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, 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)