# Overlay triplets example

## Building dynamic libraries on Linux

Using **vcpkg** you can build libraries for many configurations out of the box. However, this doesn't currently include shared libraries on Linux and Mac OS.

This doesn't mean that you cannot use **vcpkg** to build your dynamic libraries on these platforms! This document will guide you through creating your own custom triplets with `--overlay-triplets` to easily build dynamic libraries on Linux.

### Step 1: Create the custom triplet files

To save time, copy the existing `x64-linux.cmake` triplet file.

```sh
~/git$ mkdir custom-triplets
~/git$ cp vcpkg/triplets/x64-linux.cmake custom-triplets/x64-linux-dynamic.cmake
```

And modify `custom-triplets/x64-linux-dynamic.cmake` to match the contents below:
```cmake
# ~/git/custom-triplets/x64-linux-dynamic.cmake
set(VCPKG_TARGET_ARCHITECTURE x64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic) # This changed from static to dynamic

set(VCPKG_CMAKE_SYSTEM_NAME Linux)
```

### Step 2: Use `--overlay-triplets` to build dynamic libraries

Use the `--overlay-triplets` option to include the triplets in the `custom-triplets` directory. 

```
~/git$ vcpkg/vcpkg install sqlite3:x64-linux-dynamic --overlay-triplets=custom-triplets
The following packages will be built and installed:
    sqlite3[core]:x64-linux-dynamic
Starting package 1/1: sqlite3:x64-linux-dynamic
Building package sqlite3[core]:x64-linux-dynamic...
-- Loading triplet configuration from: /home/victor/git/custom-triplets/x64-linux-dynamic.cmake
-- Downloading https://sqlite.org/2019/sqlite-amalgamation-3280000.zip...
-- Extracting source /home/victor/git/vcpkg/downloads/sqlite-amalgamation-3280000.zip
-- Applying patch fix-arm-uwp.patch
-- Using source at /home/victor/git/vcpkg/buildtrees/sqlite3/src/3280000-6a3ff7ce92
-- Configuring x64-linux-dynamic-dbg
-- Configuring x64-linux-dynamic-rel
-- Building x64-linux-dynamic-dbg
-- Building x64-linux-dynamic-rel
-- Performing post-build validation
-- Performing post-build validation done
Building package sqlite3[core]:x64-linux-dynamic... done
Installing package sqlite3[core]:x64-linux-dynamic...
Installing package sqlite3[core]:x64-linux-dynamic... done
Elapsed time for package sqlite3:x64-linux-dynamic: 44.82 s

Total elapsed time: 44.82 s

The package sqlite3:x64-linux-dynamic provides CMake targets:

    find_package(sqlite3 CONFIG REQUIRED)
    target_link_libraries(main PRIVATE sqlite3)
```

Overlay triplets enables your custom triplet files when using `vcpkg install`, `vcpkg update`, `vcpkg upgrade`, and `vcpkg remove`.

When using the `--overlay-triplets` option, a message like the following lets you know that a custom triplet is being used: 

```
-- Loading triplet configuration from: /home/custom-triplets/x64-linux-dynamic.cmake
```

## Overriding default triplets

As you may have noticed, the default triplets for Windows (`x86-windows` and `x64-windows`) install dynamic libraries, while a suffix (`-static`) is needed for static libraries. This is different with Linux and Mac OS where static libraries are built by `x64-linux` and `x64-osx`.

Using `--overlay-triplets` it is possible to override the default triplets to accomplish the same behavior on Linux:

* `x64-linux`: Builds dynamic libraries,
* `x64-linux-static`: Builds static libraries.

### Step 1: Create the overlay triplets

Using the custom triplet created in the previous example, rename `custom-triplets/x64-linux-dynamic.cmake` to `custom-triplets/x64-linux.cmake`. Then, copy the default `x64-linux` triplet (which builds static libraries) in your `custom-triplets` folder and rename it to `x64-linux-static.cmake`.

```sh
~/git$ mv custom-triplets/x64-linux-dynamic.cmake custom-triplets/x64-linux.cmake
~/git$ cp vcpkg/triplets/x64-linux.cmake custom-triplets/x64-linux-static.cmake
```

### Step 2: Use `--overlay-triplets` to override default triplets

Use the `--overlay-triplets` option to include the triplets in the `custom-triplets` directory.

```
~/git$ vcpkg/vcpkg install sqlite3:x64-linux --overlay-triplets=custom-triplets
The following packages will be built and installed:
    sqlite3[core]:x64-linux
Starting package 1/1: sqlite3:x64-linux
Building package sqlite3[core]:x64-linux...
-- Loading triplet configuration from: /home/victor/git/custom-triplets/x64-linux.cmake
-- Downloading https://sqlite.org/2019/sqlite-amalgamation-3280000.zip...
-- Extracting source /home/victor/git/vcpkg/downloads/sqlite-amalgamation-3280000.zip
-- Applying patch fix-arm-uwp.patch
-- Using source at /home/victor/git/vcpkg/buildtrees/sqlite3/src/3280000-6a3ff7ce92
-- Configuring x64-linux-dbg
-- Configuring x64-linux-rel
-- Building x64-linux-dbg
-- Building x64-linux-rel
-- Performing post-build validation
-- Performing post-build validation done
Building package sqlite3[core]:x64-linux... done
Installing package sqlite3[core]:x64-linux...
Installing package sqlite3[core]:x64-linux... done
Elapsed time for package sqlite3:x64-linux: 44.82 s

Total elapsed time: 44.82 s

The package sqlite3:x64-linux provides CMake targets:

    find_package(sqlite3 CONFIG REQUIRED)
    target_link_libraries(main PRIVATE sqlite3)
```

Note that the default triplet is masked by your custom triplet:

```
-- Loading triplet configuration from: /home/victor/git/custom-triplets/x64-linux.cmake
```