最小化的lldb使用方法

写代码难免出错，以前都是print的方式调试。但发现不是很方便，于是想了解一下lldb的使用。

1. 载入并运行

lldb test
(lldb) target create "test"
Current executable set to '/data/data/com.termux/files/home/src/chong/llvm-src/002_Kaleidoscope/my/bb/test' (aarch64).

(lldb) run
Process 28856 launched: '/data/data/com.termux/files/home/src/chong/llvm-src/002_Kaleidoscope/my/bb/test' (aarch64)
ready> 89+90;
ready> Process 28856 stopped
* thread #1, name = 'test', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
    frame #0: 0x0000007fbd756654 libc++_shared.so`std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>>::basic_string(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&) + 20
libc++_shared.so`std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>>::basic_string:
->  0x7fbd756654 <+20>: ldrb   w8, [x1]
    0x7fbd756658 <+24>: mov    x19, x0
    0x7fbd75665c <+28>: tbnz   w8, #0x0, 0x7fbd756684    ; <+68>
    0x7fbd756660 <+32>: ldr    x8, [x1, #0x10]

已经出现异常了。

2. 查看函数调用栈

(lldb) thread backtrace 
* thread #1, name = 'test', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
  * frame #0: 0x0000007fbd756654 libc++_shared.so`std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>>::basic_string(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&) + 20
    frame #1: 0x0000005555d63ba4 test`FunctionAST::codegen(this=0xb400007dbd62c330) at ast.cpp:159:48
    frame #2: 0x0000005555d6699c test`Parser::HandleTopLevelExpression() at parse.cpp:254:29
    frame #3: 0x0000005555d66e3c test`Parser::MainLoop() at parse.cpp:301:7
    frame #4: 0x0000005555d60e2c test`main at main.cpp:36:3
    frame #5: 0x0000007fbd9645e0 libc.so`__libc_init + 100
(lldb) 

frame #0 表示出现异常的位置，但那是一个库函数，我自己写的代码是在frame #1那里。

3. 切换frame到我的代码

(lldb) frame select 1
frame #1: 0x0000005555d63ba4 test`FunctionAST::codegen(this=0xb400007dbd62c330) at ast.cpp:159:48
   156     
   157       // First, check for an existing function from a previous 'extern' declaration.
   158       // Function *TheFunction = ExprAST::getFunction(P.getName());
-> 159       Function *TheFunction = ExprAST::getFunction(Proto->getName());
   160     
   161       if (!TheFunction)
   162         TheFunction = Proto->codegen();
(lldb) 

已经有我的源码了。

4. 检查变量

(lldb) var
Available completions:
    this                                
    TheFunction                         
    BB                                  
    P                                   
    __begin1                            
    __end1                              
    __range1                            
    Arg                                 
    RetVal                              
    llvm::VerifyDisableABIBreakingChecks
    std::__ndk1::piecewise_construct    
    ExprAST::TheContext                 
    ExprAST::ExitOnErr                  
    ExprAST::TheJIT                     
    ExprAST::TheModule                  
    ExprAST::TheFPM                     
    ExprAST::Builder                    
    ExprAST::FunctionProtos             
    ExprAST::NamedValues                
(lldb) frame variable 
(FunctionAST *) this = 0xb400007dbd62c330
(llvm::Function *) TheFunction = <variable not available>

(llvm::BasicBlock *) BB = <variable not available>

(PrototypeAST &) P = <no location, value may have been optimized out>

(lldb) 

到这里已经基本上能解决问题了。

5. 学会使用help

我觉得调试和游泳差不多，并不需要记住什么，而是应该多用多练。

help是一个不错的命令。

6. 查看源码

如果你想了解lldb的底层原理，只能通过看源码了。