# Using the Extensible SDK

This chapter describes the extensible SDK and how to install it. Information covers the pieces of the SDK, how to install it, and presents a look at using the `devtool` functionality. The extensible SDK makes it easy to add new applications and libraries to an image, modify the source for an existing component, test changes on the target hardware, and ease integration into the rest of the OpenEmbedded build system.

For a side-by-side comparison of main features supported for an extensible SDK as compared to a standard SDK, see the "Introduction" section.

In addition to the functionality available through `devtool`, you can alternatively make use of the toolchain directly, for example from Makefile, Autotools, and Eclipse-based projects. See the "Using the SDK Toolchain Directly" chapter for more information.

## Why use the Extensible SDK and What is in It?

The extensible SDK provides a cross-development toolchain and libraries tailored to the contents of a specific image. You would use the Extensible SDK if you want a toolchain experience supplemented with the powerful set of `devtool` commands tailored for the Yocto Project environment.

The installed extensible SDK consists of several files and directories. Basically, it contains an SDK environment setup script, some configuration files, an internal build system, and the `devtool` functionality.

## Setting Up to Use the Extensible SDK

The first thing you need to do is install the SDK on your host development machine by running the `*.sh` installation script.

You can download a tarball installer, which includes the pre-built toolchain, the `runqemu` script, the internal build system, `devtool`, and support files from the appropriate directory under . Toolchains are available for 32-bit and 64-bit x86 development systems from the `i686` and `x86_64` directories, respectively. The toolchains the Yocto Project provides are based off the `core-image-sato` image and contain libraries appropriate for developing against that image. Each type of development system supports five or more target architectures.

The names of the tarball installer scripts are such that a string representing the host system appears first in the filename and then is immediately followed by a string representing the target architecture. An extensible SDK has the string "-ext" as part of the name.

         poky-glibc-host_system-image_type-arch-toolchain-ext-release_version.sh

         Where:
             host_system is a string representing your development system:

                        i686 or x86_64.

             image_type is the image for which the SDK was built.

             arch is a string representing the tuned target architecture:

                        i586, x86_64, powerpc, mips, armv7a or armv5te

             release_version is a string representing the release number of the
                    Yocto Project:

                        &DISTRO;, &DISTRO;+snapshot

For example, the following SDK installer is for a 64-bit development host system and a i586-tuned target architecture based off the SDK for `core-image-sato` and using the current &DISTRO; snapshot:

         poky-glibc-x86_64-core-image-sato-i586-toolchain-ext-&DISTRO;.sh

As an alternative to downloading an SDK, you can build the SDK installer. For information on building the installer, see the "Building an SDK Installer" section. Another helpful resource for building an installer is the Cookbook guide to Making an Eclipse Debug Capable Image wiki page. This wiki page focuses on development when using the Eclipse IDE.

The SDK and toolchains are self-contained and by default are installed into the `poky_sdk` folder in your home directory. You can choose to install the extensible SDK in any location when you run the installer. However, the location you choose needs to be writable for whichever users need to use the SDK, since files will need to be written under that directory during the normal course of operation.

The following command shows how to run the installer given a toolchain tarball for a 64-bit x86 development host system and a 64-bit x86 target architecture. The example assumes the SDK installer is located in `~/Downloads/`.

If you do not have write permissions for the directory into which you are installing the SDK, the installer notifies you and exits. Be sure you have write permissions in the directory and run the installer again.

         $ ./poky-glibc-x86_64-core-image-minimal-core2-64-toolchain-ext-&DISTRO;.sh
         Poky (Yocto Project Reference Distro) Extensible SDK installer version &DISTRO;
         ===================================================================================
         Enter target directory for SDK (default: ~/poky_sdk):
         You are about to install the SDK to "/home/scottrif/poky_sdk". Proceed[Y/n]? Y
         Extracting SDK......................................................................done
         Setting it up...
         Extracting buildtools...
         Preparing build system...
         done
         SDK has been successfully set up and is ready to be used.
         Each time you wish to use the SDK in a new shell session, you need to source the environment setup script e.g.
          $ . /home/scottrif/poky_sdk/environment-setup-core2-64-poky-linux

## Running the Extensible SDK Environment Setup Script

Once you have the SDK installed, you must run the SDK environment setup script before you can actually use it. This setup script resides in the directory you chose when you installed the SDK, which is either the default `poky_sdk` directory or the directory you chose during installation.

