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16  <h1>The LLDB Debugger</h1>
17  <!--*********************************************************************-->
18  
19  <p>LLDB is a next generation, high-performance debugger. It is built as a set
20     of reusable components which highly leverage existing libraries in the
21     larger LLVM Project, such as the Clang expression parser and LLVM
22     disassembler.</p>
23   <p>LLDB is in early development, but is mature enough to support basic
24      debugging scenarios on Mac OS X in C, Objective-C and C++.</p>
25
26  <p>All of the code in the LLDB project is available under the standard
27     <a href="http://llvm.org/docs/DeveloperPolicy.html#license">LLVM
28     License</a>, an open source "BSD-style" license.</p>
29  
30  <!--=====================================================================-->
31  <h2 id="goals">Goals</h2>
32  <!--=====================================================================-->
33
34  <p>The current state of the art in open source debuggers are that
35     they work in the common cases for C applications, but don't
36     handle many "hard cases" properly.  For example, C++ expression
37     parsing, handling overloading, templates, multi-threading, and
38     other non-trivial scenarios all work in some base cases, but
39     don't work reliably.</p>
40
41  <p>The goal of LLDB is to provide an amazing debugging experience that "just
42      works".  We aim to solve these long-standing problems where debuggers get
43      confused, so that you can think about debugging your problem, not
44      about deficiencies in the debugger.</p>
45
46  <p>With a long view, there is no good reason for a debugger to
47      reinvent its own C/C++ parser, type system, know all the
48      target calling convention details, implement its own disassembler,
49      etc.  By using the existing libraries vended by the LLVM
50      project, we believe that many of these problems will be defined
51      away, and the debugger can focus on important issues like
52      process control, efficient symbol reading and indexing, thread
53      management, and other debugger-specific problems.</p>
54
55  <p>Some more specific goals include:</p>
56  
57  <ul>
58  <li>Build libraries for inclusion in IDEs, command line tools, and
59      other analysis tools</li>
60  <li>High performance and efficient memory use</li>
61  <li>Extensible: Python scriptable and use a plug-in architecture</li>
62  <li>Reuse existing compiler technology where it makes sense</li>
63  <li>Excellent multi-threaded debugging support</li>
64  <li>Great support for C, Objective-C and C++</li>
65  <li>Retargetable to support multiple platforms</li>
66  <li>Provide a base for debugger research and other innovation</li> 
67  </ul>
68
69  <!--=====================================================================-->
70  <h2 id="why">Why a new debugger?</h2>
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72
73  <p>In order to achieve our goals we decided to start with a fresh architecture
74     that would support modern multi-threaded programs, handle debugging symbols
75     in an efficient manner, use compiler based code knowledge and have plug-in
76     support for functionality and extensions. Additionally we want the debugger
77     capabilities to be available to other analysis tools, be they scripts or
78     compiled programs, without requiring them to be GPL.</p>
79
80  <!--=====================================================================-->
81  <h2 id="features">Features</h2>
82  <!--=====================================================================-->
83
84   <p>LLDB supports a broad variety of basic debugging features such as
85      reading DWARF, supporting step, next, finish, backtraces, etc.  Some
86      more interested bits are:</p>
87
88   <ul>
89   <li>Plug-in architecture for portability and extensibility:
90   <ul>
91   <li>Object file parsers for executable file formats. Support currently
92       includes Mach-O (32 and 64-bit) &amp; ELF (32-bit).</li>
93   <li>Object container parsers to extract object files contained within a file.
