## Using Targets to Compile for Different Platforms

Target descriptions allow `yotta` to compile the same code for different target platforms (different desktop operating systems, or different embedded devices).

Each target description contains information both about how to run the compiler to compile (or cross-compile), and configuration information describing the hardware or environment that the target represents.

### # Finding Targets

You can search for targets that people have published using `yotta search target "<search query>"`. For mbed boards, you can find information about the targets that can be used on the mbed board pages.

### # Selecting the Target

To view or change the current target, use the `yotta target` subcommand:

    # display the current target:
    yotta target

    # set the target to x86-osx-native
    yotta target x86-osx-native

By default the target is set in the current directory only. To set it globally, append ` -g`to the second command. You can switch targets back and forth to test building the same module for different targets without removing the build directory or recompiling unnecessarily, as builds are carried out a separate directory for each target:` ./build/<targetname>/`.</targetname>

## # Writing Targets

The description contains information on:

- a Toolchain File, which describes how to run the compiler
- Any supporting files required to compile (such as link scripts)
- Configuration information associated with this target, for example describing what hardware functionality is available.
- What other targets this target should be considered “similar to”. This information is used to resolve dependencies.

And optionally provides information on:

- how to run the debugger (`yotta debug` support)
- how to run tests, where they cannot be run natively (`yotta test` support)

Each yotta target contains a target.json file, which describes where to find this other information.

### # Inheriting from an Existing Target

Target descriptions usually inherit from an existing description. This allows you to extend the description that already exists instead of writing everything from scratch. With inheritance you override or add only the things that need changing.

For example, this is a simple target hierarchy from mbed OS:

- base: mbed-gcc: this target description describes how to run the arm-none-eabi-gcc cross-compiler, but doesn’t say anything about which chip or board is being compiled for.
- derived: frdm-k64f-gcc: this inherits from mbed-gcc, and adds compilation flags specific to compiling for the FRDM-K64F development board. This target description could actually be further split so that the description of the compilation flags for the main MCU on the board is separate from the description related to the board’s peripherals.

If you were building a product derived from the FRDM-K64F development board schematic you would choose to inherit from the existing frdm-k64f target.

To make your target description inherit from an existing one define the `inherits` property in your target.json file:

      ...
      "inherits": {
          "some-base-target": "^1.2.3"
      }
      ...

Note the version specification next to the name of the description you inherit from. Target descriptions use semantic versioning just like modules do: it’s normally a good idea to use the `^` version specification which allows any compatible version to be used. Just like module version specifications this can also be a github, git or mercurial reference if your target inherits from another target that hasn’t been published yet.

### # The Toolchain File

`yotta` uses the CMake build system, and targets describe how the compiler should be run by providing a CMake Toolchain File.

To use the system’s native compiler, the toolchain doesn’t need to do anything, but cross-compiling toolchain descriptions can get a lot more complicated, and must define every command necessary to compile programs.

By convention, the toolchain file is placed in a CMake subdirectory of the target:

    .
    ├── CMake
    │   └── toolchain.cmake
    ├── readme.md
    └── target.json

Though the path is actually specified in the `toolchain` property in target.json:

      "toolchain": "CMake/toolchain.cmake"

### # yotta Config Information

In addition to the `similarTo` data, your target can also define arbitrary JSON configuration data that can be used as the basis for including dependencies and providing configuration to the modules being built. See the config system reference for details.

### # The similarTo List and Target Specific Dependencies

The similarTo list in a target description is the list of targets that the target should be considered “similar to”. This is defined by target.json:

      "similarTo": <list of strings>,

This list is used (in addition to the config information) to choose which dependencies to pick when a module has target-specific dependencies (defined by the `targetDependencies` property in module.json). Target-specific dependencies can be used to depend on different implementations of the functionality that a module needs on different target platforms, which is especially useful when compiling for embedded hardware.

Note that now that config info can be defined by target descriptions, and target descriptions inherit from other ones, the use of `similarTo` should generally be avoided.

When compiling, CMake variables and preprocessor definitions are defined by `yotta` for each thing in the similarTo list (and the name of the target itself):

    TARGET_LIKE_FOO

Where `FOO` is the name in the similarTo list, converted to uppercase, and with any non-alphanumeric characters converted to underscores.

### # `yotta debug` Support

Targets can optionally provide a command that yotta will use to start a debugger when the user runs `yotta debug`. They do this by providing a `debug` script in target.json. This should be an array of commmand arguments, and use $program for the name of the program that’s being debugged, for example:

      "scripts": {
          "debug": [
            "lldb", "$program"
          ]
      }

Debuggers that attach to embedded devices often need to run a debug server program in the background before running the debugger itself. ARM mbed compilation targets use the `valinor` program to achieve this (valinor also detects which debugger is installed on the local system, and chooses the preferred one).

### # `yotta test` Support

To support the `yotta test` command, targets must provide a way of running tests, to do this, implement `scripts.test` in target.json. For native compilation targets, this can simply run the program in question, for a cross-compilation target it should be a wrapper script that loads the program onto the target device, runs it, prints the program’s output to stdout, and exits with the processes return code.

For example:

      "scripts": {
          "test": [
            "mbed-test-wrapper", "-t", "K64F", "-i", 10, "$program"
          ]
      }

### # Testing targets

To test a target locally, without publishing it, you can use `yotta link-target` to link it into an existing module, and use it for compilation.

First run `yotta link-target` in the directory in which you’re editing your target:

    cd path/to/my-target
    yotta link-target

Then in the module that you want to compile, run `yotta link-target <targetname>`:

    cd ../path/to/my/module
    yotta link-target my-target

You should also select your new target from the module directory before attempting to build for it for the first time.

    yotta target my-target

Now when you build, your new target description will be used. Note that currently you need to remove the build directory after editing the target’s toolchain file, as CMake does not add dependency rules on the toolchain:

    rm -Rf build/my-target
    yotta build

### # Publishing Targets

Once you’ve written your target you can publish it:

    # change into the target's direcory:
    cd path/to/my/target

    # tag a new version:
    yotta version minor

    # publish!
    yotta publish

After publishing, whenever someone sets the target to your target with the `yotta target <targetname>` command, your target description will be automatically downloaded and used.