Before running the script, be sure it is the one that matches the architecture for which you are developing. Environment setup scripts begin with the string "`environment-setup`" and include as part of their name the tuned target architecture. As an example, the following commands set the working directory to where the SDK was installed and then source the environment setup script. In this example, the setup script is for an IA-based target machine using i586 tuning:

         $ cd /home/scottrif/poky_sdk
         $ source environment-setup-core2-64-poky-linux
         SDK environment now set up; additionally you may now run devtool to perform development tasks.
         Run devtool --help for further details.

When you run the setup script, many environment variables are defined:

         SDKTARGETSYSROOT - The path to the sysroot used for cross-compilation
         PKG_CONFIG_PATH - The path to the target pkg-config files
         CONFIG_SITE - A GNU autoconf site file preconfigured for the target
         CC - The minimal command and arguments to run the C compiler
         CXX - The minimal command and arguments to run the C++ compiler
         CPP - The minimal command and arguments to run the C preprocessor
         AS - The minimal command and arguments to run the assembler
         LD - The minimal command and arguments to run the linker
         GDB - The minimal command and arguments to run the GNU Debugger
         STRIP - The minimal command and arguments to run 'strip', which strips symbols
         RANLIB - The minimal command and arguments to run 'ranlib'
         OBJCOPY - The minimal command and arguments to run 'objcopy'
         OBJDUMP - The minimal command and arguments to run 'objdump'
         AR - The minimal command and arguments to run 'ar'
         NM - The minimal command and arguments to run 'nm'
         TARGET_PREFIX - The toolchain binary prefix for the target tools
         CROSS_COMPILE - The toolchain binary prefix for the target tools
         CONFIGURE_FLAGS - The minimal arguments for GNU configure
         CFLAGS - Suggested C flags
         CXXFLAGS - Suggested C++ flags
         LDFLAGS - Suggested linker flags when you use CC to link
         CPPFLAGS - Suggested preprocessor flags

## Using `devtool` in Your SDK Workflow

The cornerstone of the extensible SDK is a command-line tool called `devtool`. This tool provides a number of features that help you build, test and package software within the extensible SDK, and optionally integrate it into an image built by the OpenEmbedded build system.

The `devtool` command line is organized similarly to Git in that it has a number of sub-commands for each function. You can run `devtool --help` to see all the commands.

Three `devtool` subcommands that provide entry-points into development are:

- `devtool add`: Assists in adding new software to be built.

- `devtool modify`: Sets up an environment to enable you to modify the source of an existing component.

- `devtool upgrade`: Updates an existing recipe so that you can build it for an updated set of source files.

As with the OpenEmbedded build system, "recipes" represent software packages within `devtool`. When you use `devtool add`, a recipe is automatically created. When you use `devtool modify`, the specified existing recipe is used in order to determine where to get the source code and how to patch it. In both cases, an environment is set up so that when you build the recipe a source tree that is under your control is used in order to allow you to make changes to the source as desired. By default, both new recipes and the source go into a "workspace" directory under the SDK.

The remainder of this section presents the `devtool add`, `devtool modify`, and `devtool upgrade` workflows.

### Use `devtool add` to Add an Application

The `devtool add` command generates a new recipe based on existing source code. This command takes advantage of the workspace layer that many `devtool` commands use. The command is flexible enough to allow you to extract source code into both the workspace or a separate local Git repository and to use existing code that does not need to be extracted.

Depending on your particular scenario, the arguments and options you use with `devtool add` form different combinations. The following diagram shows common development flows you would use with the `devtool add` command:

1. Generating the New Recipe: The top part of the flow shows three scenarios by which you could use `devtool add` to generate a recipe based on existing source code.

   In a shared development environment, it is typical where other developers are responsible for various areas of source code. As a developer, you are probably interested in using that source code as part of your development using the Yocto Project. All you need is access to the code, a recipe, and a controlled area in which to do your work.

   Within the diagram, three possible scenarios feed into the `devtool add` workflow:

   - Left: The left scenario represents a common situation where the source code does not exist locally and needs to be extracted. In this situation, you just let it get extracted to the default workspace - you do not want it in some specific location outside of the workspace. Thus, everything you need will be located in the workspace:

              $ devtool add recipe fetchuri

     With this command, `devtool` creates a recipe and an append file in the workspace as well as extracts the upstream source files into a local Git repository also within the `sources` folder.