94       Support currently includes universal Mach-O files &amp; BSD Archives.
95       </li>
96   <li>Debug symbol file parsers to incrementally extract debug information from
97       object files. Support currently includes DWARF &amp; Mach-O symbol
98       tables.</li>
99   <li>Symbol vendor plug-ins collect data from a variety of different sources
100       for an executable object.</li>
101   <li>Disassembly plug-ins for each architecture. Support currently includes
102       an LLVM disassembler for <a
103       href="http://blog.llvm.org/2010/01/x86-disassembler.html">i386,
104       x86_64</a>, &amp; ARM/Thumb.</li>
105    <li>Debugger plug-ins implement the host and target specific functions
106        required to debug.</li>
107    </ul>
108    <li>SWIG-generated script bridging allows Python to access and control the
109        public API of the debugger library.</li>
110    <li>A remote protocol server, debugserver, implements Mac OS X debugging on
111        i386 and x86_64.</li>
112    <li>A command line debugger - the lldb executable itself.</li>
113    <li>A framework API to the library.</li>
114   </ul>
115
116  <!--=====================================================================-->
117  <h2 id="requirements">Platform Support</h2>
118  <!--=====================================================================-->
119
120   <p>LLDB is known to work on the following platforms, but ports to new
121      platforms are welcome:</p>
122
123   <li>Machine Architectures:
124    <ul>
125     <li>Mac OS X i386 and X86-64</li>
126    </ul></li>
127
128  <!--=====================================================================-->
129  <h2 id="status">Current Status</h2>
130  <!--=====================================================================-->
131
132   <p>LLDB is in early development and supports basic debugging scenarios on
133      Mac OS X. The public API has not been finalized, and different parts are
134      at different levels of maturity.  We welcome any help fleshing out missing
135      pieces and improving the code.</p>
136
137   <p>What works well:</p>
138   <ul>   
139   <li>Process control, including external process control via debugserver
140       (which is included as part of the lldb project)</li>
141   <li>Breakpoints: Source-line, symbolic, C++ mangled names, module
142       scoping</li>
143   <li>Symbol reading and object file introspection</li>
144   <li>Script bridging</li>
145   <li>Thread inspection and stepping</li>
146   <li>Disassembly of i386, x86_64, &amp; ARM/Thumb machine code, and
147       backtracing on i386 &amp; x86_64</li>
148   <li>The basic command line prompt system, shared library tracking,
149       source listings.</li>
150   </ul>
151    
152   <p>What is still pretty new:</p>
153   <ul>   
154   <li>The public API to the library</li>
155   <li>Expression evaluation</li>
156   <li>Objective-C support: stepping into/over, printing the description of
157       an object ("po")</li>
158   <li>Breakpoint actions &amp; scripts</li>
159   <li>Attaching to existing processes</li>
160   </ul>
161
162   <p>What isn't there yet:</p>
163   <ul>   
164   <li>Regression test suite</li>
165   <li>Operating system support hasn't been fully modularized yet</li>
166   <li><a href="http://clang.llvm.org/docs/LanguageExtensions.html#blocks">Blocks</a> support</li>
167   <li>Calling functions in expressions</li>
168   <li>Objective-C 2.0 Support: Printing properties, synthetic properties,
169       Objective-C expressions, KVO, dynamic types, dot syntax, runtime data</li>
170   <li>C++ support: Method access, handling demangled names, dynamic types</li>
171   <li>Exception support: Breaking by name, thrown object, thrower</li>
172</li>
173   </ul>
174
175  <!--=====================================================================-->
176  <h2>Get it and get involved!</h2>
177  <!--=====================================================================-->
178  
179  <p>To check out the code, use:</p>
180  
181  <ul>
182  <li>svn co http://llvm.org/svn/llvm-project/lldb/trunk lldb</li>
183  </ul>
184  
185  <p>Note that LLDB currently only builds out of the box on Mac OS X with
186     Xcode, but patches to improve portability are definitely welcome.</p>
187  
188  <p>Discussions about LLDB should go to the <a
189    href="http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev">lldb-dev</a> mailing
190    list.  Commit messages for the lldb SVN module are automatically sent to the
191    <a 
192  href="http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits">lldb-commits</a>
193    mailing list, and this is also the preferred mailing list for patch
194    submissions.</p>
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