Scripting Bridge API

The SB APIs constitute the stable C++ API that lldb presents to external clients, and which get processed by SWIG to produce the Python bindings to lldb. As such it is important that they not suffer from the binary incompatibilities that C++ is so susceptible to. We’ve established a few rules to ensure that this happens.

Extending the SB API

The classes in the SB API’s are all called SB<SomeName>, where SomeName is in CamelCase starting with an upper case letter. The method names are all CamelCase with initial capital letter as well.

All the SB API classes are non-virtual, single inheritance classes. They should only include SBDefines.h or other SB headers as needed. There should be no inlined method implementations in the header files, they should all be in the implementation files. And there should be no direct ivar access.

You also need to choose the ivars for the class with care, since you can’t add or remove ivars without breaking binary compatibility. In some cases, the SB class is a thin wrapper around an internal lldb_private object. In that case, the class can have a single ivar, which is either a pointer, shared_ptr or unique_ptr to the object in the lldb_private API. All the lldb_private classes that get used this way are declared as opaque classes in lldb_forward.h, which is included in SBDefines.h. So if you need an SB class to wrap an lldb_private class that isn’t in lldb_forward.h, add it there rather than making a direct opaque declaration in the SB classes .h file.

If the SB Class needs some state of its own, as well as the backing object, don’t include that as a direct ivar in the SB Class. Instead, make an Impl class in the SB’s .cpp file, and then make the SB object hold a shared or unique pointer to the Impl object. The theory behind this is that if you need more state in the SB object, those needs are likely to change over time, and this way the Impl class can pick up members without changing the size of the object. An example of this is the SBValue class. Please note that you should not put this Impl class in the lldb namespace. Failure to do so leads to leakage of weak-linked symbols in the SBAPI.

In order to fit into the Python API’s, we need to be able to default construct all the SB objects. Since the ivars of the classes are all pointers of one sort or other, this can easily be done, but it means all the methods must be prepared to handle their opaque implementation pointer being empty, and doing something reasonable. We also always have an “IsValid” method on all the SB classes to report whether the object is empty or not.

Another piece of the SB API infrastructure is the Python (or other script interpreter) customization. SWIG allows you to add property access, iterators and documentation to classes. We place the property accessors and iterators in a file dedicated to extensions to existing SB classes at “bindings/interface/SB<ClassName>Extensions.i”. The documentation is similarly located at “bindings/interface/SB<ClassName>Docstrings.i”. These two files, in addition to the actual header SB<ClassName>.h, forms the interface that lldb exposes to users through the scripting languages.

There are some situations where you may want to add functionality to the SB API only for use in C++. To prevent SWIG from generating bindings to these functions, you can use a C macro guard, like so:

#ifndef SWIG
int GetResourceCPPOnly() const;
#endif

In this case, GetResourceCPPOnly will not be generated for Python or other scripting languages. If you wanted to add a resource specifically only for the SWIG case, you can invert the condition and use #ifdef SWIG instead. When building the LLDB framework for macOS, the headers are processed with unifdef prior to being copied into the framework bundle to remove macros involving SWIG.

Another good principle when adding SB API methods is: if you find yourself implementing a significant algorithm in the SB API method, you should not do that, but instead look for and then add it - if not found - as a method in the underlying lldb_private class, and then call that from your SB API method. If it was a useful algorithm, it’s very likely it already exists because the lldb_private code also needed to do it. And if it doesn’t at present, if it was a useful thing to do, it’s likely someone will later need it in lldb_private and then we end up with two implementations of the same algorithm. If we keep the SB API code to just what’s needed to manage the SB objects and requests, we won’t get into this situation.

Lifetime

Many SB API methods will return strings in the form of const char * values. Once created, these strings are guaranteed to live until the end of the debugging session. LLDB owns these strings, clients should not attempt to free them. Doing so may cause LLDB to crash. Note that this only affects the C++ API as scripting languages usually will usually create native string types from the const char * value.

API Instrumentation

The reproducer infrastructure requires API methods to be instrumented so that they can be captured and replayed. Instrumentation consists of two macros, LLDB_REGISTER and LLDB_RECORD. Both can be automatically generated with the lldb-instr utility.

To add instrumentation for a given file, pass it to the lldb-instr tool. Like other clang-based tools it requires a compilation database (compile_commands.json) to be present in the current working directory.

$ ./bin/lldb-instr /path/to/lldb/source/API/SBDebugger.cpp

The tool will automatically insert LLDB_RECORD macros inline, however you will need to run clang-format over the processed file, as the tool (intentionally) makes no attempt to get that right.

The LLDB_REGISTER macros are printed to standard out between curly braces. You’ll have to copy-paste those into the corresponding RegisterMethods function in the implementation file. This function is fully specialized in the corresponding type.

template <> void RegisterMethods<SBDebugger>(Registry &R) {
  ...
}

When adding a new class, you’ll also have to add a call to RegisterMethods in the SBRegistry constructor.

The tool can be used incrementally. However, it will ignore existing macros even if their signature is wrong. It will only generate a LLDB_REGISTER if it emitted a corresponding LLDB_RECORD macro.