# Using the Yocto Project

This chapter describes common usage for the Yocto Project. The information is introductory in nature as other manuals in the Yocto Project documentation set provide more details on how to use the Yocto Project.

## Running a Build

This section provides a summary of the build process and provides information for less obvious aspects of the build process. For general information on how to build an image using the OpenEmbedded build system, see the "Building an Image" section of the Yocto Project Quick Start.

### Build Overview

The first thing you need to do is set up the OpenEmbedded build environment by sourcing an environment setup script (i.e. `&OE_INIT_FILE;` or `oe-init-build-env-memres`). Here is an example:

         $ source &OE_INIT_FILE; [build_dir]

The build_dir argument is optional and specifies the directory the OpenEmbedded build system uses for the build - the Build Directory. If you do not specify a Build Directory, it defaults to a directory named `build` in your current working directory. A common practice is to use a different Build Directory for different targets. For example, `~/build/x86` for a `qemux86` target, and `~/build/arm` for a `qemuarm` target.

Once the build environment is set up, you can build a target using:

         $ bitbake target

The target is the name of the recipe you want to build. Common targets are the images in `meta/recipes-core/images`, `meta/recipes-sato/images`, etc. all found in the Source Directory. Or, the target can be the name of a recipe for a specific piece of software such as BusyBox. For more details about the images the OpenEmbedded build system supports, see the "Images" chapter.

Building an image without GNU General Public License Version 3 (GPLv3), or similarly licensed, components is supported for only minimal and base images. See the "Images" chapter for more information.

### Building an Image Using GPL Components

When building an image using GPL components, you need to maintain your original settings and not switch back and forth applying different versions of the GNU General Public License. If you rebuild using different versions of GPL, dependency errors might occur due to some components not being rebuilt.

## Installing and Using the Result

Once an image has been built, it often needs to be installed. The images and kernels built by the OpenEmbedded build system are placed in the Build Directory in `tmp/deploy/images`. For information on how to run pre-built images such as `qemux86` and `qemuarm`, see the "Using Pre-Built Binaries and QEMU" section in the Yocto Project Quick Start. For information about how to install these images, see the documentation for your particular board or machine.

## Debugging Build Failures

The exact method for debugging build failures depends on the nature of the problem and on the system's area from which the bug originates. Standard debugging practices such as comparison against the last known working version with examination of the changes and the re-application of steps to identify the one causing the problem are valid for the Yocto Project just as they are for any other system. Even though it is impossible to detail every possible potential failure, this section provides some general tips to aid in debugging.

A useful feature for debugging is the error reporting tool. Configuring the Yocto Project to use this tool causes the OpenEmbedded build system to produce error reporting commands as part of the console output. You can enter the commands after the build completes to log error information into a common database, that can help you figure out what might be going wrong. For information on how to enable and use this feature, see the "Using the Error Reporting Tool" section in the Yocto Project Development Manual.

For discussions on debugging, see the "Debugging With the GNU Project Debugger (GDB) Remotely" and "Working within Eclipse" sections in the Yocto Project Development Manual.

The remainder of this section presents many examples of the `bitbake` command. You can learn about BitBake by reading the BitBake User Manual.

### Task Failures

The log file for shell tasks is available in `${WORKDIR}/temp/log.do_taskname.pid`. For example, the `do_compile` task for the QEMU minimal image for the x86 machine (`qemux86`) might be `tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/temp/log.do_compile.20830`. To see what BitBake runs to generate that log, look at the corresponding `run.do_taskname.pid` file located in the same directory.

Presently, the output from Python tasks is sent directly to the console.

### Running Specific Tasks

Any given package consists of a set of tasks. The standard BitBake behavior in most cases is: `do_fetch`, `do_unpack`, `do_patch`, `do_configure`, `do_compile`, `do_install`, `do_package`, `do_package_write_*`, and `do_build`. The default task is `do_build` and any tasks on which it depends build first. Some tasks, such as `do_devshell`, are not part of the default build chain. If you wish to run a task that is not part of the default build chain, you can use the `-c` option in BitBake. Here is an example:

         $ bitbake matchbox-desktop -c devshell

If you wish to rerun a task, use the `-f` force option. For example, the following sequence forces recompilation after changing files in the work directory.

