How to Log Both JavaScript and C++ in Node.js Using LLDB

Introduction

Node.js is a powerful platform that bridges the gap between JavaScript and C++ implementations. While working with the Node.js core or debugging specific issues, you may need to log data from both JavaScript and C++ layers. This guide explains how to achieve that using LLDB, a robust debugger for C++.

Prerequisites

Before we begin, ensure Node.js is built in debug mode to access debugging features.

Building Node.js in Debug Mode

Run the following commands in your terminal to configure and build Node.js in debug mode:

./configure --debug --ninja
make

Adding Logs in C++ Code

In Node.js core, you can use std::cout to log messages in the C++ layer. For example:

std::cout << "read in c++" << std::endl;

path: node/src/node_file.cc

example:

static void Read(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
 
  std::cout << "read in c++" << std::endl; // Added log
  
  const int argc = args.Length();
  CHECK_GE(argc, 5);

This log will print whenever the related C++ function is executed, helping you trace the flow in the native layer.

Adding Logs in JavaScript Code

In the JavaScript layer, you can add logs using console.log. For example:

console.log('read in js');

path: lib/fs.js

example:

function read(fd, buffer, offsetOrOptions, length, position, callback) {
  fd = getValidatedFd(fd);
 
   console.log('read in js'); // Added log
 
  let offset = offsetOrOptions;
  let params = null;

This will log messages to the console whenever this part of the JavaScript code is executed.

Debugging with LLDB

After adding the logs, you can use LLDB to run and debug the Node.js application.

Steps to Debug

Launch LLDB:
```
lldb ./out/Release/node
```
Set the Target:
```
target create "./out/Release/node"
```
Run Your Test File: Use the run command to execute a specific test file:
```
run test/parallel/test-fs-read.js
```
Observe Logs: As the test runs, LLDB will display logs from both C++ and JavaScript layers.

Example Output

Below is an example output showing logs from both layers:

read in c++
read in js
read in c++
read in js
read in c++
Process exited with status = 0 (0x00000000)

Conclusion

By combining std::cout in C++ and console.log in JavaScript, you can effectively debug Node.js applications. Using LLDB, you can trace logs across both layers, making it easier to identify and resolve issues in your application.

Happy debugging!