6 Hello World Example

6.1 BitBake Hello World

The simplest example commonly used to demonstrate any new programming language or tool is the “Hello World” example. This appendix demonstrates, in tutorial form, Hello World within the context of BitBake. The tutorial describes how to create a new project and the applicable metadata files necessary to allow BitBake to build it.

6.2 Obtaining BitBake

See the Obtaining BitBake section for information on how to obtain BitBake. Once you have the source code on your machine, the BitBake directory appears as follows:

$ ls -al
total 100
drwxrwxr-x. 9 wmat wmat  4096 Jan 31 13:44 .
drwxrwxr-x. 3 wmat wmat  4096 Feb  4 10:45 ..
-rw-rw-r--. 1 wmat wmat   365 Nov 26 04:55 AUTHORS
drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 bin
drwxrwxr-x. 4 wmat wmat  4096 Jan 31 13:44 build
-rw-rw-r--. 1 wmat wmat 16501 Nov 26 04:55 ChangeLog
drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 classes
drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 conf
drwxrwxr-x. 3 wmat wmat  4096 Nov 26 04:55 contrib
-rw-rw-r--. 1 wmat wmat 17987 Nov 26 04:55 COPYING
drwxrwxr-x. 3 wmat wmat  4096 Nov 26 04:55 doc
-rw-rw-r--. 1 wmat wmat    69 Nov 26 04:55 .gitignore
-rw-rw-r--. 1 wmat wmat   849 Nov 26 04:55 HEADER
drwxrwxr-x. 5 wmat wmat  4096 Jan 31 13:44 lib
-rw-rw-r--. 1 wmat wmat   195 Nov 26 04:55 MANIFEST.in
-rw-rw-r--. 1 wmat wmat  2887 Nov 26 04:55 TODO

At this point, you should have BitBake cloned to a directory that matches the previous listing except for dates and user names.

6.3 Setting Up the BitBake Environment

First, you need to be sure that you can run BitBake. Set your working directory to where your local BitBake files are and run the following command:

$ ./bin/bitbake --version
BitBake Build Tool Core version 1.23.0, bitbake version 1.23.0

The console output tells you what version you are running.

The recommended method to run BitBake is from a directory of your choice. To be able to run BitBake from any directory, you need to add the executable binary to your binary to your shell’s environment PATH variable. First, look at your current PATH variable by entering the following:

$ echo $PATH

Next, add the directory location for the BitBake binary to the PATH. Here is an example that adds the /home/scott-lenovo/bitbake/bin directory to the front of the PATH variable:

$ export PATH=/home/scott-lenovo/bitbake/bin:$PATH

You should now be able to enter the bitbake command from the command line while working from any directory.

6.4 The Hello World Example

The overall goal of this exercise is to build a complete “Hello World” example utilizing task and layer concepts. Because this is how modern projects such as OpenEmbedded and the Yocto Project utilize BitBake, the example provides an excellent starting point for understanding BitBake.

To help you understand how to use BitBake to build targets, the example starts with nothing but the bitbake command, which causes BitBake to fail and report problems. The example progresses by adding pieces to the build to eventually conclude with a working, minimal “Hello World” example.

While every attempt is made to explain what is happening during the example, the descriptions cannot cover everything. You can find further information throughout this manual. Also, you can actively participate in the https://lists.openembedded.org/g/bitbake-devel discussion mailing list about the BitBake build tool.

Note

This example was inspired by and drew heavily from Mailing List post - The BitBake equivalent of “Hello, World!”.

As stated earlier, the goal of this example is to eventually compile “Hello World”. However, it is unknown what BitBake needs and what you have to provide in order to achieve that goal. Recall that BitBake utilizes three types of metadata files: Configuration Files, Classes, and Recipes. But where do they go? How does BitBake find them? BitBake’s error messaging helps you answer these types of questions and helps you better understand exactly what is going on.

Following is the complete “Hello World” example.

  1. Create a Project Directory: First, set up a directory for the “Hello World” project. Here is how you can do so in your home directory:

    $ mkdir ~/hello
    $ cd ~/hello
    

    This is the directory that BitBake will use to do all of its work. You can use this directory to keep all the metafiles needed by BitBake. Having a project directory is a good way to isolate your project.

  2. Run BitBake: At this point, you have nothing but a project directory. Run the bitbake command and see what it does:

    $ bitbake
    The BBPATH variable is not set and bitbake did not
    find a conf/bblayers.conf file in the expected location.
    Maybe you accidentally invoked bitbake from the wrong directory?
    DEBUG: Removed the following variables from the environment:
    GNOME_DESKTOP_SESSION_ID, XDG_CURRENT_DESKTOP,
    GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG, no_proxy,
    XDG_SESSION_PATH, XAUTHORITY, SESSION_MANAGER, SHLVL,
    MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, WINDOWID, EDITOR,
    GPG_AGENT_INFO, SSH_AUTH_SOCK, GDMSESSION, GNOME_KEYRING_PID,
    XDG_SEAT_PATH, XDG_CONFIG_DIRS, LESSOPEN, DBUS_SESSION_BUS_ADDRESS,
    _, XDG_SESSION_COOKIE, DESKTOP_SESSION, LESSCLOSE, DEFAULTS_PATH,
    UBUNTU_MENUPROXY, OLDPWD, XDG_DATA_DIRS, COLORTERM, LS_COLORS
    

    The majority of this output is specific to environment variables that are not directly relevant to BitBake. However, the very first message regarding the BBPATH variable and the conf/bblayers.conf file is relevant.