   - Middle: The middle scenario also represents a situation where the source code does not exist locally. In this case, the code is again upstream and needs to be extracted to some local area - this time outside of the default workspace. If required, `devtool` always creates a Git repository locally during the extraction. Furthermore, the first positional argument srctree in this case identifies where the `devtool add` command will locate the extracted code outside of the workspace:

              $ devtool add recipe srctree fetchuri

     In summary, the source code is pulled from fetchuri and extracted into the location defined by srctree as a local Git repository.

     Within workspace, `devtool` creates both the recipe and an append file for the recipe.

   - Right: The right scenario represents a situation where the source tree (srctree) has been previously prepared outside of the `devtool` workspace.

     The following command names the recipe and identifies where the existing source tree is located:

              $ devtool add recipe srctree

     The command examines the source code and creates a recipe for it placing the recipe into the workspace.

     Because the extracted source code already exists, `devtool` does not try to relocate it into the workspace - just the new the recipe is placed in the workspace.

     Aside from a recipe folder, the command also creates an append folder and places an initial `*.bbappend` within.

2. Edit the Recipe: At this point, you can use `devtool edit-recipe` to open up the editor as defined by the `$EDITOR` environment variable and modify the file:

            $ devtool edit-recipe recipe

   From within the editor, you can make modifications to the recipe that take affect when you build it later.

3. Build the Recipe or Rebuild the Image: At this point in the flow, the next step you take depends on what you are going to do with the new code.

   If you need to take the build output and eventually move it to the target hardware, you would use `devtool build`:

            $ devtool build recipe

   On the other hand, if you want an image to contain the recipe's packages for immediate deployment onto a device (e.g. for testing purposes), you can use the `devtool build-image` command:

            $ devtool build-image image

4. Deploy the Build Output: When you use the `devtool build` command to build out your recipe, you probably want to see if the resulting build output works as expected on target hardware.

   This step assumes you have a previously built image that is already either running in QEMU or running on actual hardware. Also, it is assumed that for deployment of the image to the target, SSH is installed in the image and if the image is running on real hardware that you have network access to and from your development machine.

   You can deploy your build output to that target hardware by using the `devtool deploy-target` command:

            $ devtool deploy-target recipe target

   The target is a live target machine running as an SSH server.

   You can, of course, also deploy the image you build using the `devtool build-image` command to actual hardware. However, `devtool` does not provide a specific command that allows you to do this.

5. Finish Your Work With the Recipe: The `devtool finish` command creates any patches corresponding to commits in the local Git repository, moves the new recipe to a more permanent layer, and then resets the recipe so that the recipe is built normally rather than from the workspace.

            $ devtool finish recipe layer

   Any changes you want to turn into patches must be committed to the Git repository in the source tree.

   As mentioned, the `devtool finish` command moves the final recipe to its permanent layer.

   As a final process of the `devtool finish` command, the state of the standard layers and the upstream source is restored so that you can build the recipe from those areas rather than the workspace.

   You can use the `devtool reset` command to put things back should you decide you do not want to proceed with your work. If you do use this command, realize that the source tree is preserved.

### Use `devtool modify` to Modify the Source of an Existing Component

The `devtool modify` command prepares the way to work on existing code that already has a recipe in place. The command is flexible enough to allow you to extract code, specify the existing recipe, and keep track of and gather any patch files from other developers that are associated with the code.

Depending on your particular scenario, the arguments and options you use with `devtool modify` form different combinations. The following diagram shows common development flows you would use with the `devtool modify` command:

1. Preparing to Modify the Code: The top part of the flow shows three scenarios by which you could use `devtool modify` to prepare to work on source files. Each scenario assumes the following:

   - The recipe exists in some layer external to the `devtool` workspace.

   - The source files exist upstream in an un-extracted state or locally in a previously extracted state.

   The typical situation is where another developer has created some layer for use with the Yocto Project and their recipe already resides in that layer. Furthermore, their source code is readily available either upstream or locally.

   - Left: The left scenario represents a common situation where the source code does not exist locally and needs to be extracted. In this situation, the source is extracted into the default workspace location. The recipe, in this scenario, is in its own layer outside the workspace (i.e. `meta-`layername).

     The following command identifies the recipe and by default extracts the source files:

              $ devtool modify recipe

     Once `devtool`locates the recipe, it uses the `SRC_URI` variable to locate the source code and any local patch files from other developers are located.

     You cannot provide an URL for srctree when using the `devtool modify` command.

     With this scenario, however, since no srctree argument exists, the `devtool modify` command by default extracts the source files to a Git structure. Furthermore, the location for the extracted source is the default area within the workspace. The result is that the command sets up both the source code and an append file within the workspace with the recipe remaining in its original location.

