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C++ and C

The C++ indexer ("the clang indexer") is a wrapper over clang. The clang indexer is a drop in replacement for the C++ compiler that emits Glean facts instead of code. The wrapper is linked against libclang and libllvm.

Glean supports simple cmake builds out of the box. For other build systems you'll need to run clang-index manually on compilation targets. The indexer has been designed to scale up to very large repos, indexing compilation units in parallel across multiple machines, however support for parallel indexing isn't in the open source release. The indexer performance on C++ code is similar to running the clang frontend.

To build the indexer, you will need clang, libclang, llvm. See the github CI workflow for the specific dependencies we test with. We tend to test on one or two specific versions of clang and llvm (currently clang 11 and 12), but other versions may work.

The indexer consists of two parts:

  • clang-index

which compiles the C++ source and emits binary thrift data to write to Glean.

  • clang-derive

which computes derived facts on the result (e.g. find-references tables).

To build the indexer:

make glean-clang

Run the indexer

A simple cmake-based indexer can run via the main glean CLI tool.

> cabal build exe:glean

And index your c++ repository with:

glean index cpp-cmake DIR --indexer clang-index --deriver clang-derive --repo name/hash


  • DIR is the root directory containing the CMake manifest.
  • and indexer and deriver are the paths to the clang-index and clang-derive binaries

In the shell

C++ source in cmake projects can also be indexed directly from the Glean shell:

:index cpp-cmake DIR

Note that clang-index and clang-derive should be built and accessible in the PATH variable for this to succeed, or in the build tree.


The schema is in glean/schema/source/cxx1.angle

The schema is quite rich and captures C++, C, Objective-C and C pre-processor symbols, the semantic structure of C++ symbols, and is precise enough to do automated analysis of C++ code.