         $ bitbake matchbox-desktop
                   .
                   .
            make some changes to the source code in the work directory
                   .
                   .
         $ bitbake matchbox-desktop -c compile -f
         $ bitbake matchbox-desktop

This sequence first builds and then recompiles `matchbox-desktop`. The last command reruns all tasks (basically the packaging tasks) after the compile. BitBake recognizes that the `do_compile` task was rerun and therefore understands that the other tasks also need to be run again.

You can view a list of tasks in a given package by running the `do_listtasks` task as follows:

         $ bitbake matchbox-desktop -c listtasks

The results appear as output to the console and are also in the file `${WORKDIR}/temp/log.do_listtasks`.

### Dependency Graphs

Sometimes it can be hard to see why BitBake wants to build other packages before building a given package you have specified. The `bitbake -g targetname` command creates the `pn-buildlist`, `pn-depends.dot`, `package-depends.dot`, and `task-depends.dot` files in the current directory. These files show what will be built and the package and task dependencies, which are useful for debugging problems. You can use the `bitbake -g -u depexp targetname` command to display the results in a more human-readable form.

### General BitBake Problems

You can see debug output from BitBake by using the `-D` option. The debug output gives more information about what BitBake is doing and the reason behind it. Each `-D` option you use increases the logging level. The most common usage is `-DDD`.

The output from `bitbake -DDD -v` targetname can reveal why BitBake chose a certain version of a package or why BitBake picked a certain provider. This command could also help you in a situation where you think BitBake did something unexpected.

### Development Host System Issues

Sometimes issues on the host development system can cause your build to fail. Following are known, host-specific problems. Be sure to always consult the Release Notes for a look at all release-related issues.

- `eglibc-initial` fails to build: If your development host system has the unpatched `GNU Make 3.82`, the `do_install` task fails for `eglibc-initial` during the build.

  Typically, every distribution that ships `GNU Make 3.82` as the default already has the patched version. However, some distributions, such as Debian, have `GNU Make 3.82` as an option, which is unpatched. You will see this error on these types of distributions. Switch to `GNU Make 3.81` or patch your `make` to solve the problem.

### Building with No Dependencies

To build a specific recipe (`.bb` file), you can use the following command form:

         $ bitbake -b somepath/somerecipe.bb

This command form does not check for dependencies. Consequently, you should use it only when you know existing dependencies have been met.

You can also specify fragments of the filename. In this case, BitBake checks for a unique match.

### Variables

You can use the `-e` BitBake option to display the parsing environment for a configuration. The following displays the general parsing environment:

         $ bitbake -e

This next example shows the parsing environment for a specific recipe:

         $ bitbake -e recipename

### Recipe Logging Mechanisms

Best practices exist while writing recipes that both log build progress and act on build conditions such as warnings and errors. Both Python and Bash language bindings exist for the logging mechanism:

- Python: For Python functions, BitBake supports several loglevels: `bb.fatal`, `bb.error`, `bb.warn`, `bb.note`, `bb.plain`, and `bb.debug`.

- Bash: For Bash functions, the same set of loglevels exist and are accessed with a similar syntax: `bbfatal`, `bberror`, `bbwarn`, `bbnote`, `bbplain`, and `bbdebug`.

For guidance on how logging is handled in both Python and Bash recipes, see the `logging.bbclass` file in the `meta/classes` folder of the Source Directory.

#### Logging With Python

When creating recipes using Python and inserting code that handles build logs, keep in mind the goal is to have informative logs while keeping the console as "silent" as possible. Also, if you want status messages in the log, use the "debug" loglevel.

