The LLDB Debugger

What is LLDB?

LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser and LLVM disassembler.

LLDB is the default debugger in Xcode on Mac OS X and supports debugging C, Objective-C and C++ on the desktop and iOS devices and simulator.

All of the code in the LLDB project is available under the standard LLVM License, an open source "BSD-style" license.

Why a new debugger?

In order to achieve our goals we decided to start with a fresh architecture that would support modern multi-threaded programs, handle debugging symbols in an efficient manner, use compiler based code knowledge and have plug-in support for functionality and extensions. Additionally we want the debugger capabilities to be available to other analysis tools, be they scripts or compiled programs, without requiring them to be GPL.

Compiler Integration Benefits

LLDB currently converts debug information into clang types so that it can leverage the clang compiler infrastructure. This allows LLDB to support the latest C, C++, Objective C and Objective C++ language features and runtimes in expressions without having to reimplement any of this functionality. It also leverages the compiler to take care of all ABI details when making functions calls for expressions, when disassembling instructions and extracting instruction details, and much more.

The major benefits include:

  • Up to date language support for C, C++, Objective C
  • Multi-line expressions that can declare local variables and types
  • Utilitize the JIT for expressions when supported
  • Evaluate expression Intermediate Representation (IR) when JIT can't be used

Reusability

The LLDB debugger APIs are exposed as a C++ object oriented interface in a shared library. The lldb command line tool links to, and uses this public API. On Mac OS X the shared library is exposed as a framework named LLDB.framework, and unix systems expose it as lldb.so. The entire API is also then exposed through Python script bindings which allow the API to be used within the LLDB embedded script interpreter, and also in any python script that loads the lldb.py module in standard python script files. See the Python Reference page for more details on how and where Python can be used with the LLDB API.

Sharing the LLDB API allows LLDB to not only be used for debugging, but also for symbolication, disassembly, object and symbol file introspection, and much more.

Platform Support

LLDB is known to work on the following platforms, but ports to new platforms are welcome:

  • Mac OS X desktop user space debugging for i386 and x86-64
  • iOS simulator debugging on i386
  • iOS device debugging on ARM
  • Linux local user-space debugging for i386 and x86-64
  • FreeBSD local user-space debugging for i386 and x86-64

Get it and get involved!

To check out the code, use:

  • svn co http://llvm.org/svn/llvm-project/lldb/trunk lldb

Note that LLDB generally builds from top-of-trunk on Mac OS X with Xcode and on Linux (with clang and libstdc++/libc++).

Discussions about LLDB should go to the lldb-dev mailing list. Commit messages for the lldb SVN module are automatically sent to the lldb-commits mailing list, and this is also the preferred mailing list for patch submissions.