# Building s2n-tls

s2n-tls can be built as follows:

<details open>
<summary>Ubuntu</summary>

```bash
# install build dependencies
sudo apt update
sudo apt install cmake

# install a libcrypto
sudo apt install libssl-dev

# build s2n-tls
cmake . -Bbuild \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=./s2n-tls-install
cmake --build build -j $(nproc)
CTEST_PARALLEL_LEVEL=$(nproc) ctest --test-dir build
cmake --install build
```
</details>

<details>
<summary>MacOS</summary>

```bash
# install build dependencies
brew install cmake

# install a libcrypto
brew install openssl@3

# build s2n-tls
cmake . -Bbuild \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_PREFIX_PATH=$(dirname $(dirname $(brew list openssl@3|grep libcrypto.dylib))) \
    -DCMAKE_INSTALL_PREFIX=./s2n-tls-install
cmake --build build -j $(sysctl -n hw.ncpu)
CTEST_PARALLEL_LEVEL=$(sysctl -n hw.ncpu) ctest --test-dir build
cmake --install build
```
</details>

<details>
<summary>AL2</summary>

```bash
# install build dependencies
sudo yum groupinstall "Development Tools"
sudo yum install cmake3

# install a libcrypto
sudo yum install openssl-devel

# build s2n-tls
cmake . -Bbuild \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=./s2n-tls-install \
    -DCMAKE_EXE_LINKER_FLAGS="-lcrypto -lz"
cmake --build build -j $(nproc)
CTEST_PARALLEL_LEVEL=$(nproc) ctest --test-dir build
cmake --install build
```
</details>

Note that we currently do not support building on Windows. See https://github.com/aws/s2n-tls/issues/497 for more information.

## Consuming s2n-tls via CMake

s2n-tls ships with modern CMake finder scripts if CMake is used for the build. To take advantage of this from your CMake script, all you need to do to compile and link against s2n-tls in your project is:

````bash
find_package(s2n)

....

target_link_libraries(yourExecutableOrLibrary AWS::s2n)
````

And when invoking CMake for your project, do one of two things:
 1. Set the `CMAKE_INSTALL_PREFIX` variable with the path to your s2n-tls build.
 2. If you have globally installed s2n-tls, do nothing, it will automatically be found.

## Configuring the s2n-tls build

s2n-tls can be configured with the following CMake options. Each option can be set by passing a `-D<option>=<value>` flag to CMake.
- [**`CMAKE_BUILD_TYPE`**](https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html): Sets the build type. Some of the possible build types are as follows:
  - `Release`: Produce an optimized s2n-tls library artifact with no debug info. This option should be used when building s2n-tls for use in production.
  - `Debug`: Produce an unoptimized library artifact with debug info. This option can be used when developing for or with s2n-tls. The debug symbols produced with this build can be used with GDB and other utilities to help with debugging.
- [**`CMAKE_INSTALL_PREFIX`**](https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_PREFIX.html): Specifies where the s2n-tls library artifacts are placed when installing s2n-tls.
- [**`CMAKE_PREFIX_PATH`**](https://cmake.org/cmake/help/latest/variable/CMAKE_PREFIX_PATH.html): Specifies install locations used by CMake to search for library dependencies. This option can be used to link s2n-tls to a specific libcrypto. See the [Building with a specific libcrypto](#building-with-a-specific-libcrypto) section for more information on building with different libcryptos.
- [**`BUILD_SHARED_LIBS`**](https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html): Specifies whether a static or shared s2n-tls library artifact will be produced during the build. Defaults to `OFF`, building a static library. If set to `ON`, a shared library will be produced instead.

