Ubuntu Manpage: sd_event_add_io, sd_event_source_get_io_events, sd_event_source_set_io

Provided by: libsystemd-dev_256.5-2ubuntu3_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_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. 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.

The handler is a function to call when the event source is triggered or NULL. 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. The handler may return negative to signal an
error (see below), other return values are ignored. If handler is NULL, a default handler
that calls sd_event_exit(3) will be used.

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 either be disabled after the invocation, even if the SD_EVENT_ON mode
was requested before, or it will cause the loop to terminate, see
sd_event_source_set_exit_on_failure(3). 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.

If the handler to sd_event_add_io() is 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 sd_event_exit(3).

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 as parameters. If the event source owned the previous file descriptor,
that is if sd_event_source_set_io_fd_own() had been called for the event source with a
non-zero value, then the previous file descriptor will be closed and the event source will
also take the ownership of the new file descriptor on success.

sd_event_source_set_io_fd_own() controls whether the file descriptor of the event source
shall take ownership of the file descriptor. Takes a boolean parameter b. When true
(nonzero), the file descriptor will be closed automatically when the event source is freed
or when the file descriptor is replaced by sd_event_source_set_io_fd(). By default the
descriptor is not owned by the event source, and the application has to do close it on its
own if needed.

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, library or module instance.

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

       -EPERM
           The passed file descriptor does not support the epoll(7) API, for example because it
           is a regular file or directory. See epoll_ctl(2) for details.

           Added in version 255.

NOTES

       Functions described here are available as a shared library, which can be compiled against
       and linked to with the libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be not multi-thread-safe.
       This means that the code calling the functions described here must not call setenv(3) from
       a parallel thread. It is recommended to only do calls to setenv() from an early phase of
       the program when no other threads have been started.

HISTORY