SD_EVENT_GET_FD(3)

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

SYNOPSIS

#include <systemd/sd-event.h>

int sd_event_get_fd(sd_event *event);

DESCRIPTION

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

      Copyright 2014 Tom Gundersen

      Permission is hereby granted, free of charge, to any person
      obtaining a copy of this software and associated documentation files
      (the "Software"), to deal in the Software without restriction,
      including without limitation the rights to use, copy, modify, merge,
      publish, distribute, sublicense, and/or sell copies of the Software,
      and to permit persons to whom the Software is furnished to do so,
      subject to the following conditions:

      The above copyright notice and this permission notice shall be
      included in all copies or substantial portions of the Software.

      THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
      EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
      MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
      NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
      BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
      ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
      CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
      SOFTWARE.
    ***/

    #include <stdlib.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 libsystemd pkg-config(1) file.

SEE ALSO

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