Following is an example written in Python. The code handles logging for a function that determines the number of tasks needed to be run. See the "`do_listtasks`" section for additional information:

         python do_listtasks() {
             bb.debug(2, "Starting to figure out the task list")
             if noteworthy_condition:
                 bb.note("There are 47 tasks to run")
             bb.debug(2, "Got to point xyz")
             if warning_trigger:
                 bb.warn("Detected warning_trigger, this might be a problem later.")
             if recoverable_error:
                 bb.error("Hit recoverable_error, you really need to fix this!")
             if fatal_error:
                 bb.fatal("fatal_error detected, unable to print the task list")
             bb.plain("The tasks present are abc")
             bb.debug(2, "Finished figuring out the tasklist")
         }

#### Logging With Bash

When creating recipes using Bash and inserting code that handles build logs, you have the same goals - informative with minimal console output. The syntax you use for recipes written in Bash is similar to that of recipes written in Python described in the previous section.

Following is an example written in Bash. The code logs the progress of the `do_my_function` function.

         do_my_function() {
             bbdebug 2 "Running do_my_function"
             if [ exceptional_condition ]; then
                 bbnote "Hit exceptional_condition"
             fi
             bbdebug 2  "Got to point xyz"
             if [ warning_trigger ]; then
                 bbwarn "Detected warning_trigger, this might cause a problem later."
             fi
             if [ recoverable_error ]; then
                 bberror "Hit recoverable_error, correcting"
             fi
             if [ fatal_error ]; then
                 bbfatal "fatal_error detected"
             fi
             bbdebug 2 "Completed do_my_function"
         }

### Other Tips

Here are some other tips that you might find useful:

- When adding new packages, it is worth watching for undesirable items making their way into compiler command lines. For example, you do not want references to local system files like `/usr/lib/` or `/usr/include/`.

- If you want to remove the `psplash` boot splashscreen, add `psplash=false` to the kernel command line. Doing so prevents `psplash` from loading and thus allows you to see the console. It is also possible to switch out of the splashscreen by switching the virtual console (e.g. Fn+Left or Fn+Right on a Zaurus).

## Maintaining Build Output Quality

Many factors can influence the quality of a build. For example, if you upgrade a recipe to use a new version of an upstream software package or you experiment with some new configuration options, subtle changes can occur that you might not detect until later. Consider the case where your recipe is using a newer version of an upstream package. In this case, a new version of a piece of software might introduce an optional dependency on another library, which is auto-detected. If that library has already been built when the software is building, the software will link to the built library and that library will be pulled into your image along with the new software even if you did not want the library.

The `buildhistory` class exists to help you maintain the quality of your build output. You can use the class to highlight unexpected and possibly unwanted changes in the build output. When you enable build history, it records information about the contents of each package and image and then commits that information to a local Git repository where you can examine the information.

The remainder of this section describes the following:

- How you can enable and disable build history

- How to understand what the build history contains

- How to limit the information used for build history

- How to examine the build history from both a command-line and web interface

### Enabling and Disabling Build History

Build history is disabled by default. To enable it, add the following `INHERIT` statement and set the `BUILDHISTORY_COMMIT` variable to "1" at the end of your `conf/local.conf` file found in the Build Directory:

         INHERIT += "buildhistory"
         BUILDHISTORY_COMMIT = "1"

Enabling build history as previously described causes the build process to collect build output information and commit it to a local Git repository.

Enabling build history increases your build times slightly, particularly for images, and increases the amount of disk space used during the build.

You can disable build history by removing the previous statements from your `conf/local.conf` file.

### Understanding What the Build History Contains

Build history information is kept in `${``TOPDIR``}/buildhistory` in the Build Directory as defined by the `BUILDHISTORY_DIR` variable. The following is an example abbreviated listing:

At the top level, there is a `metadata-revs` file that lists the revisions of the repositories for the layers enabled when the build was produced. The rest of the data splits into separate `packages`, `images` and `sdk` directories, the contents of which are described below.

