_fd()</B>, which
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<B>sd_bus_get_fd()</B>
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cannot return.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>NOTES</H2>
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<P>
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These APIs are implemented as a shared library, which can be compiled and linked to with the
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<B>libsystemd</B> <B><A HREF="/cgi-bin/man/man2html?1+pkg-config">pkg-config</A></B>(1)
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file.
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<A NAME="lbAH"> </A>
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<H2>SEE ALSO</H2>
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<P>
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<B><A HREF="/cgi-bin/man/man2html?1+systemd">systemd</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?3+sd-bus">sd-bus</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_set_fd">sd_bus_set_fd</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_process">sd_bus_process</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_attach_event">sd_bus_attach_event</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+sd_bus_wait">sd_bus_wait</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+poll">poll</A></B>(3)
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="8"><A HREF="#lbAB">NAME</A><DD>
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<DT id="9"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="10"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="11"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DL>
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<DT id="12"><A HREF="#lbAF">Errors</A><DD>
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</DL>
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<DT id="13"><A HREF="#lbAG">NOTES</A><DD>
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<DT id="14"><A HREF="#lbAH">SEE ALSO</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:54 GMT, March 31, 2021
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</BODY>
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</HTML>
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