(getting_started)=

# Getting Started

This guide provides a single, robust path for new users and contributors to
build, test, and verify LLVM-libc. We use the **runtimes build** (see
{ref}`build_concepts` for more information) because it is faster and sufficient
for most development tasks.

## 1. Install Dependencies

To build LLVM-libc, you will need a recent version of Clang (v15+) and basic
build tools. On a Debian/Ubuntu-based system, you can install these using
`apt-get`:

```sh
sudo apt-get update
sudo apt-get install git cmake ninja-build clang gcc-multilib
```

## 2. Clone and Configure

The following command clones the complete LLVM project and configures the
build for LLVM-libc. We include `compiler-rt` to enable the Scudo memory
allocator.

```sh
git clone --depth=1 https://github.com/llvm/llvm-project.git
cd llvm-project
cmake -G Ninja -S runtimes -B build \
  -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
  -DLLVM_LIBC_FULL_BUILD=ON \
  -DCMAKE_BUILD_TYPE=Debug \
  -DCMAKE_C_COMPILER=clang \
  -DCMAKE_CXX_COMPILER=clang++ \
  -DLLVM_LIBC_INCLUDE_SCUDO=ON \
  -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=ON \
  -DCOMPILER_RT_BUILD_GWP_ASAN=OFF \
  -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=OFF \
  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
```

## 3. Build and Test

After configuring, you can build the library, the math library (libm), and
run all unit tests:

```sh
ninja -C build libc libm check-libc
```

To run a specific test, such as `isalpha` in `ctype.h`:

```sh
ninja -C build libc.test.src.ctype.isalpha_test
```

## 4. Verify with Hello World

To verify your build, create a simple `hello.c` file:

```c
#include <stdio.h>

int main() {
  printf("Hello world from LLVM-libc!\n");
  return 0;
}
```

Compile it using the build artifacts:

```sh
clang -nostdinc -nostdlib hello.c -o hello \
  -I build/libc/include \
  -I $(clang -print-resource-dir)/include \
  build/libc/startup/linux/crt1.o \
  build/libc/lib/libc.a \
  build/libc/lib/libm.a
```

Finally, run the executable:

```sh
./hello
# Output: Hello world from LLVM-libc!
```

This setup builds LLVM-libc as a standalone library using the
recommended set of flags. From here, you can visit {ref}`full_host_build`
for advanced sysroot setup, {ref}`overlay_mode` to learn about using
LLVM-libc to augment your system's C library, or {ref}`build_concepts`
to understand other build scenarios.