   - Middle: The middle scenario represents a situation where the source code also does not exist locally. In this case, the code is again upstream and needs to be extracted to some local area as a Git repository. The recipe, in this scenario, is again in its own layer outside the workspace.

     The following command tells `devtool` what recipe with which to work and, in this case, identifies a local area for the extracted source files that is outside of the default workspace:

              $ devtool modify recipe srctree

     As with all extractions, the command uses the recipe's `SRC_URI` to locate the source files. Once the files are located, the command by default extracts them. Providing the srctree argument instructs `devtool` where place the extracted source.

     Within workspace, `devtool` creates an append file for the recipe. The recipe remains in its original location but the source files are extracted to the location you provided with srctree.

   - Right: The right scenario represents a situation where the source tree (srctree) exists as a previously extracted Git structure outside of the `devtool` workspace. In this example, the recipe also exists elsewhere in its own layer.

     The following command tells `devtool` the recipe with which to work, uses the "-n" option to indicate source does not need to be extracted, and uses srctree to point to the previously extracted source files:

              $ devtool modify -n recipe srctree

     Once the command finishes, it creates only an append file for the recipe in the workspace. The recipe and the source code remain in their original locations.

2. Edit the Source: Once you have used the `devtool modify` command, you are free to make changes to the source files. You can use any editor you like to make and save your source code modifications.

3. Build the Recipe: Once you have updated the source files, you can build the recipe.

4. Deploy the Build Output: When you use the `devtool build` command to build out your recipe, you probably want to see if the resulting build output works as expected on target hardware.

   This step assumes you have a previously built image that is already either running in QEMU or running on actual hardware. Also, it is assumed that for deployment of the image to the target, SSH is installed in the image and if the image is running on real hardware that you have network access to and from your development machine.

   You can deploy your build output to that target hardware by using the `devtool deploy-target` command:

            $ devtool deploy-target recipe target

   The target is a live target machine running as an SSH server.

   You can, of course, also deploy the image you build using the `devtool build-image` command to actual hardware. However, `devtool` does not provide a specific command that allows you to do this.

5. Finish Your Work With the Recipe: The `devtool finish` command creates any patches corresponding to commits in the local Git repository, updates the recipe to point to them (or creates a `.bbappend` file to do so, depending on the specified destination layer), and then resets the recipe so that the recipe is built normally rather than from the workspace.

            $ devtool finish recipe layer

   Any changes you want to turn into patches must be committed to the Git repository in the source tree.

   Because there is no need to move the recipe, `devtool finish` either updates the original recipe in the original layer or the command creates a `.bbappend` in a different layer as provided by layer.

   As a final process of the `devtool finish` command, the state of the standard layers and the upstream source is restored so that you can build the recipe from those areas rather than the workspace.

   You can use the `devtool reset` command to put things back should you decide you do not want to proceed with your work. If you do use this command, realize that the source tree is preserved.

### Use `devtool upgrade` to Create a Version of the Recipe that Supports a Newer Version of the Software

The `devtool upgrade` command updates an existing recipe so that you can build it for an updated set of source files. The command is flexible enough to allow you to specify source code revision and versioning schemes, extract code into or out of the `devtool` workspace, and work with any source file forms that the fetchers support.

Depending on your particular scenario, the arguments and options you use with `devtool upgrade` form different combinations. The following diagram shows a common development flow you would use with the `devtool modify` command:

1. Initiate the Upgrade: The top part of the flow shows a typical scenario by which you could use `devtool upgrade`. The following conditions exist:

   - The recipe exists in some layer external to the `devtool` workspace.

   - The source files for the new release exist adjacent to the same location pointed to by `SRC_URI` in the recipe (e.g. a tarball with the new version number in the name, or as a different revision in the upstream Git repository).

   A common situation is where third-party software has undergone a revision so that it has been upgraded. The recipe you have access to is likely in your own layer. Thus, you need to upgrade the recipe to use the newer version of the software:

            $ devtool upgrade -V version recipe

   By default, the `devtool upgrade` command extracts source code into the `sources` directory in the workspace. If you want the code extracted to any other location, you need to provide the srctree positional argument with the command as follows:

            $ devtool upgrade -V version recipe srctree

   Also, in this example, the "-V" option is used to specify the new version. If the source files pointed to by the `SRC_URI` statement in the recipe are in a Git repository, you must provide the "-S" option and specify a revision for the software.

