sd_event_source(3) Add an I/O event source to an event loop

Other Alias

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_io_handler_t

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

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

SEE ALSO

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