Creating Custom Simulation Extensions

Custom simulation extensions allow you to simulate specialized hardware, implement custom protocols, or create unique testing environments for your robot code.

Extension Structure

A simulation extension is a dynamically loaded library (.dll on Windows, .so on Linux, .dylib on macOS) with a specific entry point that the robot program calls during initialization.

Minimum Extension Code

Every extension must implement the HALSIM_InitExtension function:

#include <hal/Extensions.h>
#include <cstdio>

extern "C" {
#if defined(WIN32) || defined(_WIN32)
__declspec(dllexport)
#endif
int HALSIM_InitExtension(void) {
  std::puts("My Extension Initializing");
  
  // Initialize your extension here
  // Register callbacks, set up connections, etc.
  
  std::puts("My Extension Initialized");
  return 0; // Success
}
}

Function Requirements

Return value: 0 for success, -1 for failure
extern “C”: Prevents C++ name mangling
__declspec(dllexport): Required on Windows for DLL exports
No parameters: The function takes no arguments

Complete Extension Example

Here’s a more complete example based on the XRP extension:

// MyCustomExtension.cpp
#include <cstdio>
#include <memory>
#include <hal/Extensions.h>
#include <hal/simulation/DriverStationData.h>

class MySimExtension {
public:
  bool Initialize() {
    // Register callback for robot state changes
    HALSIM_RegisterDriverStationEnabledCallback(
        [](const char* name, void* param, const HAL_Value* value) {
          bool enabled = value->data.v_boolean;
          printf("Robot %s\n", enabled ? "enabled" : "disabled");
        },
        nullptr,
        true);
    
    return true;
  }
  
  void Shutdown() {
    std::puts("Extension shutting down");
  }
};

static std::unique_ptr<MySimExtension> gExtension;

extern "C" {
#if defined(WIN32) || defined(_WIN32)
__declspec(dllexport)
#endif
int HALSIM_InitExtension(void) {
  std::puts("My Custom Extension Initializing");
  
  // Register shutdown callback
  HAL_OnShutdown(nullptr, [](void*) { 
    gExtension.reset(); 
  });
  
  gExtension = std::make_unique<MySimExtension>();
  if (!gExtension->Initialize()) {
    return -1;
  }
  
  std::puts("My Custom Extension Initialized");
  return 0;
}
}

Using HALSIM Callback Functions

Available Device APIs

Simulation functions are available for many HAL devices in hal/simulation/:

AccelerometerData: Update accelerometer X, Y, Z values
AnalogInData: Set analog input voltages
DIOData: Control digital I/O states
EncoderData: Update encoder counts and rates
GyroData: Set gyro angles and rates
I2CData: Simulate I2C device communication
PWMData: Monitor PWM outputs
SPIData: Simulate SPI device communication
SimDeviceData: Create custom simulation devices

Registering Callbacks

Callbacks allow you to respond to HAL data changes:

#include <hal/simulation/EncoderData.h>

int32_t callbackId = HALSIM_RegisterEncoderCountCallback(
    encoderIndex,
    [](const char* name, void* param, const HAL_Value* value) {
      int32_t count = value->data.v_int;
      printf("Encoder count: %d\n", count);
    },
    nullptr,  // User parameter
    true);    // Initial callback

Setting HAL Values

Update simulated hardware values from your extension:

#include <hal/simulation/AnalogInData.h>
#include <hal/simulation/DIOData.h>

// Set analog input voltage
HALSIM_SetAnalogInVoltage(channel, 3.3);

// Set digital input state
HALSIM_SetDIOValue(channel, true);

// Update gyro angle
HALSIM_SetGyroAngle(0, 45.0);

Building Your Extension

Setup build.gradle

Create a build.gradle file for your extension. Use the XRP extension as a template:

description = "My Custom Simulation Extension"

ext {
    includeWpiutil = true
    pluginName = 'halsim_my_extension'
}

apply from: "${rootDir}/shared/plugins/setupBuild.gradle"

model {
    binaries {
        all {
            // Don't build for roboRIO
            if (it.targetPlatform.name == nativeUtils.wpi.platforms.roborio) {
                it.buildable = false
                return
            }
            
            // Link with WPILib libraries
            lib project: ':wpinet', library: 'wpinet', linkage: 'shared'
            
            // If you need websockets support
            // lib project: ":simulation:halsim_ws_core", 
            //     library: "halsim_ws_core", linkage: "static"
        }
    }
}

Key Configuration

pluginName: Must match your extension’s library name
includeWpiutil: Include WPIUtil library
linkage: ‘shared’: Link with shared libraries (important!)
roborio check: Disable building for roboRIO platform

Building the Extension

Place in allwpilib

Put your extension directory in allwpilib/simulation/

Build WPILib

Build the entire WPILib project:

./gradlew build

Publish Locally

Publish to your local Maven repository:

./gradlew publish

Using Your Custom Extension

In a Robot Project

After publishing WPILib locally, add your extension to a robot project’s build.gradle:

wpi.sim.addDep(
    "My Custom Extension",           // Display name
    "edu.wpi.first.halsim",          // Group ID
    "halsim_my_extension"             // Plugin name from build.gradle
)

