sd_event_add_io() adds a new I/O event source to an event loop. The
event loop object is specified in the event parameter, the event
source object is returned in the source parameter. The fd parameter
takes the UNIX file descriptor to watch, which may refer to a socket,
a FIFO, a message queue, a serial connection, a character device, or
any other file descriptor compatible with Linux epoll(7). The events
parameter takes a bit mask of events to watch for, a combination of
the following event flags: EPOLLIN, EPOLLOUT, EPOLLRDHUP, EPOLLPRI,
and EPOLLET, see epoll_ctl(2) for details. The handler shall
reference a function to call when the event source is triggered. The
userdata pointer will be passed to the handler function, and may be
chosen freely by the caller. The handler will also be passed the file
descriptor the event was seen on, as well as the actual event flags.
It's generally a subset of the events watched, however may
additionally include EPOLLERR and EPOLLHUP.
By default, an event source will stay enabled continuously
(SD_EVENT_ON), but this may be changed with
sd_event_source_set_enabled(3). If the handler function returns a
negative error code, it will be disabled after the invocation, even
if the SD_EVENT_ON mode was requested before. Note that an event
source set to SD_EVENT_ON will fire continuously unless data is read
from or written to the file descriptor to reset the mask of events
Setting the I/O event mask to watch for to 0 does not mean that the
event source won't be triggered anymore, as EPOLLHUP and EPOLLERR may
be triggered even with a zero event mask. To temporarily disable an
I/O event source use sd_event_source_set_enabled(3) with SD_EVENT_OFF
To destroy an event source object use sd_event_source_unref(3), but
note that the event source is only removed from the event loop when
all references to the event source are dropped. To make sure an event
source does not fire anymore, even if it is still referenced, disable
the event source using sd_event_source_set_enabled(3) with
If the second parameter of sd_event_add_io() is NULL no reference to
the event source object is returned. In this case the event source is
considered "floating", and will be destroyed implicitly when the
event loop itself is destroyed.
It is recommended to use sd_event_add_io() only in conjunction with
file descriptors that have O_NONBLOCK set, to ensure that all I/O
operations from invoked handlers are properly asynchronous and
non-blocking. Using file descriptors without O_NONBLOCK might result
in unexpected starvation of other event sources. See fcntl(2) for
details on enabling O_NONBLOCK mode.
sd_event_source_get_io_events() retrieves the configured mask of
watched I/O events of an event source created previously with
sd_event_add_io(). It takes the event source object and a pointer to
a variable to store the mask in.
sd_event_source_set_io_events() configures the mask of watched I/O
events of an event source created previously with sd_event_add_io().
It takes the event source object and the new event mask.
sd_event_source_get_io_revents() retrieves the I/O event mask of
currently seen but undispatched events from an event source created
previously with sd_event_add_io(). It takes the event source object
and a pointer to a variable to store the event mask in. When called
from a handler function on the handler's event source object this
will return the same mask as passed to the handler's revents
parameter. This call is primarily useful to check for undispatched
events of an event source from the handler of an unrelated (possibly
higher priority) event source. Note the relation between
sd_event_source_get_pending() and sd_event_source_get_io_revents():
both functions will report non-zero results when there's an event
pending for the event source, but the former applies to all event
source types, the latter only to I/O event sources.
sd_event_source_get_io_fd() retrieves the UNIX file descriptor of an
event source created previously with sd_event_add_io(). It takes the
event source object and returns the non-negative file descriptor or a
negative error number on error (see below).
sd_event_source_set_io_fd() changes the UNIX file descriptor of an
I/O event source created previously with sd_event_add_io(). It takes
the event source object and the new file descriptor.
Returned values may indicate the following problems:
Not enough memory to allocate an object.
An invalid argument has been passed.
The event loop is already terminated.
The event loop has been created in a different process.
The passed event source is not an I/O event source.
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
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-03-13. If you dis‐
cover 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
systemd 233 SD_EVENT_ADD_IO(3)