    When you run BitBake, it begins looking for metadata files. The BBPATH variable is what tells BitBake where to look for those files. BBPATH is not set and you need to set it. Without BBPATH, BitBake cannot find any configuration files (.conf) or recipe files (.bb) at all. BitBake also cannot find the bitbake.conf file.

  3. Setting BBPATH: For this example, you can set BBPATH in the same manner that you set PATH earlier in the appendix. You should realize, though, that it is much more flexible to set the BBPATH variable up in a configuration file for each project.

    From your shell, enter the following commands to set and export the BBPATH variable:

    $ BBPATH="projectdirectory"
    $ export BBPATH
    

    Use your actual project directory in the command. BitBake uses that directory to find the metadata it needs for your project.

    Note

    When specifying your project directory, do not use the tilde (“~”) character as BitBake does not expand that character as the shell would.

  4. Run BitBake: Now that you have BBPATH defined, run the bitbake command again:

    $ bitbake
    ERROR: Traceback (most recent call last):
      File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 163, in wrapped
        return func(fn, *args)
      File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 173, in parse_config_file
        return bb.parse.handle(fn, data, include)
      File "/home/scott-lenovo/bitbake/lib/bb/parse/__init__.py", line 99, in handle
        return h['handle'](fn, data, include)
      File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/ConfHandler.py", line 120, in handle
        abs_fn = resolve_file(fn, data)
      File "/home/scott-lenovo/bitbake/lib/bb/parse/__init__.py", line 117, in resolve_file
        raise IOError("file %s not found in %s" % (fn, bbpath))
    IOError: file conf/bitbake.conf not found in /home/scott-lenovo/hello
    
    ERROR: Unable to parse conf/bitbake.conf: file conf/bitbake.conf not found in /home/scott-lenovo/hello
    

    This sample output shows that BitBake could not find the conf/bitbake.conf file in the project directory. This file is the first thing BitBake must find in order to build a target. And, since the project directory for this example is empty, you need to provide a conf/bitbake.conf file.

  5. Creating conf/bitbake.conf: The conf/bitbake.conf includes a number of configuration variables BitBake uses for metadata and recipe files. For this example, you need to create the file in your project directory and define some key BitBake variables. For more information on the bitbake.conf file, see http://git.openembedded.org/bitbake/tree/conf/bitbake.conf.

    Use the following commands to create the conf directory in the project directory:

    $ mkdir conf
    

    From within the conf directory, use some editor to create the bitbake.conf so that it contains the following:

    PN  = "${@bb.parse.vars_from_file(d.getVar('FILE', False),d)[0] or 'defaultpkgname'}"
    
    TMPDIR  = "${TOPDIR}/tmp"
    CACHE   = "${TMPDIR}/cache"
    STAMP   = "${TMPDIR}/${PN}/stamps"
    T       = "${TMPDIR}/${PN}/work"
    B       = "${TMPDIR}/${PN}"
    

    Note

    Without a value for PN , the variables STAMP , T , and B , prevent more than one recipe from working. You can fix this by either setting PN to have a value similar to what OpenEmbedded and BitBake use in the default bitbake.conf file (see previous example). Or, by manually updating each recipe to set PN . You will also need to include PN as part of the STAMP , T , and B variable definitions in the local.conf file.

    The TMPDIR variable establishes a directory that BitBake uses for build output and intermediate files other than the cached information used by the Setscene process. Here, the TMPDIR directory is set to hello/tmp.

    Tip

    You can always safely delete the tmp directory in order to rebuild a BitBake target. The build process creates the directory for you when you run BitBake.

    For information about each of the other variables defined in this example, check PN, TOPDIR, CACHE, STAMP, T or B to take you to the definitions in the glossary.

  6. Run BitBake: After making sure that the conf/bitbake.conf file exists, you can run the bitbake command again:

    $ bitbake
    ERROR: Traceback (most recent call last):
      File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 163, in wrapped
        return func(fn, *args)
      File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 177, in _inherit
        bb.parse.BBHandler.inherit(bbclass, "configuration INHERITs", 0, data)
      File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/BBHandler.py", line 92, in inherit
        include(fn, file, lineno, d, "inherit")
      File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/ConfHandler.py", line 100, in include
        raise ParseError("Could not %(error_out)s file %(fn)s" % vars(), oldfn, lineno)
    ParseError: ParseError in configuration INHERITs: Could not inherit file classes/base.bbclass
    
    ERROR: Unable to parse base: ParseError in configuration INHERITs: Could not inherit file classes/base.bbclass
    

    In the sample output, BitBake could not find the classes/base.bbclass file. You need to create that file next.