Configure Robot Project

Follow the development builds guide to configure your robot project to use your local WPILib build. Add to build.gradle:

wpi.maven.useDevelopment = true
wpi.versions.wpilibVersion = '2025.0.0-development'

Extension Communication

Registering Your Extension

Allow other extensions to detect your extension:

struct MyExtensionData {
  void (*SendCommand)(const char* cmd);
  int (*GetStatus)();
};

MyExtensionData data = {
  .SendCommand = &SendCommandImpl,
  .GetStatus = &GetStatusImpl
};

HAL_RegisterExtension("my_extension", &data);

Listening for Other Extensions

Detect when specific extensions are loaded:

HAL_RegisterExtensionListener(
    nullptr, 
    [](void*, const char* name, void* data) {
      if (std::string_view{name} == "halsim_gui") {
        // GUI extension loaded, can interact with it
        auto* guiData = static_cast<GuiExtensionData*>(data);
        guiData->RegisterWindow("My Window", &CreateWindow);
      }
    });

Advanced: I2C/SPI Simulation

Simulate real I2C or SPI devices:

#include <hal/simulation/I2CData.h>

// Register I2C read callback
HALSIM_RegisterI2CReadCallback(
    port,
    [](const char* name, void* param, 
       const uint8_t* buffer, uint32_t count) -> int32_t {
      // Simulate device response
      if (buffer[0] == 0x00) { // Read register 0
        uint8_t response[2] = {0x12, 0x34};
        HALSIM_SetI2CReadData(port, response, 2);
        return 2;
      }
      return 0;
    },
    nullptr);

Performance Considerations

Callback Performance

Callbacks run synchronously in the robot program thread:

Keep callbacks fast - avoid heavy computation
Don’t block - no long delays or I/O waits
Process data in background threads if needed

// GOOD: Quick callback
HALSIM_RegisterEncoderCountCallback(
    index,
    [](const char* name, void* param, const HAL_Value* value) {
      quickUpdate(value->data.v_int);
    },
    nullptr, true);

// BAD: Slow callback - blocks robot thread!
HALSIM_RegisterEncoderCountCallback(
    index,
    [](const char* name, void* param, const HAL_Value* value) {
      std::this_thread::sleep_for(100ms); // Don't do this!
      expensiveCalculation();
    },
    nullptr, true);

Use Background Processing

For heavy operations:

std::queue<int32_t> dataQueue;
std::mutex queueMutex;

// Quick callback - just queue the data
HALSIM_RegisterCallback(..., 
    [](const char* name, void* param, const HAL_Value* value) {
      std::lock_guard<std::mutex> lock(queueMutex);
      dataQueue.push(value->data.v_int);
    }, ...);

// Process in background thread
std::thread([&]() {
  while (running) {
    std::lock_guard<std::mutex> lock(queueMutex);
    while (!dataQueue.empty()) {
      processData(dataQueue.front());
      dataQueue.pop();
    }
  }
}).detach();

Example Extensions in WPILib

Study these built-in extensions for reference:

halsim_xrp: Simple client protocol implementation
halsim_ws_server: WebSocket server with HTTP support
halsim_gui: Complex GUI with ImGui integration
halsim_ds_socket: Driver Station protocol implementation

Source code: allwpilib/simulation/halsim_*/

Next Steps

Simulation Overview

Review the simulation architecture

Running Examples

Test your extension with example programs

Getting Started

Core Concepts

Programming Guide

Simulation

Advanced Topics

Tools

Creating Custom Simulation Extensions

Extension Structure

Minimum Extension Code

Function Requirements

Complete Extension Example

Using HALSIM Callback Functions

Available Device APIs

Registering Callbacks

Setting HAL Values

Building Your Extension

Setup build.gradle

Key Configuration

Building the Extension

Using Your Custom Extension

In a Robot Project

Configure Robot Project

Extension Communication

Registering Your Extension

Listening for Other Extensions

Advanced: I2C/SPI Simulation

Performance Considerations

Callback Performance

Use Background Processing

Example Extensions in WPILib

Next Steps

Simulation Overview

Running Examples

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Programming Guide

Simulation

Advanced Topics

Tools

​Extension Structure

​Minimum Extension Code

​Function Requirements

​Complete Extension Example

​Using HALSIM Callback Functions

​Available Device APIs

​Registering Callbacks

​Setting HAL Values

​Building Your Extension

​Setup build.gradle

​Key Configuration

​Building the Extension

​Using Your Custom Extension

​In a Robot Project

​Configure Robot Project

​Extension Communication

​Registering Your Extension

​Listening for Other Extensions

​Advanced: I2C/SPI Simulation

​Performance Considerations

​Callback Performance

​Use Background Processing

​Example Extensions in WPILib

​Next Steps

Simulation Overview

Running Examples

Build docs developers (and LLMs) love

Extension Structure

Minimum Extension Code

Function Requirements

Complete Extension Example

Using HALSIM Callback Functions

Available Device APIs

Registering Callbacks

Setting HAL Values

Building Your Extension

Setup build.gradle

Key Configuration

Building the Extension

Using Your Custom Extension

In a Robot Project

Configure Robot Project

Extension Communication

Registering Your Extension

Listening for Other Extensions

Advanced: I2C/SPI Simulation

Performance Considerations

Callback Performance

Use Background Processing

Example Extensions in WPILib

Next Steps