LLDB's Architecture

# Architecture

LLDB is a large and complex codebase. This section will help you become more familiar with the pieces that make up LLDB and give a general overview of the general architecture.

# Code Layout

LLDB has many code groupings that makeup the source base:

- API
- Breakpoint
- Commands
- Core
- DataFormatters
- Expression
- Host
- Interpreter
- Symbol
- Target
- Utility

# API

The API folder contains the public interface to LLDB.

We are currently vending a C++ API. In order to be able to add methods to this API and allow people to link to our classes, we have certain rules that we must follow:

- Classes can't inherit from any other classes.
- Classes can't contain virtual methods.
- Classes should be compatible with script bridging utilities like swig.
- Classes should be lightweight and be backed by a single member. Pointers (or shared pointers) are the preferred choice since they allow changing the contents of the backend without affecting the public object layout.
- The interface should be as minimal as possible in order to give a complete API.

By adhering to these rules we should be able to continue to vend a C++ API, and make changes to the API as any additional methods added to these classes will just be a dynamic loader lookup and they won't affect the class layout (since they aren't virtual methods, and no members can be added to the class).

# Breakpoint

A collection of classes that implement our breakpoint classes. Breakpoints are resolved symbolically and always continue to resolve themselves as your program runs. Whether settings breakpoints by file and line, by symbol name, by symbol regular expression, or by address, breakpoints will keep trying to resolve new locations each time shared libraries are loaded. Breakpoints will of course unresolve themselves when shared libraries are unloaded. Breakpoints can also be scoped to be set only in a specific shared library. By default, breakpoints can be set in any shared library and will continue to attempt to be resolved with each shared library load.

Breakpoint options can be set on the breakpoint, or on the individual locations. This allows flexibility when dealing with breakpoints and allows us to do what the user wants.

# Commands

The command source files represent objects that implement the functionality for all textual commands available in our command line interface.

Every command is backed by a lldb_private::CommandObject or lldb_private::CommandObjectMultiword object.

lldb_private::CommandObjectMultiword are commands that have subcommands and allow command line commands to be logically grouped into a hierarchy.

lldb_private::CommandObject command line commands are the objects that implement the functionality of the command. They can optionally define options for themselves, as well as group those options into logical groups that can go together. The help system is tied into these objects and can extract the syntax and option groupings to display appropriate help for each command.

# Core

The Core source files contain basic functionality that is required in the debugger as well as the class represeting the debugger it self (Debugger). A wide variety of classes are implemented:

- Address (section offset addressing)
- AddressRange
- Architecture specification
- Broadcaster / Event / Listener
- Communication classes that use Connection objects
- Mangled names
- Source manager
- Value objects

# DataFormatters

A collection of classes that implement the data formatters subsystem.

Data formatters provide a set of user-tweakable hooks in the ValueObjects world that allow to customize presentation aspects of variables. While users interact with formatters mostly through the `type` command, inside LLDB there are a few layers to the implementation: DataVisualization at the highest end of the spectrum, backed by classes implementing individual formatters, matching rules, ...

For a general user-level introduction to data formatters, you can look here.

More details on the architecture are to be found here.

# Expression

Expression parsing files cover everything from evaluating DWARF expressions, to evaluating expressions using Clang.

The DWARF expression parser has been heavily modified to support type promotion, new opcodes needed for evaluating expressions with symbolic variable references (expression local variables, program variables), and other operators required by typical expressions such as assign, address of, float/double/long double floating point values, casting, and more. The DWARF expression parser uses a stack of lldb_private::Value objects. These objects know how to do the standard C type promotion, and allow for symbolic references to variables in the program and in the LLDB process (expression local and expression global variables).

The expression parser uses a full instance of the Clang compiler in order to accurately evaluate expressions. Hooks have been put into Clang so that the compiler knows to ask about identifiers it doesn't know about. Once expressions have be compiled into an AST, we can then traverse this AST and either generate a DWARF expression that contains simple opcodes that can be quickly re-evaluated each time an expression needs to be evaluated, or JIT'ed up into code that can be run on the process being debugged.

# Host

LLDB tries to abstract itself from the host upon which it is currently running by providing a host abstraction layer. This layer includes functionality, whose implementation varies wildly from host to host.

Host functionality includes abstraction layers for:

- Information about the host system (triple, list of running processes, etc.)
- Launching processes
- Various OS primitives like pipes and sockets

It also includes the base classes of the NativeProcess/Thread hierarchy, which is used by lldb-server.

# Interpreter

The interpreter classes are the classes responsible for being the base classes needed for each command object, and is responsible for tracking and running command line commands.

# Symbol

Symbol classes involve everything needed in order to parse object files and debug symbols. All the needed classes for compilation units (code and debug info for a source file), functions, lexical blocks within functions, inlined functions, types, declaration locations, and variables are in this section.

# Target

Classes that are related to a debug target include:

- Target
- Process
- Thread
- Stack frames
- Stack frame registers
- ABI for function calling in process being debugged
- Execution context batons

# Utility

This module contains the lowest layers of LLDB. A lot of these classes don't really have anything to do with debugging -- they are just there because the higher layers of the debugger use these clasess to implement their functionality. Others are data structures used in many other parts of the debugger (TraceOptions). Most of the functionality in this module could be useful in an application that is not a debugger; however, providing a general purpose C++ library is an explicit non-goal of this module.

This module provides following functionality:

- Abstract path manipulation (FileSpec)
- Data buffers (DataBuffer, DataEncoder, DataExtractor)
- Logging
- Structured data manipulation (JSON)
- Streams
- Timers
- etc.

For historic reasons, some of this functionality overlaps that which is provided by the LLVM support library.