The C API for extending and developing with Guacamole is libguac. All native components produced by the Guacamole project link with this library, and this library provides the common basis for extending the native functionality of those native components (by implementing client plugins).

libguac is used mainly for developing client plugins like libguac-client-vnc or libguac-client-rdp, or for developing a proxy supporting the Guacamole protocol like guacd. This chapter is intended to give an overview of how libguac is used, and how to use it for general communication with the Guacamole protocol.

Error handling

Most functions within libguac handle errors by returning a zero or non-zero value, whichever is appropriate for the function at hand. If an error is encountered, the guac_error variable is set appropriately, and guac_error_message contains a statically-allocated human-readable string describing the context of the error. These variables intentionally mimic the functionality provided by errno and errno.h.

Both guac_error and guac_error_message are defined within error.h. A human-readable string describing the error indicated by guac_error can be retrieved using guac_status_string(), which is also statically allocated.

If functions defined within client plugins set guac_error and guac_error_message appropriately when errors are encountered, the messages logged to syslog by guacd will be more meaningful for both users and developers.

Client plugins

Client plugins are libraries which follow specific conventions such that they can be loaded dynamically by guacd. All client plugins must:

  1. Follow a naming convention, where the name of the library is libguac-client-PROTOCOL. This is necessary for guacd to locate the library for a requested protocol.

  2. Be linked against libguac, the library used by guacd to handle the Guacamole protocol. The structures which are given to functions invoked by guacd are defined by libguac, and are expected to be manipulated via the functions provided by libguac or as otherwise documented within the structure itself. Communication between guacd and client plugins is only possible if they share the same core structural and functional definitions provided by libguac.

  3. Implement the standard entry point for client plugins, guac_client_init(), described in more detail below. It is this function which initializes the structures provided by guacd such that users can join and interact with the connection.

Entry point

All client plugins must provide a function named guac_client_init which accepts, as its sole argument, a pointer to a guac_client structure. This function is similar in principle to the main() function of a C program, and it is the responsibility of this function to initialize the provided structure as necessary to begin the actual remote desktop connection, allow users to join/leave, etc.

The provided guac_client will already have been initialized with handlers for logging, the broadcast socket, etc. The absolutely critical pieces which must be provided by guac_client_init are:

  1. A handler for users which join the connection (join_handler). The join handler is also usually the most appropriate place for the actual remote desktop connection to be established.

  2. A NULL-terminated set of argument names which the client plugin accepts, assigned to the args property of the given guac_client. As the handshake procedure is completed for each connecting user, these argument names will be presented as part of the handshake, and the values for those arguments will be passed to the join handler once the handshake completes.

  3. A handler for users leaving the connection (leave_handler), if any cleanup, updates, etc. are required.

  4. A handler for freeing the data associated with the guac_client after the connection has terminated (free_handler). If your plugin will allocate any data at all, implementing the free handler is necessary to avoid memory leaks.

If guac_client_init returns successfully, guacd will proceed with allowing the first use to join the connection, and the rest of the plugin lifecycle commences.

Joining/leaving a connection

Whenever a user joins a connection, including the very first user of a connection (the user which is establishing the remote desktop connection in the first place), the join handler of the guac_client will be invoked. This handler is provided with the guac_user structure representing the user that just joined, along with the arguments provided during the handshake procedure:

int join_handler(guac_user* user, int argc, char** argv) {
    /* Synchronize display state, init the user, etc. */


/* Within guac_client_init  */
client->join_handler = join_handler;

As the parameters and user information provided during the Guacamole protocol handshake are often required to be known before the remote desktop connection can be established, the join handler is usually the best place to create a thread which establishes the remote desktop connection and updates the display accordingly.

If necessary, the user which first established the connection can be distinguished from all other users by the owner flag of guac_user, which will be set to a non-zero value.