   Once `devtool` locates the recipe, it uses the `SRC_URI` variable to locate the source code and any local patch files from other developers are located. The result is that the command sets up the source code, the new version of the recipe, and an append file all within the workspace.

2. Resolve any Conflicts created by the Upgrade: At this point, there could be some conflicts due to the software being upgraded to a new version. This would occur if your recipe specifies some patch files in `SRC_URI` that conflict with changes made in the new version of the software. If this is the case, you need to resolve the conflicts by editing the source and following the normal `git rebase` conflict resolution process.

   Before moving onto the next step, be sure to resolve any such conflicts created through use of a newer or different version of the software.

3. Build the Recipe: Once you have your recipe in order, you can build it. You can either use `devtool build` or `bitbake`. Either method produces build output that is stored in `TMPDIR`.

4. Deploy the Build Output: When you use the `devtool build` command or `bitbake` to build out your recipe, you probably want to see if the resulting build output works as expected on target hardware.

   This step assumes you have a previously built image that is already either running in QEMU or running on actual hardware. Also, it is assumed that for deployment of the image to the target, SSH is installed in the image and if the image is running on real hardware that you have network access to and from your development machine.

   You can deploy your build output to that target hardware by using the `devtool deploy-target` command:

            $ devtool deploy-target recipe target

   The target is a live target machine running as an SSH server.

   You can, of course, also deploy the image you build using the `devtool build-image` command to actual hardware. However, `devtool` does not provide a specific command that allows you to do this.

5. Finish Your Work With the Recipe: The `devtool finish` command creates any patches corresponding to commits in the local Git repository, moves the new recipe to a more permanent layer, and then resets the recipe so that the recipe is built normally rather than from the workspace. If you specify a destination layer that is the same as the original source, then the old version of the recipe and associated files will be removed prior to adding the new version.

            $ devtool finish recipe layer

   Any changes you want to turn into patches must be committed to the Git repository in the source tree.

   As a final process of the `devtool finish` command, the state of the standard layers and the upstream source is restored so that you can build the recipe from those areas rather than the workspace.

   You can use the `devtool reset` command to put things back should you decide you do not want to proceed with your work. If you do use this command, realize that the source tree is preserved.

## A Closer Look at `devtool add`

The `devtool add` command automatically creates a recipe based on the source tree with which you provide it. Currently, the command has support for the following:

- Autotools (`autoconf` and `automake`)

- CMake

- Scons

- `qmake`

- Plain `Makefile`

- Out-of-tree kernel module

- Binary package (i.e. "-b" option)

- Node.js module

- Python modules that use `setuptools` or `distutils`

Apart from binary packages, the determination of how a source tree should be treated is automatic based on the files present within that source tree. For example, if a `CMakeLists.txt` file is found, then the source tree is assumed to be using CMake and is treated accordingly.

In most cases, you need to edit the automatically generated recipe in order to make it build properly. Typically, you would go through several edit and build cycles until you can build the recipe. Once the recipe can be built, you could use possible further iterations to test the recipe on the target device.

The remainder of this section covers specifics regarding how parts of the recipe are generated.

### Name and Version

If you do not specify a name and version on the command line, `devtool add` attempts to determine the name and version of the software being built from various metadata within the source tree. Furthermore, the command sets the name of the created recipe file accordingly. If the name or version cannot be determined, the `devtool add` command prints an error and you must re-run the command with both the name and version or just the name or version specified.

Sometimes the name or version determined from the source tree might be incorrect. For such a case, you must reset the recipe:

         $ devtool reset -n recipename

After running the `devtool reset` command, you need to run `devtool add` again and provide the name or the version.

### Dependency Detection and Mapping

The `devtool add` command attempts to detect build-time dependencies and map them to other recipes in the system. During this mapping, the command fills in the names of those recipes in the `DEPENDS` value within the recipe. If a dependency cannot be mapped, then a comment is placed in the recipe indicating such. The inability to map a dependency might be caused because the naming is not recognized or because the dependency simply is not available. For cases where the dependency is not available, you must use the `devtool add` command to add an additional recipe to satisfy the dependency and then come back to the first recipe and add its name to `DEPENDS`.

If you need to add runtime dependencies, you can do so by adding the following to your recipe:

         RDEPENDS_${PN} += "dependency1 dependency2 ..."

The `devtool add` command often cannot distinguish between mandatory and optional dependencies. Consequently, some of the detected dependencies might in fact be optional. When in doubt, consult the documentation or the configure script for the software the recipe is building for further details. In some cases, you might find you can substitute the dependency for an option to disable the associated functionality passed to the configure script.