The entire list of s2n-tls CMake options can be viewed with the following command:

```bash
cmake . -LH
```

## Building with a specific libcrypto

s2n-tls has a dependency on a libcrypto library. A supported libcrypto must be linked to s2n-tls when building. The following libcrypto libraries are currently supported:
- [AWS-LC](https://github.com/aws/aws-lc)
- [OpenSSL](https://www.openssl.org/) (versions 1.0.2 - 3.0)
  - ChaChaPoly is not supported before Openssl-1.1.1.
  - RSA-PSS is not supported before Openssl-1.1.1.
  - RC4 is not supported with Openssl-3.0 or later.
- [BoringSSL](https://boringssl.googlesource.com/boringssl)
  - OCSP features are not supported with BoringSSL.
- [LibreSSL](https://www.libressl.org/)

By default, s2n-tls will attempt to find a system libcrypto to link with when building. However, this search can be overridden to any of the above libcryptos by specifying the install directory with the `CMAKE_PREFIX_PATH` flag.

For help building a desired libcrypto on your platform, please consult the project's build documentation.

### AWS-LC

[AWS-LC](https://github.com/aws/aws-lc) is the recommended libcrypto to use with s2n-tls due to increased performance and security. See the [AWS-LC build documentation](https://github.com/aws/aws-lc/blob/main/BUILDING.md) for information on building AWS-LC.

The `CMAKE_INSTALL_PREFIX` option can be provided when building AWS-LC to specify where AWS-LC will be installed. The install path for AWS-LC should be provided when building s2n-tls, via the `CMAKE_PREFIX_PATH` option. This will ensure that s2n-tls is able to find the AWS-LC library artifact to link with.

## Other build methods

### Ninja build system

[Ninja](https://ninja-build.org/) can be specified as the build system with CMake, which can increase build performance:

<details open>
<summary>Ubuntu</summary>

```bash
# install ninja
sudo apt update
sudo apt install ninja-build

# build s2n-tls with ninja
cmake . -Bbuild -GNinja \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_INSTALL_PREFIX=./s2n-tls-install
ninja -C build -j $(nproc)
CTEST_PARALLEL_LEVEL=$(nproc) ninja -C build test
ninja -C build install
```
</details>

### Traditional Makefile

CMake is the preferred build system for s2n-tls since it includes updated build features and supports the most platforms. However, building s2n-tls with a traditional Makefile is also supported on some platforms. With make, the desired libcrypto install path must be set with the `LIBCRYPTO_ROOT` environment variable.

<details open>
<summary>Ubuntu</summary>

```bash
LIBCRYPTO_ROOT=/usr/local/ssl make
```
</details>

### Nix

s2n-tls supports building with Nix, which can be used for s2n-tls development to set up an environment that closely matches our CI. If Nix is installed, s2n-tls can be built with the following:

```bash
nix develop
configure; build
```

For more information on installing Nix and using Nix with s2n-tls, see the [s2n-tls Nix documentation](../nix/README.md).

## mlock() and system limits

Internally s2n-tls uses mlock() to prevent memory from being swapped to disk. The
s2n-tls build tests may fail in some environments where the default limit on locked
memory is too low. To check this limit, run:

```bash
ulimit -l
```

to raise the limit, consult the documentation for your platform.

### Disabling mlock()

To disable s2n-tls's mlock behavior, run your application with the `S2N_DONT_MLOCK` environment variable set to 1.
s2n-tls also reads this for unit tests. Try setting this environment variable before running the unit tests if you're having mlock failures.

## Cross Compiling for 32 Bit Platforms

There is an example toolchain for 32 bit cross-compiling in [`cmake/toolchains/32-bit.toolchain`](../cmake/toolchains/32-bit.toolchain).

First, you will need access to a 32 bit version of libcrypto. Many linux distributions are [multi-arch](https://help.ubuntu.com/community/MultiArch) compatible which allows you to download 32 bit packages on a 64 bit platform. This can be done with the following:

<details open>
<summary>Ubuntu</summary>

```bash
# install build dependencies
sudo apt update
sudo apt install cmake clang gcc-multilib

# we're interested in i386 (32 bit) architectures
sudo dpkg --add-architecture i386

# install the 32 bit (i386) version of libcrypto
sudo apt install libssl-dev:i386

# wipe the build directory to clear the CMake cache
rm -rf build

# build with the toolchain
cmake . -Bbuild -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/32-bit.toolchain
cmake --build ./build -j $(nproc)
CTEST_PARALLEL_LEVEL=$(nproc) ctest --test-dir build
```
</details>
