# Client API

## Introduction

The open-source reference implementation of Wayland protocol is split in two C libraries, libwayland-client and libwayland-server. Their main responsibility is to handle the Inter-process communication (IPC) with each other, therefore guaranteeing the protocol objects marshaling and messages synchronization.

A client uses libwayland-client to communicate with one or more wayland servers. A wl_display object is created and manages each open connection to a server. At least one wl_event_queue object is created for each wl_display, this holds events as they are received from the server until they can be processed. Multi-threading is supported by creating an additional wl_event_queue for each additional thread, each object can have it's events placed in a particular queue, so potentially a different thread could be made to handle the events for each object created.

Though some convenience functions are provided, libwayland-client is designed to allow the calling code to wait for events, so that different polling mechanisms can be used. A file descriptor is provided, when it becomes ready for reading the calling code can ask libwayland-client to read the available events from it into the wl_event_queue objects.

The library only provides low-level access to the wayland objects. Each object created by the client is represented by a wl_proxy object that this library creates. This includes the id that is actually communicated over the socket to the server, a void* data pointer that is intended to point at a client's representation of the object, and a pointer to a static wl_interface object, which is generated from the xml and identifies the object's class and can be used for introspection into the messages and events.

Messages are sent by calling wl_proxy_marshal. This will write a message to the socket, by using the message id and the wl_interface to identify the types of each argument and convert them into stream format. Most software will call type-safe wrappers generated from the xml description of the Wayland protocols. For instance the C header file generated from the xml defines the following inline function to transmit the wl_surface::attach message:

    static inline void
    wl_surface_attach(struct wl_surface *wl_surface, struct wl_buffer *buffer, int32_t x, int32_t y)
    {
      wl_proxy_marshal((struct wl_proxy *) wl_surface, WL_SURFACE_ATTACH, buffer, x, y);
    }

Events (messages from the server) are handled by calling a "dispatcher" callback the client stores in the wl_proxy for each event. A language binding for a string-based interpreter, such as CPython, might have a dispatcher that uses the event name from the wl_interface to identify the function to call. The default dispatcher uses the message id number to index an array of functions pointers, called a wl_listener, and the wl_interface to convert data from the stream into arguments to the function. The C header file generated from the xml defines a per-class structure that forces the function pointers to be of the correct type, for instance the wl_surface::enter event defines this pointer in the wl_surface_listener object:

    struct wl_surface_listener {
      void (*enter)(void *data, struct wl_surface *, struct wl_output *);
      ...
    }

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