sd_event_get_fd - Obtain a file descriptor to poll for event loop events

NAME SYNOPSIS DESCRIPTION RETURN VALUE Errors EXAMPLES NOTES SEE ALSO

NAME

sd_event_get_fd − Obtain a file descriptor to poll for event loop events

SYNOPSIS

#include <elogind/sd−event.h>

int sd_event_get_fd(sd_event *event);

sd_event_get_fd() returns the file descriptor that an event loop object returned by the sd_event_new(3) function uses to wait for events. This file descriptor may itself be polled for POLLIN/EPOLLIN events. This makes it possible to embed an sd-event(3) event loop into another, possibly foreign, event loop.

The returned file descriptor refers to an epoll(7) object. It is recommended not to alter it by invoking epoll_ctl(2) on it, in order to avoid interference with the event loop's inner logic and assumptions.

RETURN VALUE

On success, sd_event_get_fd() returns a non−negative file descriptor. On failure, it returns a negative errno−style error code.

Errors

Returned errors may indicate the following problems:

−EINVAL

event is not a valid pointer to an sd_event structure.

−ECHILD

The event loop has been created in a different process.

EXAMPLES

Example 1. Integration in the GLib event loop

/* SPDX−License−Identifier: MIT−0 */

#include <stdlib.h>
#include <glib.h>
#include <elogind/sd−event.h>

typedef struct SDEventSource {
GSource source;
GPollFD pollfd;
sd_event *event;
} SDEventSource;

static gboolean event_prepare(GSource *source, gint *timeout_) {
return sd_event_prepare(((SDEventSource *)source)−>event) > 0;
}

static gboolean event_check(GSource *source) {
return sd_event_wait(((SDEventSource *)source)−>event, 0) > 0;
}

static gboolean event_dispatch(GSource *source, GSourceFunc callback, gpointer user_data) {
return sd_event_dispatch(((SDEventSource *)source)−>event) > 0;
}

static void event_finalize(GSource *source) {
sd_event_unref(((SDEventSource *)source)−>event);
}

static GSourceFuncs event_funcs = {
.prepare = event_prepare,
.check = event_check,
.dispatch = event_dispatch,
.finalize = event_finalize,
};

GSource *g_sd_event_create_source(sd_event *event) {
SDEventSource *source;

source = (SDEventSource *)g_source_new(&event_funcs, sizeof(SDEventSource));

source−>event = sd_event_ref(event);
source−>pollfd.fd = sd_event_get_fd(event);
source−>pollfd.events = G_IO_IN | G_IO_HUP | G_IO_ERR;

g_source_add_poll((GSource *)source, &source−>pollfd);

return (GSource *)source;
}

NOTES

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