|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | HISTORY | SEE ALSO | COLOPHON |
|
|
|
SD_BUS_GET_FD(3) sd_bus_get_fd SD_BUS_GET_FD(3)
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 absolute timeout in μs, from
which the relative timeout 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 does
not 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.
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.
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.
sd_bus_get_fd(), sd_bus_get_events(), and sd_bus_get_timeout()
were added in version 221.
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)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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
man-pages@man7.org
systemd 258~rc2 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)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|