### License Detection

The `devtool add` command attempts to determine if the software you are adding is able to be distributed under a common open-source license and sets the `LICENSE` value accordingly. You should double-check this value against the documentation or source files for the software you are building and update that `LICENSE` value if necessary.

The `devtool add` command also sets the `LIC_FILES_CHKSUM` value to point to all files that appear to be license-related. However, license statements often appear in comments at the top of source files or within documentation. Consequently, you might need to amend the `LIC_FILES_CHKSUM` variable to point to one or more of those comments if present. Setting `LIC_FILES_CHKSUM` is particularly important for third-party software. The mechanism attempts to ensure correct licensing should you upgrade the recipe to a newer upstream version in future. Any change in licensing is detected and you receive an error prompting you to check the license text again.

If the `devtool add` command cannot determine licensing information, the `LICENSE` value is set to "CLOSED" and the `LIC_FILES_CHKSUM` value remains unset. This behavior allows you to continue with development but is unlikely to be correct in all cases. Consequently, you should check the documentation or source files for the software you are building to determine the actual license.

### Adding Makefile-Only Software

The use of `make` by itself is very common in both proprietary and open source software. Unfortunately, Makefiles are often not written with cross-compilation in mind. Thus, `devtool add` often cannot do very much to ensure that these Makefiles build correctly. It is very common, for example, to explicitly call `gcc` instead of using the `CC` variable. Usually, in a cross-compilation environment, `gcc` is the compiler for the build host and the cross-compiler is named something similar to `arm-poky-linux-gnueabi-gcc` and might require some arguments (e.g. to point to the associated sysroot for the target machine).

When writing a recipe for Makefile-only software, keep the following in mind:

- You probably need to patch the Makefile to use variables instead of hardcoding tools within the toolchain such as `gcc` and `g++`.

- The environment in which `make` runs is set up with various standard variables for compilation (e.g. `CC`, `CXX`, and so forth) in a similar manner to the environment set up by the SDK's environment setup script. One easy way to see these variables is to run the `devtool build` command on the recipe and then look in `oe-logs/run.do_compile`. Towards the top of this file you will see a list of environment variables that are being set. You can take advantage of these variables within the Makefile.

- If the Makefile sets a default for a variable using "=", that default overrides the value set in the environment, which is usually not desirable. In this situation, you can either patch the Makefile so it sets the default using the "?=" operator, or you can alternatively force the value on the `make` command line. To force the value on the command line, add the variable setting to `EXTRA_OEMAKE` or `PACKAGECONFIG_CONFARGS` within the recipe. Here is an example using `EXTRA_OEMAKE`:

           EXTRA_OEMAKE += "'CC=${CC}' 'CXX=${CXX}'"

  In the above example, single quotes are used around the variable settings as the values are likely to contain spaces because required default options are passed to the compiler.

- Hardcoding paths inside Makefiles is often problematic in a cross-compilation environment. This is particularly true because those hardcoded paths often point to locations on the build host and thus will either be read-only or will introduce contamination into the cross-compilation by virtue of being specific to the build host rather than the target. Patching the Makefile to use prefix variables or other path variables is usually the way to handle this.

- Sometimes a Makefile runs target-specific commands such as `ldconfig`. For such cases, you might be able to simply apply patches that remove these commands from the Makefile.

### Adding Native Tools

Often, you need to build additional tools that run on the build host system as opposed to the target. You should indicate this using one of the following methods when you run `devtool add`:

- Specify the name of the recipe such that it ends with "-native". Specifying the name like this produces a recipe that only builds for the build host.

- Specify the "&dash;&dash;also-native" option with the `devtool add` command. Specifying this option creates a recipe file that still builds for the target but also creates a variant with a "-native" suffix that builds for the build host.

If you need to add a tool that is shipped as part of a source tree that builds code for the target, you can typically accomplish this by building the native and target parts separately rather than within the same compilation process. Realize though that with the "&dash;&dash;also-native" option, you can add the tool using just one recipe file.

### Adding Node.js Modules

You can use the `devtool add` command two different ways to add Node.js modules: 1) Through `npm` and, 2) from a repository or local source.

