sd_bus_get_events(3) — Linux manual page


SD_BUS_GET_FD(3)                sd_bus_get_fd               SD_BUS_GET_FD(3)

NAME         top

       sd_bus_get_fd, sd_bus_set_fd, sd_bus_get_events, sd_bus_get_timeout -
       Get the file descriptor, I/O events and timeout to wait for from a
       message bus object

SYNOPSIS         top

       #include <systemd/sd-bus.h>

       int sd_bus_get_fd(sd_bus *bus);

       int sd_bus_set_fd(sd_bus *bus, int input_fd, int output_fd);

       int sd_bus_get_events(sd_bus *bus);

       int sd_bus_get_timeout(sd_bus *bus, uint64_t *timeout_usec);

DESCRIPTION         top

       sd_bus_get_fd() returns the file descriptor used to communicate from
       a message bus object. This descriptor can be used with poll(3) or a
       similar function to wait for I/O events on the specified bus
       connection object. If the bus object was configured with the
       sd_bus_set_fd() function, then the input_fd file descriptor used in
       that call is returned.

       sd_bus_set_fd() sets the file descriptors used to communicate from a
       message bus object. Both input_fd and output_fd must be valid file
       descriptors. The same file descriptor may be used as both the input
       and the output file descriptor. This function must be called before
       the bus is started.

       sd_bus_get_events() returns the I/O events to wait for, suitable for
       passing to poll() or a similar call. Returns a combination of POLLIN,
       POLLOUT, ... events, or negative on error.

       sd_bus_get_timeout() returns the timeout in µs to pass to poll() 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 UINT64_MAX 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 relative and
       specified in microseconds. When converting this value in order to
       pass it as third argument to poll() (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 ppoll(3)
       instead of plain poll(), which understands timeouts with nano-second

       These three functions are useful to hook up a bus connection object
       with an external or manual event loop involving poll() 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
       sd_bus_get_fd() should be polled for the events indicated by
       sd_bus_get_events(), and the I/O call should block for that up to the
       timeout returned by sd_bus_get_timeout(). After each I/O polling call
       the bus connection needs to process incoming or outgoing data, by
       invoking sd_bus_process(3).

       Note that these functions are only one of three supported ways to
       implement I/O event handling for bus connections. Alternatively use
       sd_bus_attach_event(3) to attach a bus connection to an sd-event(3)
       event loop. Or use sd_bus_wait(3) as a simple synchronous, blocking
       I/O waiting call.

RETURN VALUE         top

       On success, sd_bus_get_fd() returns the file descriptor used for
       communication. On failure, it returns a negative errno-style error

       On success, sd_bus_set_fd() returns a non-negative integer. On
       failure, it returns a negative errno-style error code.

       On success, sd_bus_get_events() returns the I/O event mask to use for
       I/O event watching. On failure, it returns a negative errno-style
       error code.

       On success, sd_bus_get_timeout() returns a non-negative integer. On
       failure, it returns a negative errno-style error code.

       Returned errors may indicate the following problems:

           An invalid bus object was passed.

           The bus connection was allocated in a parent process and is being
           reused in a child process after fork().

           The bus connection has been terminated.

           Two distinct file descriptors were passed for input and output
           using sd_bus_set_fd(), which sd_bus_get_fd() cannot return.

           An invalid file descriptor was passed to sd_bus_set_fd().

           The bus cannot be resolved.

NOTES         top

       These APIs are implemented as a shared library, which can be compiled
       and linked to with the libsystemd pkg-config(1) file.

SEE ALSO         top

       systemd(1), sd-bus(3), sd_bus_process(3), sd_bus_attach_event(3),
       sd_bus_wait(3), poll(3)

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
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       report for this manual page, see
       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on 2020-09-18.  (At that
       time, the date of the most recent commit that was found in the repos‐
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systemd 246                                                 SD_BUS_GET_FD(3)

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