sd_event_get_fd(3) — Linux manual page


SD_EVENT_GET_FD(3)           sd_event_get_fd          SD_EVENT_GET_FD(3)

NAME         top

       sd_event_get_fd - Obtain a file descriptor to poll for event loop

SYNOPSIS         top

       #include <systemd/sd-event.h>

       int sd_event_get_fd(sd_event *event);

DESCRIPTION         top

       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

       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

RETURN VALUE         top

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

       Returned errors may indicate the following problems:

           event is not a valid pointer to an sd_event structure.

           The event loop has been created in a different process,
           library or module instance.

EXAMPLES         top

       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-> = G_IO_IN | G_IO_HUP | G_IO_ERR;

             g_source_add_poll((GSource *)source, &source->pollfd);

             return (GSource *)source;

NOTES         top

       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

HISTORY         top

       sd_event_get_fd() was added in version 217.

SEE ALSO         top

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

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
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       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
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       (which is not part of the original manual page), send a mail to

systemd 255                                           SD_EVENT_GET_FD(3)

Pages that refer to this page: sd-event(3)sd_event_run(3)sd_event_wait(3)systemd.directives(7)systemd.index(7)