'\" t
.TH "SD_EVENT_GET_FD" "3" "" "systemd 255" "sd_event_get_fd"
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.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
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.nh
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.\" * MAIN CONTENT STARTS HERE *
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.SH "NAME"
sd_event_get_fd \- Obtain a file descriptor to poll for event loop events
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-event\&.h>
.fi
.ft
.HP \w'int\ sd_event_get_fd('u
.BI "int sd_event_get_fd(sd_event\ *" "event" ");"
.SH "DESCRIPTION"
.PP
\fBsd_event_get_fd()\fR
returns the file descriptor that an event loop object returned by the
\fBsd_event_new\fR(3)
function uses to wait for events\&. This file descriptor may itself be polled for
\fBPOLLIN\fR/\fBEPOLLIN\fR
events\&. This makes it possible to embed an
\fBsd-event\fR(3)
event loop into another, possibly foreign, event loop\&.
.PP
The returned file descriptor refers to an
\fBepoll\fR(7)
object\&. It is recommended not to alter it by invoking
\fBepoll_ctl\fR(2)
on it, in order to avoid interference with the event loop\*(Aqs inner logic and assumptions\&.
.SH "RETURN VALUE"
.PP
On success,
\fBsd_event_get_fd()\fR
returns a non\-negative file descriptor\&. On failure, it returns a negative errno\-style error code\&.
.SS "Errors"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-EINVAL\fR
.RS 4
\fIevent\fR
is not a valid pointer to an
sd_event
structure\&.
.RE
.PP
\fB\-ECHILD\fR
.RS 4
The event loop has been created in a different process, library or module instance\&.
.RE
.SH "EXAMPLES"
.PP
\fBExample\ \&1.\ \&Integration in the GLib event loop\fR
.sp
.if n \{\
.RS 4
.\}
.nf
/* 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;
}
.fi
.if n \{\
.RE
.\}
.SH "NOTES"
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.PP
The code described here uses
\fBgetenv\fR(3), which is declared to be not multi\-thread\-safe\&. This means that the code calling the functions described here must not call
\fBsetenv\fR(3)
from a parallel thread\&. It is recommended to only do calls to
\fBsetenv()\fR
from an early phase of the program when no other threads have been started\&.
.SH "HISTORY"
.PP
\fBsd_event_get_fd()\fR
was added in version 217\&.
.SH "SEE ALSO"
.PP
\fBsd-event\fR(3),
\fBsd_event_new\fR(3),
\fBsd_event_wait\fR(3),
\fBepoll_ctl\fR(2),
\fBepoll\fR(7)