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_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

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);

       int sd_event_source_get_io_fd_own(sd_event_source *source);

       int sd_event_source_set_io_fd_own(sd_event_source *source, int b);

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.

       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 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.

       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.

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

       These APIs are implemented as a shared library, which can be compiled and linked to with
       the libsystemd pkg-config(1) file.

SEE ALSO

       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)