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ClangQL: query C++ codebases using SQLite and clangd

What is it?

ClangQL is a proof-of-concept SQLite extension for querying C++ codebases that have been indexed using clangd.

How does it work?

It employs SQLite's virtual table system to act as an intermediary between SQLite and clangd's gRPC interface

How do I use it?

Once the module has been built, you can load it in the sqlite3 CLI via the usual .load clangql.

Afterwards, you can connect to a codebase by instantiating the various virtual tables:

sqlite> CREATE VIRTUAL TABLE llvm_symbols USING clangql (symbols,;
sqlite> CREATE VIRTUAL TABLE llvm_base_of USING clangql (base_of,;
sqlite> CREATE VIRTUAL TABLE llvm_overridden_by USING clangql (overridden_by,;
sqlite> CREATE VIRTUAL TABLE llvm_refs USING clangql (refs,;

You can then query the codebase as if it was a regular table (some caveats apply, read the last point to learn more):

sqlite> SELECT Name, Scope, DefPath FROM llvm_symbols WHERE Name = "Foo"
Name  Scope                                        DefPath
----  -------------------------------------------  --------------------------------------------------------
Foo   clang::clangd::                              clang-tools-extra/clangd/unittests/LSPBinderTests.cpp
Foo   STLExtras_MoveRange_Test::TestBody()::Foo::  llvm/unittests/ADT/STLExtrasTest.cpp
Foo   STLExtras_MoveRange_Test::TestBody()::Foo::  llvm/unittests/ADT/STLExtrasTest.cpp
Foo   SizelessTypeTester::                         clang/unittests/AST/SizelessTypesTest.cpp
Foo                                                llvm/unittests/ADT/TypeTraitsTest.cpp
Foo                                                llvm/unittests/ADT/STLExtrasTest.cpp
Foo   Class::                                      lldb/unittests/Utility/ReproducerInstrumentationTest.cpp
Foo   llvm::orc::CoreAPIsBasedStandardTest::       llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.h
Foo   llvm::TrailingObjects::                      llvm/include/llvm/Support/TrailingObjects.h
Foo                                                llvm/unittests/Support/BinaryStreamTest.cpp
Foo   STLExtras_MoveRange_Test::TestBody()::Foo::  llvm/unittests/ADT/STLExtrasTest.cpp
Foo                                                llvm/unittests/Support/BinaryStreamTest.cpp
Foo   clang::                                      clang/unittests/AST/ASTTypeTraitsTest.cpp
Foo                                                lldb/unittests/Utility/ReproducerInstrumentationTest.cpp

As another example, searching all the subclasses of a particular class:

sqlite> SELECT subclass.Name, subclass.Scope, subclass.DefPath FROM llvm_symbols AS superclass INNER JOIN llvm_base_of AS rel ON rel.Subject = superclass.Id INNER JOIN llvm_symbols AS subclass ON subclass.Id = rel.Object WHERE superclass.Name = "MCAsmInfo";
Name               Scope   DefPath
-----------------  ------  ---------------------------------------------------
NVPTXMCAsmInfo     llvm::  llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXMCAsmInfo.h
MCAsmInfoWasm      llvm::  llvm/include/llvm/MC/MCAsmInfoWasm.h
BPFMCAsmInfo       llvm::  llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h
MockedUpMCAsmInfo          llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
AVRMCAsmInfo       llvm::  llvm/lib/Target/AVR/MCTargetDesc/AVRMCAsmInfo.h
MCAsmInfoXCOFF     llvm::  llvm/include/llvm/MC/MCAsmInfoXCOFF.h
MCAsmInfoDarwin    llvm::  llvm/include/llvm/MC/MCAsmInfoDarwin.h
HackMCAsmInfo              llvm/unittests/CodeGen/TestAsmPrinter.cpp
MCAsmInfoELF       llvm::  llvm/include/llvm/MC/MCAsmInfoELF.h
MCAsmInfoCOFF      llvm::  llvm/include/llvm/MC/MCAsmInfoCOFF.h

Searching for all declarations inside of the std namespace:

sqlite> SELECT decl.Name FROM llvm_symbols AS decl INNER JOIN llvm_refs AS ref ON ref.SymbolId = decl.Id WHERE decl.Scope = "std::" AND ref.Declaration = 1;

In general, for each codebase four different virtual tables can be queried: a symbols table will contain information about every symbol in the codebase, a base_of table will contain information about what symbols are base classes of what symbols, a overridden_by table will contain information about what symbols are overridden by what symbols, and a refs table will contain information about symbol references.