Use the following form to add Node.js modules through `npm`:

         $ devtool add "npm://registry.npmjs.org;name=forever;version=0.15.1"

The name and version parameters are mandatory. Lockdown and shrinkwrap files are generated and pointed to by the recipe in order to freeze the version that is fetched for the dependencies according to the first time. This also saves checksums that are verified on future fetches. Together, these behaviors ensure the reproducibility and integrity of the build.

#### Notes

- You must use quotes around the URL. The `devtool add` does not require the quotes, but the shell considers ";" as a splitter between multiple commands. Thus, without the quotes, `devtool add` does not receive the other parts, which results in several "command not found" errors.

- In order to support adding Node.js modules, a `nodejs` recipe must be part of your SDK in order to provide Node.js itself.

As mentioned earlier, you can also add Node.js modules directly from a repository or local source tree. To add modules this way, use `devtool add` in the following form:

         $ devtool add https://github.com/diversario/node-ssdp

In this example, `devtool` fetches the specified Git repository, detects that the code is Node.js code, fetches dependencies using `npm`, and sets `SRC_URI` accordingly.

## Working With Recipes

When building a recipe with `devtool build` the typical build progression is as follows:

1. Fetch the source

2. Unpack the source

3. Configure the source

4. Compiling the source

5. Install the build output

6. Package the installed output

For recipes in the workspace, fetching and unpacking is disabled as the source tree has already been prepared and is persistent. Each of these build steps is defined as a function, usually with a "do_" prefix. These functions are typically shell scripts but can instead be written in Python.

If you look at the contents of a recipe, you will see that the recipe does not include complete instructions for building the software. Instead, common functionality is encapsulated in classes inherited with the `inherit` directive, leaving the recipe to describe just the things that are specific to the software to be built. A `base` class exists that is implicitly inherited by all recipes and provides the functionality that most typical recipes need.

The remainder of this section presents information useful when working with recipes.

### Finding Logs and Work Files

When you are debugging a recipe that you previously created using `devtool add` or whose source you are modifying by using the `devtool modify` command, after the first run of `devtool build`, you will find some symbolic links created within the source tree: `oe-logs`, which points to the directory in which log files and run scripts for each build step are created and `oe-workdir`, which points to the temporary work area for the recipe. You can use these links to get more information on what is happening at each build step.

These locations under `oe-workdir` are particularly useful:

- `image/`: Contains all of the files installed at the `do_install` stage. Within a recipe, this directory is referred to by the expression `${` `D` `}`.

- `sysroot-destdir/`: Contains a subset of files installed within `do_install` that have been put into the shared sysroot. For more information, see the "Sharing Files Between Recipes" section.

- `packages-split/`: Contains subdirectories for each package produced by the recipe. For more information, see the "Packaging" section.

### Setting Configure Arguments

If the software your recipe is building uses GNU autoconf, then a fixed set of arguments is passed to it to enable cross-compilation plus any extras specified by `EXTRA_OECONF` or `PACKAGECONFIG_CONFARGS` set within the recipe. If you wish to pass additional options, add them to `EXTRA_OECONF` or `PACKAGECONFIG_CONFARGS`. Other supported build tools have similar variables (e.g. `EXTRA_OECMAKE` for CMake, `EXTRA_OESCONS` for Scons, and so forth). If you need to pass anything on the `make` command line, you can use `EXTRA_OEMAKE` or the `PACKAGECONFIG_CONFARGS` variables to do so.

You can use the `devtool configure-help` command to help you set the arguments listed in the previous paragraph. The command determines the exact options being passed, and shows them to you along with any custom arguments specified through `EXTRA_OECONF` or `PACKAGECONFIG_CONFARGS`. If applicable, the command also shows you the output of the configure script's "&dash;&dash;help" option as a reference.

### Sharing Files Between Recipes

Recipes often need to use files provided by other recipes on the build host. For example, an application linking to a common library needs access to the library itself and its associated headers. The way this access is accomplished within the extensible SDK is through the sysroot. One sysroot exists per "machine" for which the SDK is being built. In practical terms, this means a sysroot exists for the target machine, and a sysroot exists for the build host.

Recipes should never write files directly into the sysroot. Instead, files should be installed into standard locations during the `do_install` task within the `${` `D` `}` directory. A subset of these files automatically go into the sysroot. The reason for this limitation is that almost all files that go into the sysroot are cataloged in manifests in order to ensure they can be removed later when a recipe is modified or removed. Thus, the sysroot is able to remain free from stale files.

### Packaging

Packaging is not always particularly relevant within the extensible SDK. However, if you examine how build output gets into the final image on the target device, it is important to understand packaging because the contents of the image are expressed in terms of packages and not recipes.

