SD_EVENT_GET_FD(3)

  .SH "NAME" sd_event_get_fd - Obtain a file descriptor to poll for event loop events
  .SH "SYNOPSIS"

#include <systemd/sd-event.h>

      .HP u int sd_event_get_fd(sd_event *event);

  .SH "DESCRIPTION"

    .PP 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.
    .PP 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.

  .SH "RETURN VALUE"

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

  .SH "ERRORS"

    .PP Returned errors may indicate the following problems:

      .PP -EINVAL

            event is not a valid pointer to an sd_event structure.
          .RE
          .PP -ECHILD

                The event loop has been created in a different process.
              .RE

          .SH "EXAMPLES"

            .PP 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;
            }

          .SH "NOTES"

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

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