Use Debug.Log from C++

Question

When making C++ plugins in Unity it is easier to use Debug.Log to quickly view variable values but this function is only available from C# side. This makes it very hard to debug C++ plugin since Unity's debugger does not support that. std::cout is not an option since it doesn't show in the Editor.

I looked inside the Unity C++ API located at <UnityInstallationDirecory>\Editor\Data\PluginAPI but didn't find anything about logging in the API.

Any suggestions on how to display in the Editor log from C++?

Is reverse Invoking an option for you? answers.unity3d.com/questions/30620/… — Smartis has left SO again
– Smartis has left SO again, Commented May 2, 2017 at 8:06
@Smartis Thanks. That looks promising but it won't work on iOS due to AOT. It's a good start. I will post answer that works on iOS too if I get it working. — Programmer
– Programmer, Commented May 2, 2017 at 8:10

Programmer · Accepted Answer · 2017-05-09 18:37:27Z

This can be done with a callback function. Send a pointer to a function to from C# to C++ store it in a temporary variable. Put Debug.Log inside that callback function and allow it to receive strings as a pointer(IntPtr).

When this function is called from C++, convert the IntPtr to string with Marshal.PtrToStringAnsi.

To make it work on iOS you have to use the MonoPInvokeCallback attribute on the callback function.

C# (Attach to an empty GameObject):

using AOT;
using System;
using System.Runtime.InteropServices;
using UnityEngine;

public class DebugCPP : MonoBehaviour
{

    // Use this for initialization
    void OnEnable()
    {
        RegisterDebugCallback(OnDebugCallback);
    }

    //------------------------------------------------------------------------------------------------
    [DllImport("DebugLogPlugin", CallingConvention = CallingConvention.Cdecl)]
    static extern void RegisterDebugCallback(debugCallback cb);
    //Create string param callback delegate
    delegate void debugCallback(IntPtr request, int color, int size);
    enum Color { red, green, blue, black, white, yellow, orange };
    [MonoPInvokeCallback(typeof(debugCallback))]
    static void OnDebugCallback(IntPtr request, int color, int size)
    {
        //Ptr to string
        string debug_string = Marshal.PtrToStringAnsi(request, size);

        //Add Specified Color
        debug_string =
            String.Format("{0}{1}{2}{3}{4}",
            "<color=",
            ((Color)color).ToString(),
            ">",
            debug_string,
            "</color>"
            );

        UnityEngine.Debug.Log(debug_string);
    }
}

C++ (DebugCPP.h):

#pragma once
#include<stdio.h>
#include <string>
#include <stdio.h>
#include <sstream>

#define DLLExport __declspec(dllexport)

extern "C"
{
    //Create a callback delegate
    typedef void(*FuncCallBack)(const char* message, int color, int size);
    static FuncCallBack callbackInstance = nullptr;
    DLLExport void RegisterDebugCallback(FuncCallBack cb);
}

//Color Enum
enum class Color { Red, Green, Blue, Black, White, Yellow, Orange };

class  Debug
{
public:
    static void Log(const char* message, Color color = Color::Black);
    static void Log(const std::string message, Color color = Color::Black);
    static void Log(const int message, Color color = Color::Black);
    static void Log(const char message, Color color = Color::Black);
    static void Log(const float message, Color color = Color::Black);
    static void Log(const double message, Color color = Color::Black);
    static void Log(const bool message, Color color = Color::Black);

private:
    static void send_log(const std::stringstream &ss, const Color &color);
};

C++ (DebugCPP.cpp):

#include "DebugCPP.h"

#include<stdio.h>
#include <string>
#include <stdio.h>
#include <sstream>

//-------------------------------------------------------------------
void  Debug::Log(const char* message, Color color) {
    if (callbackInstance != nullptr)
        callbackInstance(message, (int)color, (int)strlen(message));
}

void  Debug::Log(const std::string message, Color color) {
    const char* tmsg = message.c_str();
    if (callbackInstance != nullptr)
        callbackInstance(tmsg, (int)color, (int)strlen(tmsg));
}

void  Debug::Log(const int message, Color color) {
    std::stringstream ss;
    ss << message;
    send_log(ss, color);
}

void  Debug::Log(const char message, Color color) {
    std::stringstream ss;
    ss << message;
    send_log(ss, color);
}

void  Debug::Log(const float message, Color color) {
    std::stringstream ss;
    ss << message;
    send_log(ss, color);
}

void  Debug::Log(const double message, Color color) {
    std::stringstream ss;
    ss << message;
    send_log(ss, color);
}

void Debug::Log(const bool message, Color color) {
    std::stringstream ss;
    if (message)
        ss << "true";
    else
        ss << "false";

    send_log(ss, color);
}

void Debug::send_log(const std::stringstream &ss, const Color &color) {
    const std::string tmp = ss.str();
    const char* tmsg = tmp.c_str();
    if (callbackInstance != nullptr)
        callbackInstance(tmsg, (int)color, (int)strlen(tmsg));
}
//-------------------------------------------------------------------