Once a user has disconnected, the leave handler of guac_client will be invoked. Just as with the join handler, this handler is provided the guac_user structure of the user that disconnected. The guac_user structure will be freed immediately after the handler completes:

int leave_handler(guac_user* user) {
    /* Free user-specific data and clean up */


/* Within guac_client_init  */
client->leave_handler = leave_handler;


Once the last user of a connection has left, guacd will begin freeing resources allocated to that connection, invoking the free handler of the guac_client. At this point, the “leave” handler has been invoked for all previous users. All that remains is for the client plugin to free any remaining data that it allocated, such that guacd can clean up the rest:

int free_handler(guac_client* client) {
    /* Disconnect, free client-specific data, etc. */


/* Within guac_client_init  */
client->free_handler = free_handler;

Layers and buffers

The main operand of all drawing instructions is the layer, represented within libguac by the guac_layer structure. Each guac_layer is normally allocated using guac_client_alloc_layer() or guac_client_alloc_buffer(), depending on whether a layer or buffer is desired, and freed with guac_client_free_layer() or guac_client_free_buffer().


Care must be taken to invoke the allocate and free pairs of each type of layer correctly. guac_client_free_layer() should only be used to free layers allocated with guac_client_alloc_layer(), and guac_client_free_buffer() should only be used to free layers allocated with guac_client_alloc_buffer(), all called using the same instance of guac_client.

If these restrictions are not observed, the effect of invoking these functions is undefined.

Using these layer management functions allows you to reuse existing layers or buffers after their original purpose has expired, thus conserving resources on the client side, as allocation of new layers within the remote client is a relatively expensive operation.

It is through layers and buffers that Guacamole provides support for hardware-accelerated compositing and cached updates. Creative use of layers and buffers leads to efficient updates on the client side, which usually translates into speed and responsiveness.

Regardless of whether you allocate new layers or buffers, there is always one layer guaranteed to be present: the default layer, represented by libguac as GUAC_DEFAULT_LAYER. If you only wish to affect the main display of the connected client somehow, this is the layer you want to use as the operand of your drawing instruction.


In addition to drawing, the Guacamole protocol supports streaming of arbitrary data. The main operand of all streaming instructions is the stream, represented within libguac by the guac_stream structure. Each guac_stream exists either at the user or client levels, depending on whether the stream is intended to be broadcast to all users or just one, and is thus allocated using either guac_client_alloc_stream() or guac_user_alloc_stream(). Explicitly-allocated streams must eventually be freed with guac_client_free_stream() or guac_user_free_stream().


Just as with layers, care must be taken to invoke the allocate and free pairs correctly for each explicitly-allocated stream. guac_client_free_stream() should only be used to free streams allocated with guac_client_alloc_stream(), and guac_user_free_stream() should only be used to free streams allocated with guac_user_alloc_stream().

If these restrictions are not observed, the effect of invoking these functions is undefined.

Streams are the means by which data is transmitted for clipboard (via the “clipboard” instruction), audio (via the “audio” instruction), and even the images which make up typical drawing operations (via the “img” instruction). They will either be allocated for you, when an inbound stream is received from a user, or allocated manually, when an outbound stream needs to be sent to a user. As with guac_client and guac_user, each guac_stream has a set of handlers which correspond to instructions received related to streams. These instructions are documented in more detail in Streams and objects and Guacamole protocol reference.

Sending instructions

All drawing in Guacamole is accomplished through the sending of instructions to the connected client using the Guacamole protocol. The same goes for streaming audio, video, or file content. All features and content supported by Guacamole ultimately reduce to one or more instructions which are part of the documented protocol.

Most drawing using libguac is done using Cairo functions on a cairo_surface_t (see the Cairo API documentation) which is later streamed to the client using an img instruction and subsequent blob instructions, sent via guac_client_stream_png(). Cairo was chosen as a dependency of libguac to provide developers an existing and stable means of drawing to image buffers which will ultimately be sent as easy-to-digest PNG images.