  7. Creating classes/base.bbclass: BitBake uses class files to provide common code and functionality. The minimally required class for BitBake is the classes/base.bbclass file. The base class is implicitly inherited by every recipe. BitBake looks for the class in the classes directory of the project (i.e hello/classes in this example).

    Create the classes directory as follows:

    $ cd $HOME/hello
    $ mkdir classes
    

    Move to the classes directory and then create the base.bbclass file by inserting this single line: addtask build The minimal task that BitBake runs is the do_build task. This is all the example needs in order to build the project. Of course, the base.bbclass can have much more depending on which build environments BitBake is supporting.

  8. Run BitBake: After making sure that the classes/base.bbclass file exists, you can run the bitbake command again:

    $ bitbake
    Nothing to do. Use 'bitbake world' to build everything, or run 'bitbake --help' for usage information.
    

    BitBake is finally reporting no errors. However, you can see that it really does not have anything to do. You need to create a recipe that gives BitBake something to do.

  9. Creating a Layer: While it is not really necessary for such a small example, it is good practice to create a layer in which to keep your code separate from the general metadata used by BitBake. Thus, this example creates and uses a layer called “mylayer”.

    Note

    You can find additional information on layers in the “Layers” section.

    Minimally, you need a recipe file and a layer configuration file in your layer. The configuration file needs to be in the conf directory inside the layer. Use these commands to set up the layer and the conf directory:

    $ cd $HOME
    $ mkdir mylayer
    $ cd mylayer
    $ mkdir conf
    

    Move to the conf directory and create a layer.conf file that has the following:

    BBPATH .= ":${LAYERDIR}"
    BBFILES += "${LAYERDIR}/*.bb"
    BBFILE_COLLECTIONS += "mylayer"
    BBFILE_PATTERN_mylayer := "^${LAYERDIR_RE}/"
    

    For information on these variables, click on BBFILES, LAYERDIR, BBFILE_COLLECTIONS or BBFILE_PATTERN_mylayer to go to the definitions in the glossary.

    You need to create the recipe file next. Inside your layer at the top-level, use an editor and create a recipe file named printhello.bb that has the following:

    DESCRIPTION = "Prints Hello World"
    PN = 'printhello'
    PV = '1'
    
    python do_build() {
       bb.plain("********************");
       bb.plain("*                  *");
       bb.plain("*  Hello, World!   *");
       bb.plain("*                  *");
       bb.plain("********************");
    }
    

    The recipe file simply provides a description of the recipe, the name, version, and the do_build task, which prints out “Hello World” to the console. For more information on DESCRIPTION, PN or PV follow the links to the glossary.

  10. Run BitBake With a Target: Now that a BitBake target exists, run

    the command and provide that target:

    $ cd $HOME/hello
    $ bitbake printhello
    ERROR: no recipe files to build, check your BBPATH and BBFILES?
    
    Summary: There was 1 ERROR message shown, returning a non-zero exit code.
    

    We have created the layer with the recipe and the layer configuration file but it still seems that BitBake cannot find the recipe. BitBake needs a conf/bblayers.conf that lists the layers for the project. Without this file, BitBake cannot find the recipe.

  11. Creating conf/bblayers.conf: BitBake uses the

    conf/bblayers.conf file to locate layers needed for the project. This file must reside in the conf directory of the project (i.e. hello/conf for this example).

    Set your working directory to the hello/conf directory and then create the bblayers.conf file so that it contains the following:

    BBLAYERS ?= " \
        /home/<you>/mylayer \
    "
    

    You need to provide your own information for you in the file.

  12. Run BitBake With a Target: Now that you have supplied the

    bblayers.conf file, run the bitbake command and provide the target:

    $ bitbake printhello
    Parsing recipes: 100% |##################################################################################|
    Time: 00:00:00
    Parsing of 1 .bb files complete (0 cached, 1 parsed). 1 targets, 0 skipped, 0 masked, 0 errors.
    NOTE: Resolving any missing task queue dependencies
    NOTE: Preparing RunQueue
    NOTE: Executing RunQueue Tasks
    ********************
    *                  *
    *  Hello, World!   *
    *                  *
    ********************
    NOTE: Tasks Summary: Attempted 1 tasks of which 0 didn't need to be rerun and all succeeded.
    

    Note

    After the first execution, re-running bitbake printhello again will not result in a BitBake run that prints the same console output. The reason for this is that the first time the printhello.bb recipe’s do_build task executes successfully, BitBake writes a stamp file for the task. Thus, the next time you attempt to run the task using that same bitbake command, BitBake notices the stamp and therefore determines that the task does not need to be re-run. If you delete the tmp directory or run bitbake -c clean printhello and then re-run the build, the “Hello, World!” message will be printed again.