Build Concepts

Most people don’t need to build their own C library — the one provided by their system works well. However, LLVM-libc’s Overlay Mode can provide key updates like faster or more consistent math functions for projects that need them.

For those who do need a full C library, LLVM-libc supports several build configurations depending on your target environment and intended usage.

The Five Build Scenarios

1. Overlay Mode (Incremental Adoption)

In Overlay Mode, LLVM-libc functions are compiled alongside the host’s existing system library (like glibc). Only the functions explicitly implemented in LLVM-libc are used; the rest “fall back” to the system library. This is the preferred method for most contributors as it is the fastest to build and test.

To configure for an overlay build, point CMake to the runtimes directory and set LLVM_LIBC_FULL_BUILD=OFF (which is the default). This will build a static archive named libllvmlibc.a:

cmake -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libc" \
      -DLLVM_LIBC_FULL_BUILD=OFF ...

2. Full Build Mode (Standalone Library)

In Full Build Mode, LLVM-libc is a complete replacement for the system library. This is used to build standalone libc.a and libm.a (with separate CMake targets) for a new operating system or to generate a sysroot for a specific target.

To configure for a full build, set LLVM_LIBC_FULL_BUILD=ON:

cmake -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
      -DLLVM_LIBC_FULL_BUILD=ON ...

3. Bootstrap Build

A bootstrap build first builds the compiler (Clang) and other LLVM tools using the host compiler, and then uses that newly-built Clang to build the libc. This ensures you are using a matched toolchain where the compiler and the library are built for each other.

To configure a bootstrap build, you point CMake to the llvm directory:

cmake -S llvm -B build -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" ...

4. Cross-compiler Build (Targeting Other Architectures)

Used when you want to build LLVM-libc for a different architecture than you are currently running on (e.g., building on x86_64 for an aarch64 target). This requires a cross-compiler or a toolchain file.

5. Bootstrap Cross-compiler (New Environment)

For users who are starting from scratch (e.g., with only Linux kernel headers) and want to generate a full C compiler and sysroot for their target. This is the most common path for those building entire environments to tinker in.