NAME

       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

       #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

       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

       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.

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           An invalid bus object was passed.

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

       -ENOTCONN
           The bus connection has been terminated.

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

       -ENOPKG
           The bus cannot be resolved.

           Added in version 246.

NOTES

       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 started.

HISTORY

       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

       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)

systemd 255                                                                                     SD_BUS_GET_FD(3)