# Getting the Sources¶

Refer to the LLVM Getting Started Guide for general instructions on how to check out source. Note that LLDB depends on having a working checkout of LLVM and Clang, so the first step is to download and build as described at the above URL. The same repository also contains LLDB.

For macOS building from Xcode, simply checkout LLDB and then build from Xcode. The Xcode project will automatically detect that it is a fresh checkout, and checkout LLVM and Clang automatically. Unlike other platforms / build systems, it will use the following directory structure.

lldb
|
-- llvm
|
+-- tools
|
-- clang


So updating your checkout will consist of updating LLDB, LLV<, and Clang in these locations. Refer to the Build Instructions for more detailed instructions on how to build for a particular platform / build system combination.

## Contributing to LLDB¶

Please refer to the LLVM Developer Policy for information about authoring and uploading a patch. LLDB differs from the LLVM Developer Policy in the following respects.

Test infrastructure. It is still important to submit tests with your patches, but LLDB uses a different system for tests. Refer to the lldb/test folder on disk for examples of how to write tests. For anything not explicitly listed here, assume that LLDB follows the LLVM policy.

## Error handling and use of assertions in LLDB¶

Contrary to Clang, which is typically a short-lived process, LLDB debuggers stay up and running for a long time, often serving multiple debug sessions initiated by an IDE. For this reason LLDB code needs to be extra thoughtful about how to handle errors. Below are a couple rules of thumb:

• Invalid input. To deal with invalid input, such as malformed DWARF, missing object files, or otherwise inconsistent debug info, LLVM’s error handling types such as llvm::Expected<T> or llvm::Optional<T> should be used. Functions that may fail should return their result using these wrapper types instead of using a bool to indicate success. Returning a default value when an error occurred is also discouraged.
• Assertions. Assertions (from assert.h) should be used liberally to assert internal consistency. Assertions shall never be used to detect invalid user input, such as malformed DWARF. An assertion should be placed to assert invariants that the developer is convinced will always hold, regardless what an end-user does with LLDB. Because assertions are not present in release builds, the checks in an assertion may be more expensive than otherwise permissible. In combination with the LLDB test suite, assertions are what allows us to refactor and evolve the LLDB code base.
• Logging. LLDB provides a very rich logging API. When recoverable errors cannot reasonably by surfaced to the end user, the error may be written to a topical log channel.
• Soft assertions. LLDB provides lldb_assert() as a soft alternative to cover the middle ground of situations that indicate a recoverable bug in LLDB. In a Debug configuration lldb_assert() behaves like assert(). In a Release configuration it will print a warning and encourage the user to file a bug report, similar to LLVM’s crash handler, and then return execution. Use these sparingly and only if error handling is not otherwise feasible. Specifically, new code should not be using lldb_assert() and existing uses should be replaced by other means of error handling.
• Fatal errors. Aborting LLDB’s process using llvm::report_fatal_error() or abort() should be avoided at all costs. It’s acceptable to use llvm_unreachable() for actually unreachable code such as the default in an otherwise exhaustive switch statement.

Overall, please keep in mind that the debugger is often used as a last resort, and a crash in the debugger is rarely appreciated by the end-user.