sd_bus_get_fd(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_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_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_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 absolute time-out in μs, from
       which the relative time-out to pass to poll() (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 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 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).

       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 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

           The bus cannot be resolved.

           Added in version 246.

NOTES         top

       Functions described here are available as a shared library, which
       can be compiled against and linked to with the
       libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be
       not multi-thread-safe. This means that the code calling the
       functions described here must not call setenv(3) from a parallel
       thread. It is recommended to only do calls to setenv() from an
       early phase of the program when no other threads have been

HISTORY         top

       sd_bus_get_fd() was added in version 231.

       sd_bus_get_events() and sd_bus_get_timeout() were added in
       version 240.

SEE ALSO         top

       systemd(1), sd-bus(3), sd_bus_process(3), sd_bus_attach_event(3),
       sd_bus_wait(3), sd_bus_set_fd(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
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 255                                             SD_BUS_GET_FD(3)

Pages that refer to this page: sd-bus(3)sd_bus_attach_event(3)sd_bus_process(3)sd_bus_set_fd(3)sd_bus_wait(3)systemd.directives(7)systemd.index(7)