//Create a callback delegate
void RegisterDebugCallback(FuncCallBack cb) {
    callbackInstance = cb;
}

Usage from C++:

Debug::Log("Hellow Red", Color::Red);
Debug::Log("Hellow Green", Color::Green);
Debug::Log("Hellow Blue", Color::Blue);
Debug::Log("Hellow Black", Color::Black);
Debug::Log("Hellow White", Color::White);
Debug::Log("Hellow Yellow", Color::Yellow);
Debug::Log("Hellow Orange", Color::Orange);

Debug::Log(true, Color::Black);
Debug::Log(false, Color::Red);

Output from the Editor:

Now, you can easily implement Debug.LogWarning and Debug.LogError.

Color is really useful to easily separate different logs. You are welcome!

Adam Yaxley · Accepted Answer · 2022-03-24 00:54:14Z

Here's a simple solution, using automatic marshalling (from string to char*).

C++ (DebugLogPlugin.cpp)

// The callback signature (shared by Unity C#)`enter code here`
using DebugLogCallback = void(const char*);

// Storage for the pointer to the Unity C# callback
static DebugLogCallback s_debugLogCallback = nullptr;

// Register the callback (called from Unity C#)
void RegisterDebugLog(DebugLogCallback callback)
{
    s_debugLogCallback = callback;
}

// Use from C++ to log the message to Unity C#
void DebugLog(const char* message)
{
    s_debugLogCallback(message);
}

// Usage example
DebugLog("Hello World");

C# (DebugLogBehaviour.cs)

// The callback signature (shared by C++). string is marshalled as const char* automatically
public delegate void DebugLogCallback(string message);

// Import our C++ register function
[DllImport("DebugLogPlugin")]
public static extern void RegisterDebugLog(DebugLogCallback callback);

// MonoBehaviour to load our plugin and register the callback
public class DebugLogBehaviour : MonoBehaviour
{
    void OnEnable()
    {
        // Call our C++ function to register the callback.
        RegisterDebugLog(DebugLog);
    }
    
    [MonoPInvokeCallback]
    static void DebugLog(string message)
    {
        Debug.Log(message);
    }
}

darkgrouptw · Accepted Answer · 2023-06-29 15:53:31Z

I find the most simplest way to do it via Unity Native API.

In native project

Please include the Unity Native API in %UNITY_LOCATION%\Editor\Data\PluginAPI
Write a code like below

// Must include this one
#include "IUnityLog.h"

extern "C" void UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API SendDataFromUnityToLog(float t)
{
    string content = "Native log: " + to_string(t);
    const char* c_content = content.c_str();

    // Do this three function below to warp the log from unity
    UNITY_LOG(mLogger, c_content);
    UNITY_LOG_WARNING(mLogger, c_content);
    UNITY_LOG_ERROR(mLogger, c_content);
}

// This event trigger once unity load your plugin
extern "C" void UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API UnityPluginLoad(IUnityInterfaces * pInterfaces)
{
    mLogger = pInterfaces->Get<IUnityLog>();
    UNITY_LOG(mLogger, "Native Plugin load");
}

// This event trigger when unity unload your plugin
extern "C" void UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API UnityPluginUnload()
{
    UNITY_LOG(mLogger, "Native Plugin unload");
    mLogger = nullptr;
}

In your unity project

    [DllImport("NativePlugin")]
    private static extern void SendDataFromUnityToLog(float _);

    private void Start()
    {
        UnityEngine.Random.InitState(System.DateTime.Now.Second);
        float randNum = UnityEngine.Random.value;
        Debug.Log("Unity Log: " + randNum.ToString());

        // You must trigger this to trigger UnityPluginLoad load
        // and test it 
        SendDataFromUnityToLog(randNum);
    }

Result: result

It costs me a lot of time to try without any unity documents. So I wrote a enample in: https://github.com/Darkgrouptw/UnityNativeLogger

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