# Full Cross Build¶

In this document, we will present a recipe to cross build a full libc. When we say cross build a full libc, we mean that we will build the libc for a target system which is not the same as the system on which the libc is being built. For example, you could be building for a bare metal aarch64 target on a Linux x86_64 host.

## Configure the full cross build of the libc¶

Below is a simple recipe to configure the libc for a cross build.

$> cd llvm-project # The llvm-project checkout$> mkdir build
$> cd build$> cmake ../llvm  \
-G Ninja \ # Generator
-DLLVM_ENABLE_PROJECTS=libc  \ # Enable the libc project
-DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++  \
-DLLVM_LIBC_FULL_BUILD=ON   \ # We are building the full libc


We will go over the special options passed to the cmake command above.

• Enabled Projects - Since we want to build the libc project, we list libc as the enabled project.
• The full build option - Since we want to build the full libc, we pass -DLLVM_LIBC_FULL_BUILD=ON.
• The target triple - This is the target triple of the target for which we are building the libc. For example, for a Linux 32-bit Arm target, one can specify it as arm-linux-eabi.

## Build and install¶

After configuring the build with the above cmake command, one can build the the libc for the target with the following command:

\$> ninja libc


The above ninja command will build the libc.a static archive for the target specified with -DLIBC_TARGET_TRIPLE to the cmake command.

## Building for bare metal¶

To build for bare metal, all one has to do is to specify the system component of the target triple as none. For example, to build for a 32-bit arm target on bare metal, one can use a target triple like arm-none-eabi.