Ubuntu Manpage: sd_event_add_io, sd_event_source_get_io_events, sd_event_source_set_io

Provided by: libsystemd-dev_229-4ubuntu21.31_amd64

NAME

       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, sd_event_io_handler_t - Add an I/O event source to an event loop

SYNOPSIS

       #include <systemd/sd-event.h>

       typedef struct sd_event_source sd_event_source;

       typedef int (*sd_event_io_handler_t)(sd_event_source *s, int fd, uint32_t revents,
                                            void *userdata);

       int sd_event_add_io(sd_event *event, sd_event_source **source, int fd, uint32_t events,
                           sd_event_io_handler_t handler, void *userdata);

       int sd_event_source_get_io_events(sd_event_source *source, uint32_t *events);

       int sd_event_source_set_io_events(sd_event_source *source, uint32_t events);

       int sd_event_source_get_io_revents(sd_event_source *source, uint32_t *revents);

       int sd_event_source_get_io_fd(sd_event_source *source);

       int sd_event_source_set_io_fd(sd_event_source *source, int fd);

DESCRIPTION

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
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 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 instead.

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
SD_EVENT_OFF.

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.

RETURN VALUE

       On success, these functions return 0 or a positive integer. On failure, they return a
       negative errno-style error code.

ERRORS

       Returned values may indicate the following problems:

       -ENOMEM
           Not enough memory to allocate an object.

       -EINVAL
           An invalid argument has been passed.

       -ESTALE
           The event loop is already terminated.

       -ECHILD
           The event loop has been created in a different process.

       -EDOM
           The passed event source is not an I/O event source.

NOTES