#### Build History Package Information

The history for each package contains a text file that has name-value pairs with information about the package. For example, `buildhistory/packages/core2-poky-linux/busybox/busybox/latest` contains the following:

         PV = 1.19.3
         PR = r3
         RDEPENDS = update-rc.d eglibc (>= 2.13)
         RRECOMMENDS = busybox-syslog busybox-udhcpc
         PKGSIZE = 564701
         FILES = /usr/bin/* /usr/sbin/* /usr/libexec/* /usr/lib/lib*.so.* \
            /etc /com /var /bin/* /sbin/* /lib/*.so.* /usr/share/busybox \
            /usr/lib/busybox/* /usr/share/pixmaps /usr/share/applications \
            /usr/share/idl /usr/share/omf /usr/share/sounds /usr/lib/bonobo/servers
         FILELIST = /etc/busybox.links /etc/init.d/hwclock.sh /bin/busybox /bin/sh

Most of these name-value pairs correspond to variables used to produce the package. The exceptions are `FILELIST`, which is the actual list of files in the package, and `PKGSIZE`, which is the total size of files in the package in bytes.

There is also a file corresponding to the recipe from which the package came (e.g. `buildhistory/packages/core2-poky-linux/busybox/latest`):

         PV = 1.19.3
         PR = r3
         DEPENDS = virtual/i586-poky-linux-gcc virtual/i586-poky-linux-compilerlibs \
            virtual/libc update-rc.d-native
         PACKAGES = busybox-httpd busybox-udhcpd busybox-udhcpc busybox-syslog \
            busybox-mdev busybox-dbg busybox busybox-doc busybox-dev \
            busybox-staticdev busybox-locale

Finally, for those recipes fetched from a version control system (e.g., Git), a file exists that lists source revisions that are specified in the recipe and lists the actual revisions used during the build. Listed and actual revisions might differ when `SRCREV` is set to `${AUTOREV}`. Here is an example assuming `buildhistory/packages/emenlow-poky-linux/linux-yocto/latest_srcrev`):

         # SRCREV_machine = "b5c37fe6e24eec194bb29d22fdd55d73bcc709bf"
         SRCREV_machine = "b5c37fe6e24eec194bb29d22fdd55d73bcc709bf"
         # SRCREV_emgd = "caea08c988e0f41103bbe18eafca20348f95da02"
         SRCREV_emgd = "caea08c988e0f41103bbe18eafca20348f95da02"
         # SRCREV_meta = "c2ed0f16fdec628242a682897d5d86df4547cf24"
         SRCREV_meta = "c2ed0f16fdec628242a682897d5d86df4547cf24"

You can use the `buildhistory-collect-srcrevs` command to collect the stored `SRCREV` values from build history and report them in a format suitable for use in global configuration (e.g., `local.conf` or a distro include file) to override floating `AUTOREV` values to a fixed set of revisions. Here is some example output from this command:

         # emenlow-poky-linux
         SRCREV_machine_pn-linux-yocto = "b5c37fe6e24eec194bb29d22fdd55d73bcc709bf"
         SRCREV_emgd_pn-linux-yocto = "caea08c988e0f41103bbe18eafca20348f95da02"
         SRCREV_meta_pn-linux-yocto = "c2ed0f16fdec628242a682897d5d86df4547cf24"
         # core2-poky-linux
         SRCREV_pn-kmod = "62081c0f68905b22f375156d4532fd37fa5c8d33"
         SRCREV_pn-blktrace = "d6918c8832793b4205ed3bfede78c2f915c23385"
         SRCREV_pn-opkg = "649"

Here are some notes on using the `buildhistory-collect-srcrevs` command:

- By default, only values where the `SRCREV` was not hardcoded (usually when `AUTOREV` was used) are reported. Use the `-a` option to see all `SRCREV` values.

- The output statements might not have any effect if overrides are applied elsewhere in the build system configuration. Use the `-f` option to add the `forcevariable` override to each output line if you need to work around this restriction.

- The script does apply special handling when building for multiple machines. However, the script does place a comment before each set of values that specifies which triplet to which they belong as shown above (e.g., `emenlow-poky-linux`).

#### Build History Image Information

The files produced for each image are as follows:

- `image-files:` A directory containing selected files from the root filesystem. The files are defined by `BUILDHISTORY_IMAGE_FILES`.

- `build-id.txt:` Human-readable information about the build configuration and metadata source revisions. This file contains the full build header as printed by BitBake.