The Guacamole protocol also supports drawing primitives similar to those present in the Cairo API and HTML5’s canvas tag. These instructions are documented individually in the Guacamole Protocol Reference in a section dedicated to drawing instructions, and like all Guacamole protocol instructions, each instruction has a corresponding function in libguac following the naming convention guac_protocol_send_OPCODE().

Each protocol function takes a guac_socket as an argument, which is the buffered I/O object used by libguac. For each active connection, there are two important types of guac_socket instance: the broadcast socket, which exists at the client level within the guac_client, and the per-user socket, which is accessible within each guac_user. Data sent along the client-level broadcast socket will be sent to all connected users beneath that guac_client, while data sent along a user-level socket will be sent only to that user.

For example, to send a “size” instruction to all connected users via the client-level broadcast socket, you could invoke:

guac_protocol_send_size(client->socket, GUAC_DEFAULT_LAYER, 1024, 768);

Or, if the instruction is only relevant to a particular user, the socket associated with that user can be used instead:

guac_protocol_send_size(user->socket, GUAC_DEFAULT_LAYER, 1024, 768);

The sockets provided by libguac are threadsafe at the protocol level. Instructions written to a socket by multiple threads are guaranteed to be written atomically with respect to that socket.

Event handling

Generally, as guacd receives instructions from the connected client, it invokes event handlers if set within the associated guac_user or guac_client, depending on the nature of the event. Most events are user-specific, and thus the event handlers reside within the guac_user structure, but there are client-specific events as well, such as a user joining or leaving the current connection. Event handlers typically correspond to Guacamole protocol instructions received over the socket by a connected user, which in turn correspond to events which occur on the client side.

Key events

When keys are pressed or released on the client side, the client sends key instructions to the server. These instructions are parsed and handled by calling the key event handler installed in the key_handler member of the guac_user. This key handler is given the keysym of the key that was changed, and a boolean value indicating whether the key was pressed or released.

int key_handler(guac_user* user, int keysym, int pressed) {
    /* Do something */


/* Within the "join" handler of guac_client */
user->key_handler = key_handler;

Mouse events

When the mouse is moved, and buttons are pressed or released, the client sends mouse instructions to the server. These instructions are parsed and handled by calling the mouse event handler installed in the mouse_handler member of the guac_user. This mouse handler is given the current X and Y coordinates of the mouse pointer, as well as a mask indicating which buttons are pressed and which are released.

int mouse_handler(guac_user* user, int x, int y, int button_mask) {
    /* Do something */


/* Within the "join" handler of guac_client */
user->mouse_handler = mouse_handler;

The file client.h also defines the mask of each button for convenience:


The left mouse button, set when pressed.


The middle mouse button, set when pressed.


The right mouse button, set when pressed.


The button corresponding to one scroll in the upwards direction of the mouse scroll wheel, set when scrolled.


The button corresponding to one scroll in the downwards direction of the mouse scroll wheel, set when scrolled.

Clipboard, file, and other stream events

If a connected user sends data which should be sent to the clipboard of the remote desktop, guacd will trigger the clipboard handler installed in the clipboard_handler member of the guac_user associated with that user.

int clipboard_handler(guac_user* user, guac_stream* stream, char* mimetype) {
    /* Do something */


/* Within the "join" handler of guac_client */
user->clipboard_handler = clipboard_handler;

The handler is expected to assign further handlers to the provided guac_stream as necessary, such that the “blob” and “end” instructions received along the stream can be handled. A similar handler is provided for received files:

int file_handler(guac_user* user, guac_stream* stream,
        char* mimetype, char* filename) {
    /* Do something */


/* Within the "join" handler of guac_client */
user->file_handler = file_handler;

This pattern continues for all other types of streams which can be received from a user. The instruction which begins the stream has a corresponding handler within guac_user, and the metadata describing that stream and provided with the instruction is included within the parameters passed to that handler.

These handlers are, of course, optional. If any type of stream lacks a corresponding handler, guacd will automatically close the stream and respond with an “ack” instruction and appropriate error code, informing the user’s Guacamole client that the stream is unsupported.