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, library or module instance.

EXAMPLES

       Example 1. Integration in the GLib event loop

           /* SPDX-License-Identifier: MIT-0 */

           #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;
           }

NOTES

       Functions described here are available as a shared library, which can be compiled against
       and linked to with the libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be not multi-thread-safe.
       This means that the code calling the functions described here must not call setenv(3) from
       a parallel thread. It is recommended to only do calls to setenv() from an early phase of
       the program when no other threads have been started.

HISTORY

       sd_event_get_fd() was added in version 217.

SEE ALSO

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