- `*.dot:` Dependency graphs for the image that are compatible with `graphviz`.

- `files-in-image.txt:` A list of files in the image with permissions, owner, group, size, and symlink information.

- `image-info.txt:` A text file containing name-value pairs with information about the image. See the following listing example for more information.

- `installed-package-names.txt:` A list of installed packages by name only.

- `installed-package-sizes.txt:` A list of installed packages ordered by size.

- `installed-packages.txt:` A list of installed packages with full package filenames.

Installed package information is able to be gathered and produced even if package management is disabled for the final image.

Here is an example of `image-info.txt`:

         DISTRO = poky
         DISTRO_VERSION = 1.1+snapshot-20120207
         USER_CLASSES = image-mklibs image-prelink
         IMAGE_CLASSES = image_types
         IMAGE_FEATURES = debug-tweaks x11-base apps-x11-core \
            package-management ssh-server-dropbear package-management
         IMAGE_LINGUAS = en-us en-gb
         IMAGE_INSTALL = packagegroup-core-boot packagegroup-base-extended
         BAD_RECOMMENDATIONS =
         ROOTFS_POSTPROCESS_COMMAND = buildhistory_get_image_installed ;   rootfs_update_timestamp ;
         IMAGE_POSTPROCESS_COMMAND = buildhistory_get_imageinfo ;
         IMAGESIZE = 171816

Other than `IMAGESIZE`, which is the total size of the files in the image in Kbytes, the name-value pairs are variables that may have influenced the content of the image. This information is often useful when you are trying to determine why a change in the package or file listings has occurred.

#### Using Build History to Gather Image Information Only

As you can see, build history produces image information, including dependency graphs, so you can see why something was pulled into the image. If you are just interested in this information and not interested in collecting specific package or SDK information, you can enable writing only image information without any history by adding the following to your `conf/local.conf` file found in the Build Directory:

         INHERIT += "buildhistory"
         BUILDHISTORY_COMMIT = "0"
         BUILDHISTORY_FEATURES = "image"

Here, you set the `BUILDHISTORY_FEATURES` variable to use the image feature only.

#### Build History SDK Information

Build history collects similar information on the contents of SDKs (e.g. `meta-toolchain` or `bitbake -c populate_sdk imagename`) as compared to information it collects for images. The following list shows the files produced for each SDK:

- `files-in-sdk.txt:` A list of files in the SDK with permissions, owner, group, size, and symlink information. This list includes both the host and target parts of the SDK.

- `sdk-info.txt:` A text file containing name-value pairs with information about the SDK. See the following listing example for more information.

- The following information appears under each of the `host` and `target` directories for the portions of the SDK that run on the host and on the target, respectively:

  - `depends.dot:` Dependency graph for the SDK that is compatible with `graphviz`.

  - `installed-package-names.txt:` A list of installed packages by name only.

  - `installed-package-sizes.txt:` A list of installed packages ordered by size.

  - `installed-packages.txt:` A list of installed packages with full package filenames.

Here is an example of `sdk-info.txt`:

         DISTRO = poky
         DISTRO_VERSION = 1.3+snapshot-20130327
         SDK_NAME = poky-eglibc-i686-arm
         SDK_VERSION = 1.3+snapshot
         SDKMACHINE =
         SDKIMAGE_FEATURES = dev-pkgs dbg-pkgs
         BAD_RECOMMENDATIONS =
         SDKSIZE = 352712

Other than `SDKSIZE`, which is the total size of the files in the SDK in Kbytes, the name-value pairs are variables that might have influenced the content of the SDK. This information is often useful when you are trying to determine why a change in the package or file listings has occurred.

#### Examining Build History Information

You can examine build history output from the command line or from a web interface.

To see any changes that have occurred (assuming you have `BUILDHISTORY_COMMIT = "1"`), you can simply use any Git command that allows you to view the history of a repository. Here is one method:

          $ git log -p

You need to realize, however, that this method does show changes that are not significant (e.g. a package's size changing by a few bytes).

