Building a Minimal Wayland Client for a Terminal Emulator

This introduces the fundamentals of the Wayland protocol and client implementation. Implementat core components of a minimal Wayland client, providing the groundwork for a graphical terminal emulator.

Learning Objectives

• Understand the structure of the Wayland display protocol and its client-side APIs.
• Work with UNIX domain sockets to establish a client–server connection.
• Implement shared memory buffers and message marshaling/unmarshaling.
• Handle Wayland events and manage state objects (surfaces, callbacks, etc.).
• Lay the foundation for a simple terminal emulator frontend.

---

Structure

The file is divided into several logical sections. Each part builds toward a functioning Wayland client capable of opening a window and rendering a surface.

1. Initialization – Connect to the Wayland display using a UNIX domain socket.
2. Registry and Globals – Discover and bind to core Wayland interfaces.
3. Shared Memory Buffers – Create and manage memory-backed buffers for drawing pixels.
4. Surface Management – Attach buffers to surfaces and display them.
5. Event Loop – Receive and handle Wayland events to keep the client responsive.

---

Implementation

1. Buffer Helper Methods

Functions:

• buf_write_u32
• buf_write_u16
• buf_write_string
• buf_read_u32
• buf_read_u16
• buf_read_n

Goal: Implement small read/write utilities to serialize and deserialize primitive data to and from byte buffers. Wayland uses little-endian encoding and 4-byte alignment.

Steps:

1. Work with raw byte arrays to store or extract integers.
2. For writing: check that the buffer has enough capacity, then copy bytes.
3. For reading: advance the buffer pointer and reduce the available size.
4. Strings should be padded to 4-byte boundaries.

---

2. Connection Setup

Function:

• wayland_display_connect

Goal: Connect to the Wayland compositor via a UNIX domain socket.

Steps:

1. Retrieve the display name using WAYLAND_DISPLAY environment variable, defaulting to "wayland-0".
2. Create a UNIX domain socket of type SOCK_STREAM.
3. Construct the path /run/user/<uid>/<display> or use the abstract socket namespace if preferred.
4. Connect to the socket; on success, return its file descriptor.

---

3. Message Marshalling

Functions:

• wayland_wl_display_get_registry
• wayland_wl_registry_bind

Goal: Build and send binary messages conforming to the Wayland wire protocol.

Steps:

1. Prepare a message buffer.
2. Write message header fields: object ID, opcode, and total message size.
3. Append arguments using your buffer helpers.
4. Send the full message to the Wayland socket.

---

4. Shared Memory Pool and Buffer Creation

Functions:

• wayland_wl_shm_create_pool
• wayland_wl_shm_pool_create_buffer

Goal: Create a shared memory pool for rendering and attach buffers to it.

Steps:

1. Create a temporary file (e.g., using memfd_create or shm_open).
2. Resize it with ftruncate to the desired size.
3. Map it into memory using mmap.
4. Send a wl_shm.create_pool message to the compositor.
5. Use the returned pool ID to create a wl_buffer with specified width, height, and stride.

---

5. Rendering Helpers

Functions:

• renderer_clear
• renderer_draw_rect
• render_frame

Goal: Draw into the shared memory buffer and display it via the Wayland surface.

Steps:

1. In renderer_clear, fill all pixels with a single color.
2. In renderer_draw_rect, color a bounded rectangular region.
3. In render_frame, combine the two above to draw the scene, then send a wl_surface.attach and wl_surface.commit to display the result.

---

6. Surface and Window Management

Functions:

• wayland_wl_surface_attach
• wayland_wl_surface_commit
• wayland_xdg_surface_get_toplevel
• wayland_wl_surface_damage_buffer

Goal: Link your buffers to visible Wayland surfaces and present them on screen.

Steps:

1. Send a wl_compositor.create_surface to obtain a wl_surface ID.
2. Attach a buffer to it and mark the damaged (changed) region.
3. Commit the surface to make changes visible.
4. Use the xdg-shell protocol to promote it to a window (xdg_surface and xdg_toplevel).

---

7. Event Handling

Function:

• wayland_handle_message

Goal: Parse messages from the compositor and update your local state.

Steps:

1. Continuously read data from the Wayland socket.
2. For each message, extract the sender, opcode, and payload.
3. Match these against known objects (registry, surface, seat, etc.).
4. Update your state_t structure accordingly.
5. Trigger re-renders when frame callbacks or pointer motion events occur.

---

8. Main Program Flow

Function:

• main

Goal: Orchestrate all steps and run the Wayland client loop.

Steps:

1. Connect to the display.
2. Retrieve and bind to globals (registry, shm, compositor, xdg_wm_base).
3. Create a shared memory pool and initial buffer.
4. Create a surface and toplevel window.
5. Enter an infinite event loop:
   • Read socket data.
   • Dispatch messages via wayland_handle_message.
   • Redraw frames using render_frame.

---

Milestones

1. Milestone 1: Implement buffer helpers and verify serialization.
2. Milestone 2: Establish Wayland socket connection.
3. Milestone 3: Create shm pool and buffer.
4. Milestone 4: Render and display your first frame.
5. Milestone 5: Add event handling and pointer interaction.

---

Debugging can be done using tools like WAYLAND_DEBUG=1 to observe protocol traffic.