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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_) {
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;
}
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.
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 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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 258~rc2 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 2025-09-06 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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