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https://github.com/systemd/systemd.git
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206 lines
7.8 KiB
C
206 lines
7.8 KiB
C
/* SPDX-License-Identifier: MIT-0 */
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/* Implements a D-Bus service that automatically reconnects when the system bus is restarted.
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*
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* Compile with 'cc sd_bus_service_reconnect.c $(pkg-config --libs --cflags libsystemd)'
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*
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* To allow the program to take ownership of the name 'org.freedesktop.ReconnectExample',
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* add the following as /etc/dbus-1/system.d/org.freedesktop.ReconnectExample.conf:
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<?xml version="1.0"?> <!--*-nxml-*-->
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<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
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"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
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<busconfig>
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<policy user="root">
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<allow own="org.freedesktop.ReconnectExample"/>
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<allow send_destination="org.freedesktop.ReconnectExample"/>
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<allow receive_sender="org.freedesktop.ReconnectExample"/>
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</policy>
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<policy context="default">
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<allow send_destination="org.freedesktop.ReconnectExample"/>
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<allow receive_sender="org.freedesktop.ReconnectExample"/>
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</policy>
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</busconfig>
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*
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* To get the property via busctl:
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*
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* $ busctl --user get-property org.freedesktop.ReconnectExample \
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* /org/freedesktop/ReconnectExample \
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* org.freedesktop.ReconnectExample \
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* Example
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* s "example"
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*/
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#include <errno.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <systemd/sd-bus.h>
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#define _cleanup_(f) __attribute__((cleanup(f)))
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#define check(x) ({ \
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int _r = (x); \
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errno = _r < 0 ? -_r : 0; \
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printf(#x ": %m\n"); \
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if (_r < 0) \
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return EXIT_FAILURE; \
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})
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typedef struct object {
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const char *example;
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sd_bus **bus;
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sd_event **event;
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} object;
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static int property_get(
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sd_bus *bus,
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const char *path,
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const char *interface,
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const char *property,
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sd_bus_message *reply,
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void *userdata,
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sd_bus_error *error) {
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object *o = userdata;
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if (strcmp(property, "Example") == 0)
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return sd_bus_message_append(reply, "s", o->example);
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return sd_bus_error_setf(error,
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SD_BUS_ERROR_UNKNOWN_PROPERTY,
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"Unknown property '%s'",
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property);
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}
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/* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html */
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static const sd_bus_vtable vtable[] = {
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SD_BUS_VTABLE_START(0),
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SD_BUS_PROPERTY(
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"Example", "s",
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property_get,
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0,
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SD_BUS_VTABLE_PROPERTY_CONST),
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SD_BUS_VTABLE_END
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};
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static int setup(object *o);
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static int on_disconnect(sd_bus_message *message, void *userdata, sd_bus_error *ret_error) {
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check(setup((object *)userdata));
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return 0;
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}
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static int setup(object *o) {
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/* If we are reconnecting, then the bus object needs to be closed, detached from
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* the event loop and recreated.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_detach_event.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_close_unref.html
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*/
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if (*o->bus) {
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check(sd_bus_detach_event(*o->bus));
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*o->bus = sd_bus_close_unref(*o->bus);
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}
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/* Set up a new bus object for the system bus, configure it to wait for D-Bus to be available
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* instead of failing if it is not, and start it. All the following operations are asynchronous
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* and will not block waiting for D-Bus to be available.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_new.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_set_address.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_set_bus_client.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_negotiate_creds.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_set_watch_bind.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_set_connected_signal.html
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* https://www.freedesktop.org/software/systemd/man/sd_bus_start.html
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*/
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check(sd_bus_new(o->bus));
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check(sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket"));
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check(sd_bus_set_bus_client(*o->bus, 1));
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check(sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS));
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check(sd_bus_set_watch_bind(*o->bus, 1));
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check(sd_bus_set_connected_signal(*o->bus, 1));
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check(sd_bus_start(*o->bus));
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/* Publish an interface on the bus, specifying our well-known object access
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* path and public interface name.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
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* https://dbus.freedesktop.org/doc/dbus-tutorial.html
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*/
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check(sd_bus_add_object_vtable(*o->bus,
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NULL,
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"/org/freedesktop/ReconnectExample",
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"org.freedesktop.ReconnectExample",
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vtable,
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o));
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/* By default the service is only assigned an ephemeral name. Also add a well-known
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* one, so that clients know whom to call. This needs to be asynchronous, as
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* D-Bus might not be yet available.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
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*/
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check(sd_bus_request_name_async(*o->bus,
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NULL,
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"org.freedesktop.ReconnectExample",
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0,
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NULL,
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NULL));
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/* When D-Bus is disconnected this callback will be invoked, which will
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* set up the connection again. This needs to be asynchronous, as D-Bus might not
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* yet be available.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_match_signal_async.html
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*/
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check(sd_bus_match_signal_async(*o->bus,
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NULL,
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"org.freedesktop.DBus.Local",
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NULL,
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"org.freedesktop.DBus.Local",
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"Disconnected",
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on_disconnect,
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NULL,
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o));
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/* Attach the bus object to the event loop so that calls and signals are processed.
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* https://www.freedesktop.org/software/systemd/man/sd_bus_attach_event.html
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*/
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check(sd_bus_attach_event(*o->bus, *o->event, 0));
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return 0;
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}
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int main(int argc, char **argv) {
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/* The bus should be relinquished before the program terminates. The cleanup
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* attribute allows us to do it nicely and cleanly whenever we exit the
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* block.
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*/
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_cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
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_cleanup_(sd_event_unrefp) sd_event *event = NULL;
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object o = {
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.example = "example",
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.bus = &bus,
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.event = &event,
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};
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/* Create an event loop data structure, with default parameters.
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* https://www.freedesktop.org/software/systemd/man/sd_event_default.html
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*/
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check(sd_event_default(&event));
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/* By default the event loop will terminate when all sources have disappeared, so
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* we have to keep it 'occupied'. Register signal handling to do so.
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* https://www.freedesktop.org/software/systemd/man/sd_event_add_signal.html
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*/
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check(sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));
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check(sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL));
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check(setup(&o));
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/* Enter the main loop, it will exit only on sigint/sigterm.
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* https://www.freedesktop.org/software/systemd/man/sd_event_loop.html
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*/
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check(sd_event_loop(event));
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/* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html */
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check(sd_bus_release_name(bus, "org.freedesktop.ReconnectExample"));
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return 0;
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}
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