# Install the C API

## Prerequisites

Before getting started, you must ensure that you have installed the [Rust
toolchain](https://www.rust-lang.org/tools/install) as well as the [clang
compiler](https://clang.llvm.org/).

Now, clone the `qiskit-fermions` repository.

```sh
git clone git@github.com:Qiskit/qiskit-fermions.git
cd qiskit-fermions
```

Next, you must also compile the Qiskit C API. You can do this in any location,
but to get started quickly, follow these steps:

```sh
git clone git@github.com:Qiskit/qiskit.git build/qiskit
cd build/qiskit
## git-checkout the latest stable tag
make c
```

#### HINT
On **Windows** only, you need to copy additional library files:

### Windows

```sh
cp target/release/qiskit_cext.dll dist/c/lib/qiskit_cext.dll
cp target/release/qiskit_cext.dll.lib dist/c/lib/qiskit_cext.dll.lib
cp target/release/qiskit_cext.dll.lib dist/c/lib/qiskit.dll.lib
```

Now you can follow the steps in the next section to compile the `qiskit-fermions` C API.

## Compile from source

To get started, configure your shell environment with the locations of your
compiled Qiskit C API library and include directory.

#### IMPORTANT
The C API links against the Qiskit **C** library, so `QISKIT_LIB` and
`QISKIT_INCLUDE` must resolve to it. If unset, the build attempts to infer
them from any installed `qiskit` Python package which bundles its C
artifacts (`qiskit.capi.get_lib()`/`get_include()`); set them explicitly,
as below, to build against a separately compiled Qiskit C API such as the
`build/qiskit` clone above.

#### HINT
This code uses the `build/qiskit` path from the code at
the top of this page. Adjust the paths according to your setup.

### UNIX

```sh
export QISKIT_LIB=$(find $(pwd)/build/qiskit/dist/c/lib -name "libqiskit.*")
export QISKIT_INCLUDE=$(pwd)/build/qiskit/dist/c/include
```

### Windows

```sh
export QISKIT_LIB=$(find $(pwd)/build/qiskit/dist/c/lib -name "qiskit_cext.dll.lib")
export QISKIT_INCLUDE=$(pwd)/build/qiskit/dist/c/include
```

Now, compile the `qiskit-fermions` C API:

```sh
make cext
```

If you want to test your installation, you now must also install [CMake](https://cmake.org/).
You can then test your compilation by running the C unit tests:

```sh
make testc
```

Finally, verify that you see these relevant files:

- `dist/c/lib/libqiskit_fermions.so` (The suffix might vary depending on your operating system)
- `dist/c/include/qiskit_fermions.h`