gdb-js

Seamless GDB wrapper for Node.js (>= 0.12) and the browser.
It can be used to build different kinds of frontends for GDB.

Documentation

Documentation is availabe here.
Reading the sources of tests is also useful.

Features

• Support of multithreaded targets.
• All methods return Promises.
• All MI & CLI commands are supported.
• Support of custom Python scripts execution.
• Support of multiple targets.

Considerations

• GDB >= 7.3 is required.
• GDB should support Python.
• GDB should be used in MI mode (i.e. gdb -i=mi).
• If Python 2.x is used then future module should be installed with pip.
• gdb-js needs babel-polyfill to work, or if you're heading to ES6 environment, just regenerator runtime.
• Although it's possible to use gdb-js in the all-stop mode, it makes much more sense for a frontend to work with the non-stop mode (together with target-async). So, it's recommended that enableAsync method should be called.
• Execution of all CLI commands is possible only after calling init method which defines some custom supportive commands in GDB. However, you can load them (scripts folder in the project repository) manually with .gdbinit for example.
• gdb-js is a seamless wrapper. It means that it doesn't have any assumptions about your goals and doesn't do anything behind the scenes. So, if something is going wrong it's probably a problem with your GDB usage (i.e. the same problem can be reproduced within a bare console).
• gdb-js has a defined API that should be convinient to use. But if it's not enough for you, it also makes it easy to use low-level stuff. You can execute any GDB/MI command with a execMI method and get a parsed JSON representation of the result. You can execute any CLI command of GDB and get a string as a result. You can also listen to events that emit raw records of GDB/MI interface.
• It's currently not posible to distingish target output and GDB output correctly. Thus, it's recommended to use --tty option with your GDB.
• For browsers it makes sense to make use of utilities that expose process streams (i.e. stdin/stdout/stderr) through WebSockets.
• All methods (where it makes sense) accept thread as the last parameter. So, you can step/continue/interrupt/inspect any thread you want.
• If you're debugging a target that spawns new processes with fork, just call attachOnFork method and you're done. If not and you still need to debug multiple targets you should attach them manually (see the official GDB documentation).

Install

$ npm install gdb-js

Usage

import { spawn } from 'child-process'
import { GDB } from 'gdb-js'

let child = spawn('gdb', ['-i=mi', 'main'])
let gdb = new GDB(child)

Note that the argument shouldn't necesserily be a Node.js child process. It can be any object that has stdin/stdout/stderr streams.

Examples

General example:

gdb.on('stopped', (data) => {
  if (data.reason === 'breakpoint-hit') {
    console.log(data.breakpoint.id + 'is hit!')
  }
})

await gdb.break('main.c', 'foo')
await gdb.run()

Multithreading:

await gdb.init()
await gdb.enableAsync()
await gdb.run()
// stop all the threads
await gdb.interrupt()
// get info about threads
let threads = await gdb.threads()
// continue execution of the first thread
await gdb.continue(threads[1])

Multiple targets:

// Get all available thread groups (i.e. processes)
let { groups } = await gdb.execMI('-list-thread-groups --available')
let bash = groups.find((p) => p.description === 'bash')
// bash.id is just a pid, it can be any other pid
await gdb.attach(bash.id)

Extending

Although gdb-js supports all CLI and MI commands, you may be interested in extending its functionality usging GDB's Python API. It's possible to add new functionality even without forking gdb-js.

Implementation details

In order to understand how to extend the functionality, it may be useful to know a little about internals of gdb-js. It distinguishes MI and CLI commands. For MI commands the logic is pretty straightforward: every result record of GDB/MI output syntax is parsed, turned to JSON and returned as a result of execMI method. However, it's not possible to do the same for CLI commands since their output is exposed to console stream. What gdb-js does is defining custom CLI commands with Python API that are framed into <gdbjs:cmd:[command_name] [JSON] [command_name]:cmd:gdbjs> where [command_name] is the command name obviously and [JSON] is the valid JSON string. This way we can extract results of such commands and return them as a result of execCMD method. One of such commands that gdb-js defines is gdbjs-exec CLI command that executes whatever you pass to it and prints <gdbjs:cmd:exec [results] exec:cmd:gdbjs> where [results] is everything that was written to console during the execution of your command (remember that string is a valid JSON). That's how it's possible to get the results of CLI commands with execCLI method. execCLI(cmd) is essentially execCMD('exec ' + cmd). And execPy(script) is just execCMD('exec python\\n' + escape(script)) (we need to escape quotes and other stuff). Also gdb-js uses events from Python API and it writes them to console stream as <gdbjs:event:[event_name] [JSON] [event_name]:event:gdbjs> where [event_name] is the name of the event and [JSON] is the contents of the event. All of these internal gdb-js messages are stripped from the consoleStream property of this wrapper. It's possible to define your own events and commands and here's how.

Defining a new command

import gdb


class ThreadIDCommand(BaseCommand):
    """Returns the ID of the thread as assigned by OS."""

    def __init__(self):
        super(ThreadIDCommand, self).__init__("thread-id")

    def action(self, arg, from_tty):
        thread = gdb.selected_thread()
        (pid, lwpid, tid) = thread.ptid
        return { "pid": pid, "lwpid": lwpid, "tid": tid }

threadIDCommand = ThreadIDCommand()

let script = fs.readFileSync('thread-id.py', { encoding: 'utf8' })

await gdb.execPy(script)

let { pid, lwpid, tid } = await gdb.execCMD('thread-id')

It's even possible to use defined commands in other defined commands.

class JustPIDCommand(BaseCommand):
    """Returns just the PID of the thread."""

    def __init__(self):
        super(JustPIDCommand, self).__init__("thread-pid")

    def action(self, arg, from_tty):
        thread = threadIDCommand.action()
        return thread.pid

justPIDCommand = JustPIDCommand()

Defining a new CLI command

If you want, you can use the bare Python API to define new CLI commands.

import gdb
import sys


class GreetCommand(gdb.Command):
    """My shiny CLI command."""

    def __init__(self, name):
        super(BaseCommand, self).__init__("greet" + name, gdb.COMMAND_USER)

    def invoke(self, arg, from_tty):
        sys.stdout.write("Hello {}!".format(arg))
        sys.stdout.flush()

GreetCommand()

let script = fs.readFileSync('greet.py', { encoding: 'utf8' })

await gdb.execPy(script)

let greetings = await gdb.execCLI('greet World') // 'Hello World!'

Defining a new event

from threading import Timer


def hour_passed_handler(arg_1, arg_2)
    """Handle the time."""
    
    base_event_handler("hour-passed", "{} {}, pal...".format(arg_1, arg_2))

Timer(3600.0, hour_passed_handler, ("That's", "sad")).start()

let script = fs.readFileSync('hour.py', { encoding: 'utf8' })

await gdb.execPy(script)

gdb.on('hour-passed', (data) => console.log(data)) // 'That's sad, pal...''

Running tests

$ npm install
$ npm run docker-pull
$ npm test

Tests require Docker to be installed.

Generating documentation

$ npm run docs

It will generate static pages in the /docs folder. It's also convinient to have the repository cloned into /docs folder with the gh-pages branch checked out. This way deploying the documentation is really easy:

$ npm run docs
$ cd docs
$ git commit -a -m "update"
$ git push