sd_event_add_io, sd_event_source_get_io_events, sd_event_source_set_io_events, sd_event_source_get_io_revents, sd_event_source_get_io_fd, sd_event_source_set_io_fd, sd_event_source_get_io_fd_own, sd_event_source_set_io_fd_own, sd_event_source, sd_event_io_handler_t — Add an I/O event source to an event loop
typedef struct sd_event_source sd_event_source;
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
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:
details. Note that not all file descriptors are compatible with epoll, for example regular file or
directories are not. If this function is called with a file descriptor that does not support epoll,
-EPERM is returned (also see below). In most cases such file descriptors may be
treated as always-readable or always-writable, so that IO event watching is unnecessary.
handler is a function to call when the event source is triggered or
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
handler may return negative to signal an error (see below), other return values are ignored. If
NULL, a default handler that calls
sd_event_exit(3) will be
By default, an event source will stay enabled continuously (
this may be changed with
If the handler function returns a negative error code, it will either be disabled after the invocation,
even if the
SD_EVENT_ON mode was requested before, or it will cause the loop to
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 seen.
Setting the I/O event mask to watch for to 0 does not mean
that the event source won't be triggered anymore, as
may be triggered even with a zero event mask. To temporarily
disable an I/O event source use
To destroy an event source object use
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
If the second parameter of
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.
NULL, and the event source fires, this will be considered a request to exit the
event loop. In this case, the
userdata parameter, cast to an integer, is passed as
the exit code parameter to
Note that this call does not take possession of the file descriptor passed in, ownership (and thus
the duty to close it when it is no longer needed) remains with the caller. However, with the
sd_event_source_set_io_fd_own() call (see below) the event source may optionally
take ownership of the file descriptor after the event source has been created. In that case the file
descriptor is closed automatically as soon as the event source is released.
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
for details on enabling
the configured mask of watched I/O events of an event source created
sd_event_add_io(). It takes
the event source object and a pointer to a variable to store the
configures the mask of watched I/O events of an event source created
sd_event_add_io(). It takes the
event source object and the new event mask.
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
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
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.
the UNIX file descriptor of an event source created previously
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).
changes the UNIX file descriptor of an I/O event source created
sd_event_add_io(). It takes
the event source object and the new file descriptor.
sd_event_source_set_io_fd_own() controls whether the file descriptor of the event source
shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
event source object. By default it is not closed automatically, and the application has to do this on its own. The
b parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
when the event source is freed, otherwise it is closed.
sd_event_source_get_io_fd_own() may be used to query the current setting of the file
descriptor ownership boolean flag as set with
sd_event_source_set_io_fd_own(). It returns
positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
negative on error.
On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code.
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, library or module instance.
The passed event source is not an I/O event source.
Functions described here are available as a shared
library, which can be compiled against and linked to with the
The code described here uses
which is declared to be not multi-thread-safe. This means that the code calling the functions described
here must not call
from a parallel thread. It is recommended to only do calls to
from an early phase of the program when no other threads have been started.
sd_event_source_set_io_fd() were added in version 229.
sd_event_source_set_io_fd_own() were added in version 239.
systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3), sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_child(3), sd_event_add_inotify(3), sd_event_add_defer(3), sd_event_source_set_enabled(3), sd_event_source_set_priority(3), sd_event_source_set_userdata(3), sd_event_source_set_description(3), sd_event_source_get_pending(3), sd_event_source_set_floating(3), epoll_ctl(2), epoll(7)