Ubuntu Manpage: sd_event_add_io, sd_event_source_get_io_events, sd_event_source_set_io

Provided by: libsystemd-dev_256.5-2ubuntu3.1_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