Index · Directives systemd 250


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


#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);


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 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 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.


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.


The bus cannot be resolved.


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

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)