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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLES | NOTES | HISTORY | SEE ALSO | COLOPHON |
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SD_EVENT_GET_FD(3) sd_event_get_fd SD_EVENT_GET_FD(3)
sd_event_get_fd - Obtain a file descriptor to poll for event loop
events
#include <systemd/sd-event.h>
int sd_event_get_fd(sd_event *event);
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.
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.
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_) {
sd_event *event = ((SDEventSource *) source)->event;
/* GLib does not guarantee that check() is always called between two
* consecutive prepare() invocations. For example, if another thread
* attaches/removes a source with poll fds to this GMainContext while
* the main thread is inside poll(), g_main_context_check_unlocked()
* returns immediately without calling any source's check().
*
* If check() was skipped, the sd_event is still in SD_EVENT_ARMED
* state. Return FALSE to let the normal poll() -> check() -> dispatch()
* path complete the cycle. */
if (sd_event_get_state(event) == SD_EVENT_ARMED)
return FALSE;
return sd_event_prepare(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;
}
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
All functions listed here are thread-agnostic and only a single
thread may operate on a given object at any given time. Different
threads may access the same object at different times. Multiple
independent objects may be used from different threads in
parallel.
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.
sd_event_get_fd() was added in version 217.
sd-event(3), sd_event_new(3), sd_event_wait(3), epoll_ctl(2),
epoll(7)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2026-08-04. (At that
time, the date of the most recent commit that was found in the
repository was 2026-08-03.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 262~devel 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)
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HTML rendering created 2026-09-09 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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