Manpages - sd_event_get_fd.3

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

  #include <systemd/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 loops inner logic and assumptions.

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

Example 1. Integration in the GLib event loop

    /* SPDX-License-Identifier: MIT */

    #include <stdlib.h>
    #include <glib.h>
    #include <systemd/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;
    }

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

*sd-event*(3), *sd_event_new*(3), *sd_event_wait*(3), *epoll_ctl*(2), *epoll*(7)