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.

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)