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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |NOTES |HISTORY |SEE ALSO |COLOPHON | |
SD_BUS_GET_FD(3) sd_bus_get_fdSD_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 withpoll(3) or a similar function to wait for I/O events on the specified bus connection object. If the bus object was configured with thesd_bus_set_fd()function, then theinput_fd file descriptor used in that call is returned.sd_bus_get_events()returns the I/O events to wait for, suitable for passing topoll()or a similar call. Returns a combination ofPOLLIN,POLLOUT, ... events, or negative on error.sd_bus_get_timeout()returns theabsolute timeout in μs, from which the relative timeout to pass topoll()(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 beUINT64_MAXin 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 ofCLOCK_MONOTONICand specified in microseconds. When converting this value in order to pass it as third argument topoll()(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, useppoll(2) instead of plainpoll(), 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 involvingpoll()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 bysd_bus_get_fd()should be polled for the events indicated bysd_bus_get_events(), and the I/O call should block for that up to the timeout returned bysd_bus_get_timeout(). After each I/O polling call the bus connection needs to process incoming or outgoing data, by invokingsd_bus_process(3). Note that these functions are only one of three supported ways to implement I/O event handling for bus connections. Alternatively usesd_bus_attach_event(3) to attach a bus connection to ansd-event(3) event loop. Or usesd_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 afterfork().-ENOTCONN The bus connection has been terminated.-EPERM Two distinct file descriptors were passed for input and output usingsd_bus_set_fd(), whichsd_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 thelibsystemd pkg-config(1) file. The code described here usesgetenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not callsetenv(3) from a parallel thread. It is recommended to only do calls tosetenv()from an early phase of the program when no other threads have been started.
sd_bus_get_fd(),sd_bus_get_events(), andsd_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 thesystemd (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 isnot part of the original manual page), send a mail to man-pages@man7.orgsystemd 258~rc2SD_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)
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