During the `do_package` task, files installed during the `do_install` task are split into one main package, which is almost always named the same as the recipe, and several other packages. This separation is done because not all of those installed files are always useful in every image. For example, you probably do not need any of the documentation installed in a production image. Consequently, for each recipe the documentation files are separated into a `-doc` package. Recipes that package software that has optional modules or plugins might do additional package splitting as well.

After building a recipe you can see where files have gone by looking in the `oe-workdir/packages-split` directory, which contains a subdirectory for each package. Apart from some advanced cases, the `PACKAGES` and `FILES` variables controls splitting. The `PACKAGES` variable lists all of the packages to be produced, while the `FILES` variable specifies which files to include in each package, using an override to specify the package. For example, `FILES_${PN}` specifies the files to go into the main package (i.e. the main package is named the same as the recipe and `${` `PN` `}` evaluates to the recipe name). The order of the `PACKAGES` value is significant. For each installed file, the first package whose `FILES` value matches the file is the package into which the file goes. Defaults exist for both the `PACKAGES` and `FILES` variables. Consequently, you might find you do not even need to set these variables in your recipe unless the software the recipe is building installs files into non-standard locations.

## Restoring the Target Device to its Original State

If you use the `devtool deploy-target` command to write a recipe's build output to the target, and you are working on an existing component of the system, then you might find yourself in a situation where you need to restore the original files that existed prior to running the `devtool deploy-target` command. Because the `devtool deploy-target` command backs up any files it overwrites, you can use the `devtool undeploy-target` to restore those files and remove any other files the recipe deployed. Consider the following example:

         $ devtool undeploy-target lighttpd root@192.168.7.2

If you have deployed multiple applications, you can remove them all at once thus restoring the target device back to its original state:

         $ devtool undeploy-target -a root@192.168.7.2

Information about files deployed to the target as well as any backed up files are stored on the target itself. This storage of course requires some additional space on the target machine.

The `devtool deploy-target` and `devtool undeploy-target` command do not currently interact with any package management system on the target device (e.g. RPM or OPKG). Consequently, you should not intermingle operations `devtool deploy-target` and the package manager operations on the target device. Doing so could result in a conflicting set of files.

## Installing Additional Items Into the Extensible SDK

The extensible SDK typically only comes with a small number of tools and libraries out of the box. If you have a minimal SDK, then it starts mostly empty and is populated on-demand. However, sometimes you will need to explicitly install extra items into the SDK. If you need these extra items, you can first search for the items using the `devtool search` command. For example, suppose you need to link to libGL but you are not sure which recipe provides it. You can use the following command to find out:

         $ devtool search libGL
         mesa                  A free implementation of the OpenGL API

Once you know the recipe (i.e. `mesa` in this example), you can install it:

         $ devtool sdk-install mesa

By default, the `devtool sdk-install` assumes the item is available in pre-built form from your SDK provider. If the item is not available and it is acceptable to build the item from source, you can add the "-s" option as follows:

         $ devtool sdk-install -s mesa

It is important to remember that building the item from source takes significantly longer than installing the pre-built artifact. Also, if no recipe exists for the item you want to add to the SDK, you must instead add it using the `devtool add` command.

## Updating the Extensible SDK

If you are working with an extensible SDK that gets occasionally updated (e.g. typically when that SDK has been provided to you by another party), then you will need to manually pull down those updates to your installed SDK.

To update your installed SDK, run the following:

         $ devtool sdk-update

The previous command assumes your SDK provider has set the default update URL for you. If that URL has not been set, you need to specify it yourself as follows:

         $ devtool sdk-update path_to_update_directory

The URL needs to point specifically to a published SDK and not an SDK installer that you would download and install.

## Creating a Derivative SDK With Additional Components

You might need to produce an SDK that contains your own custom libraries for sending to a third party (e.g., if you are a vendor with customers needing to build their own software for the target platform). If that is the case, then you can produce a derivative SDK based on the currently installed SDK fairly easily. Use these steps:

1. If necessary, install an extensible SDK that you want to use as a base for your derivative SDK.

2. Source the environment script for the SDK.

3. Add the extra libraries or other components you want by using the `devtool add` command.

4. Run the `devtool build-sdk` command.

The above procedure takes the recipes added to the workspace and constructs a new SDK installer containing those recipes and the resulting binary artifacts. The recipes go into their own separate layer in the constructed derivative SDK, leaving the workspace clean and ready for users to add their own recipes.