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# Forcing a roundtrip (Tutorial 3)

In this tutorial we show how to force a roundtrip to the server
to make sure an action completed.

We'll change our example from [Tutorial 2](tutorial2.md) slightly
and add the extra code to implement the roundtrip.

Let's take the following small method first:

```c
static int roundtrip(struct pw_core *core, struct pw_main_loop *loop)
{
        struct spa_hook core_listener;
        int pending, done = 0;

        void core_event_done(void *object, uint32_t id, int seq) {
                if (id == PW_ID_CORE && seq == pending) {
                        done = 1;
                        pw_main_loop_quit(loop);
                }
        }
        const struct pw_core_events core_events = {
                PW_VERSION_CORE_EVENTS,
                .done = core_event_done,
        };

        spa_zero(core_listener);
        pw_core_add_listener(core, &core_listener,
			&core_events, NULL);

        pending = pw_core_sync(core, PW_ID_CORE, 0);

        while (!done) {
                pw_main_loop_run(loop);
        }
        spa_hook_remove(&core_listener);
        return 0;
}
```

Let's take a look at what this method does.

```c
        struct spa_hook core_listener;
        spa_zero(core_listener);
        pw_core_add_listener(core, &core_listener,
			&core_events, NULL);
```

First of all we add a listener for the events of the core
object. We are only interested in the `done` event in this
tutorial. This is the event handler:

```c
        int pending, done = 0;

        void core_event_done(void *object, uint32_t id, int seq) {
                if (id == PW_ID_CORE && seq == pending) {
                        done = 1;
                        pw_main_loop_quit(loop);
                }
        }
        const struct pw_core_events core_events = {
                PW_VERSION_CORE_EVENTS,
                .done = core_event_done,
        };
```

When the done event is received for an object with id `PW_ID_CORE`
and a certain sequence number `seq`, this function will set the done
variable to 1 and call `pw_main_loop_quit()`.

Next we do:

```c
        pending = pw_core_sync(core, PW_ID_CORE, 0);
```

This triggers the `sync` method on the core object with id
`PW_ID_CORE` and sequence number 0.

Because this is a method on a proxy object, it will be executed
asynchronously and the returns value will reflect this. PipeWire
uses the return values of the underlying SPA (Simple Plugin API)
helper objects (See also [error codes](spa/design.md#error-codes)).

Because all messages on the PipeWire server are handled sequencially,
the sync method will be executed after all previous methods are
completed. The PipeWire server will emit a `done` event with the
same ID and the return value of the original `pw_core_sync()`
method in the sequence number.

We then run the mainloop to send the messages to the server and
receive the events:

```c
        while (!done) {
                pw_main_loop_run(loop);
        }
```

When we get the done event, we can compare it to the sync method
and then we know that we did a complete roundtrip and there are no
more pending methods on the server. We can quit the mainloop and
remove the listener:

```c
        spa_hook_remove(&core_listener);
```

If we add this roundtrip method to our code and call it instead of the
`pw_main_loop_run()` we will exit the program after all previous methods
are finished. This means that the `pw_core_get_registry()` call
completed and thus that we also received all events for the globals
on the server.


```c
#include <pipewire/pipewire.h>

static int roundtrip(struct pw_core *core, struct pw_main_loop *loop)
{
        struct spa_hook core_listener;
        int pending, done = 0;
        void core_event_done(void *object, uint32_t id, int seq) {
                if (id == PW_ID_CORE && seq == pending) {
                        done = 1;
                        pw_main_loop_quit(loop);
                }
        }
        const struct pw_core_events core_events = {
                PW_VERSION_CORE_EVENTS,
                .done = core_event_done,
        };

        spa_zero(core_listener);
        pw_core_add_listener(core, &core_listener,
                                 &core_events, NULL);

        pending = pw_core_sync(core, PW_ID_CORE, 0);

        while (!done) {
                pw_main_loop_run(loop);
        }
        spa_hook_remove(&core_listener);
        return 0;
}

static void registry_event_global(void *data, uint32_t id,
		uint32_t permissions, const char *type, uint32_t version,
		const struct spa_dict *props)
{
	printf("object: id:%u type:%s/%d\n", id, type, version);
}

static const struct pw_registry_events registry_events = {
	PW_VERSION_REGISTRY_EVENTS,
	.global = registry_event_global,
};

int main(int argc, char *argv[])
{
        struct pw_main_loop *loop;
        struct pw_context *context;
        struct pw_core *core;
        struct pw_registry *registry;
        struct spa_hook registry_listener;

        pw_init(&argc, &argv);

        loop = pw_main_loop_new(NULL /* properties */);
        context = pw_context_new(pw_main_loop_get_loop(loop),
                        NULL /* properties */,
                        0 /* user_data size */);

        core = pw_context_connect(context,
                        NULL /* properties */,
                        0 /* user_data size */);

        registry = pw_core_get_registry(core, PW_VERSION_REGISTRY,
                        0 /* user_data size */);

        spa_zero(registry_listener);
        pw_registry_add_listener(registry, &registry_listener,
                                       &registry_events, NULL);

        roundtrip(core, loop);

        pw_proxy_destroy((struct pw_proxy*)registry);
        pw_core_disconnect(core);
        pw_context_destroy(context);
        pw_main_loop_destroy(loop);

        return 0;
}
```

To compile the simple test application, copy it into a tutorial3.c file and
use:

```
gcc -Wall tutorial3.c -o tutorial3 $(pkg-config --cflags --libs libpipewire-0.3)
```

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