x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
SD_BUS_GET_FD(3) sd_bus_get_fd SD_BUS_GET_FD(3)
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 <mu>s, 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.
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.
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 254 SD_BUS_GET_FD(3)
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