The syntax for instantiating the tables is the following:

CREATE VIRTUAL TABLE my_symbols USING clangql (symbols, host:port);
CREATE VIRTUAL TABLE my_base_of USING clangql (base_of, host:port);
CREATE VIRTUAL TABLE my_overridden_by USING clangql (overridden_by, host:port);
CREATE VIRTUAL TABLE my_refs USING clangql (refs, host:port);

my_* names are not important and can be anything, the first parameter to the creation of the virtual tables is important and must be left as-is, the second parameter is the connection string. Currently, only unencrypted gRPC connections are supported.

What's the schema?

The schema of symbols tables is equivalent to the following:

  Signature TEXT, Documentation TEXT, ReturnType TEXT,
  Type TEXT, DefPath TEXT, DefStartLine INT, DefStartCol INT,
  DefEndLine INT, DefEndCol INT, DeclPath TEXT,
  DeclStartLine INT, DeclStartCol INT, DeclEndLine INT, DeclEndCol INT,
  Kind INT, SubKind INT, Language INT,
  Generic INT, TemplatePartialSpecialization INT, TemplateSpecialization INT,
  UnitTest INT, IBAnnotated INT, IBOutletCollection INT, GKInspectable INT,
  Local INT, ProtocolInterface INT)

A textual representation for the Kind, SubKind and Language columns can be obtained using the symbol_kind, symbol_subkind and symbol_language functions.

Currently, the columns from Generic to ProtocolInterface are always 0, because for some reason the server always sends a zero-valued properties field.

The schema for base_of is the same as overridden_by, and is equivalent to the following:

CREATE TABLE vtable(Subject TEXT, Object TEXT)

The meaning is as follows: if a row (S, O) is present in base_of, then S is a base class of O; if a row (S, O) is present in overridden_by, then S has been overridden by O.

Please note that it is only possible to query these two tables by their Subject, querying by Object is not possible due to limitations in the clangd protocol.

The schema of refs tables is equivalent to

CREATE TABLE vtable(SymbolId TEXT, Declaration INT,
  Definition INT, Reference INT, Spelled INT,
  Path TEXT, StartLine INT, StartCol INT,
  EndLine INT, EndCol INT)

Please note that querying refs without a SymbolId will return 0 rows.

How do I build it?

ClangQL uses CMake, Protocol Buffers and gRPC. On Windows I used vcpkg to manage the two dependencies. I'm afraid I'm not knowledgeable enough with Linux and/or macOS to give precise indications on how to build it there, but I'm guessing that as long as you have the correct development packages installed and visible on your system, CMake will be able to locate them.

Once the repository is cloned, run:

cmake -B build -S . -DCMAKE_BUILD_TYPE=Release -DVCPKG_TARGET_TRIPLET=x64-windows-static-md -DCMAKE_TOOLCHAIN_FILE=D:/vcpkg/scripts/buildsystems/vcpkg.cmake

to configure the build. Adjust CMAKE_BUILD_TYPE, VCPKG_TARGET_TRIPLET, CMAKE_TOOLCHAIN_FILE and the generator type to suit your system and needs the best. Please note that the sqlite CLI tool and the extension must have the same bitness, at least on Windows. A 32-bit CLI (such as the precompiled one from will not load a 64-bit extension.

Once configured, run:

cmake --build build

to compile the extension. First time will take a long time (on Windows), due to the need to compile Protobuf and gRPC as well. Later builds will be faster.

I have uploaded precompiled 32- and 64-bit DLLs for Windows as a GitHub release, anyways.

What constraints are available?

On symbols tables, the following constraints will generate more specific requests to the clangd server:

  • Equality on Id, Name or Scope
  • LIKE on Name, Scope, DefPath or DeclPath

The LIKE constraint on Name relies on the fuzzy search semantics of clangd. The LIKE constraint on Scope has the effect of enabling the any_scope field of the fuzzy find request to clangd. The LIKE constraint on DefPath or DeclPath has the only effect of populating the proximity_path of the fuzzy find request to clangd, which has the ultimate effect of prioritizing symbols declared or defined near the specified path.

On base_of and overridden_by tables, only equality on Subject generates specific queries to the server.

On refs tables, only equality on SubjectId generates specific queries to the server.

What works, what doesn't?

There is currently no way to i.e. obtain all possible relations between two symbols, so the relation tables are really only useful in joins. It's not a huge deal, as they are meant to be used that way anyways, but you still need to be careful when writing queries.

Not all queries are equally fast: querying on symbol id, name or scope is fast, everything else needs to happen client side and is potentially slow.

Similarly, when querying the base_of or overridden_by relations, only one of the two directions is possible, the other is not currently possible due to protocol limitations. Also, not specifying a Subject will result in 0 rows being returned.

Querying refs without specifying a SymbolId will result in 0 rows being produced. Specifying any one of Definition, Declaration, Reference or Spelled will generate more specific requests to the clangd server, all other fields are scanned client side.

Error checking is nonexistant. This is not ready for production use and was mostly made for fun, to explore to what extent the clangd interface was suitable for use with SQLite, and to learn about the SQLite virtual table system.