A command-line tool called `buildhistory-diff` does exist, though, that queries the Git repository and prints just the differences that might be significant in human-readable form. Here is an example:

         $ ~/poky/poky/scripts/buildhistory-diff . HEAD^
         Changes to images/qemux86_64/eglibc/core-image-minimal (files-in-image.txt):
            /etc/anotherpkg.conf was added
            /sbin/anotherpkg was added
            * (installed-package-names.txt):
            *   anotherpkg was added
         Changes to images/qemux86_64/eglibc/core-image-minimal (installed-package-names.txt):
            anotherpkg was added
         packages/qemux86_64-poky-linux/v86d: PACKAGES: added "v86d-extras"
            * PR changed from "r0" to "r1"
            * PV changed from "0.1.10" to "0.1.12"
         packages/qemux86_64-poky-linux/v86d/v86d: PKGSIZE changed from 110579 to 144381 (+30%)
            * PR changed from "r0" to "r1"
            * PV changed from "0.1.10" to "0.1.12"

To see changes to the build history using a web interface, follow the instruction in the `README` file here. .

Here is a sample screenshot of the interface:

## Speeding Up the Build

Build time can be an issue. By default, the build system uses three simple controls to try and maximize build efficiency:

- `BB_NUMBER_THREADS`

- `BB_NUMBER_PARSE_THREADS`

- `PARALLEL_MAKE`

These three variables all scale to the number of processor cores available on the build system. This auto-scaling ensures that the build system fundamentally takes advantage of potential parallel operations during the build based on the build machine's capabilities.

If you need to achieve even faster builds than what the build system produces by default, you can consider and implement some of the following:

- `BB_NUMBER_THREADS`, `BB_NUMBER_PARSE_THREADS`, and `PARALLEL_MAKE`: As previously mentioned, the build system scales the values for these variables. However, you can manually override them in your `local.conf` file if you are not satisfied with the defaults.

- File system type: The file system type that the build is being performed on can also influence performance. Using `ext4` is recommended as compared to `ext2` and `ext3` due to `ext4` improved features such as extents.

- Disabling the updating of access time using `noatime`: The `noatime` mount option prevents the build system from updating file and directory access times.

- Setting a longer commit: Using the "commit=" mount option increases the interval in seconds between disk cache writes. Changing this interval from the five second default to something longer increases the risk of data loss but decreases the need to write to the disk, thus increasing the build performance.

- Choosing the packaging backend: Of the available packaging backends, IPK is the fastest. Additionally, selecting a singular packaging backend also helps.

- Using `/tmp` as a temporary file system: While this can help speed up the build, the benefits are limited due to the compiler using `-pipe`. The build system goes to some lengths to avoid `sync()` calls into the file system on the principle that if there was a significant failure, the Build Directory contents could easily be rebuilt.

- Inheriting the `rm_work` class: Inheriting this class has shown to speed up builds due to significantly lower amounts of data stored in the data cache as well as on disk. Inheriting this class also makes cleanup of `TMPDIR` faster, at the expense of being easily able to dive into the source code. File system maintainers have recommended that the fastest way to clean up large numbers of files is to reformat partitions rather than delete files due to the linear nature of partitions. This, of course, assumes you structure the disk partitions and file systems in a way that this is practical.

Aside from the previous list, you should keep some trade offs in mind that can help you speed up the build:

- Exclude debug symbols and other debug information: If you do not need these symbols and other debug information, disabling the `*-dbg` package generation can speed up the build. You can disable this generation by setting the `INHIBIT_PACKAGE_DEBUG_SPLIT` variable to "1".

- Disable static library generation for recipes derived from `autoconf` or `libtool`: Following is an example showing how to disable static libraries and still provide an override to handle exceptions:

           STATICLIBCONF = "--disable-static"
           STATICLIBCONF_sqlite3-native = ""
           EXTRA_OECONF += "${STATICLIBCONF}"

  ### Notes

  - Some recipes need static libraries in order to work correctly (e.g. `pseudo-native` needs `sqlite3-native`). Overrides, as in the previous example, account for these kinds of exceptions.

  - Some packages have packaging code that assumes the presence of the static libraries. If so, you might need to exclude them as well.