7 Hello World Example
7.1 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.
7.2 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 108
drwxr-xr-x 9 fawkh 10000 4096 feb 24 12:10 .
drwx------ 36 fawkh 10000 4096 mar 2 17:00 ..
-rw-r--r-- 1 fawkh 10000 365 feb 24 12:10 AUTHORS
drwxr-xr-x 2 fawkh 10000 4096 feb 24 12:10 bin
-rw-r--r-- 1 fawkh 10000 16501 feb 24 12:10 ChangeLog
drwxr-xr-x 2 fawkh 10000 4096 feb 24 12:10 classes
drwxr-xr-x 2 fawkh 10000 4096 feb 24 12:10 conf
drwxr-xr-x 5 fawkh 10000 4096 feb 24 12:10 contrib
drwxr-xr-x 6 fawkh 10000 4096 feb 24 12:10 doc
drwxr-xr-x 8 fawkh 10000 4096 mar 2 16:26 .git
-rw-r--r-- 1 fawkh 10000 31 feb 24 12:10 .gitattributes
-rw-r--r-- 1 fawkh 10000 392 feb 24 12:10 .gitignore
drwxr-xr-x 13 fawkh 10000 4096 feb 24 12:11 lib
-rw-r--r-- 1 fawkh 10000 1224 feb 24 12:10 LICENSE
-rw-r--r-- 1 fawkh 10000 15394 feb 24 12:10 LICENSE.GPL-2.0-only
-rw-r--r-- 1 fawkh 10000 1286 feb 24 12:10 LICENSE.MIT
-rw-r--r-- 1 fawkh 10000 229 feb 24 12:10 MANIFEST.in
-rw-r--r-- 1 fawkh 10000 2413 feb 24 12:10 README
-rw-r--r-- 1 fawkh 10000 43 feb 24 12:10 toaster-requirements.txt
-rw-r--r-- 1 fawkh 10000 2887 feb 24 12:10 TODO
At this point, you should have BitBake cloned to a directory that matches the previous listing except for dates and user names.
7.3 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 2.3.1
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.
7.4 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.
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.
Run BitBake: At this point, you have nothing but a project directory. Run the
bitbake
command and see what it does:$ bitbake ERROR: 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?
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 thebitbake.conf
file.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.
Run BitBake: Now that you have BBPATH defined, run the
bitbake
command again:$ bitbake ERROR: Unable to parse /home/scott-lenovo/bitbake/lib/bb/parse/__init__.py Traceback (most recent call last): File "/home/scott-lenovo/bitbake/lib/bb/parse/__init__.py", line 127, in resolve_file(fn='conf/bitbake.conf', d=<bb.data_smart.DataSmart object at 0x7f22919a3df0>): if not newfn: > raise IOError(errno.ENOENT, "file %s not found in %s" % (fn, bbpath)) fn = newfn FileNotFoundError: [Errno 2] file conf/bitbake.conf not found in <projectdirectory>
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 aconf/bitbake.conf
file.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 thebitbake.conf
file, see https://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 thebitbake.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}/${PN}"
Note
Without a value for PN, the variables STAMP, T, and B, prevent more than one recipe from working. You can fix this by either setting PN to have a value similar to what OpenEmbedded and BitBake use in the default
bitbake.conf
file (see previous example). Or, by manually updating each recipe to set PN. You will also need to include PN as part of the STAMP, T, and B variable definitions in thelocal.conf
file.The
TMPDIR
variable establishes a directory that BitBake uses for build output and intermediate files other than the cached information used by the Setscene process. Here, theTMPDIR
directory is set tohello/tmp
.Tip
You can always safely delete the tmp directory in order to rebuild a BitBake target. The build process creates the directory for you when you run BitBake.
For information about each of the other variables defined in this example, check PN, TOPDIR, CACHE, STAMP, T or B to take you to the definitions in the glossary.
Run BitBake: After making sure that the
conf/bitbake.conf
file exists, you can run thebitbake
command again:$ bitbake ERROR: Unable to parse /home/scott-lenovo/bitbake/lib/bb/parse/parse_py/BBHandler.py Traceback (most recent call last): File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/BBHandler.py", line 67, in inherit(files=['base'], fn='configuration INHERITs', lineno=0, d=<bb.data_smart.DataSmart object at 0x7fab6815edf0>): if not os.path.exists(file): > raise ParseError("Could not inherit file %s" % (file), fn, lineno) bb.parse.ParseError: 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.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. Thebase
class is implicitly inherited by every recipe. BitBake looks for the class in theclasses
directory of the project (i.ehello/classes
in this example).Create the
classes
directory as follows:$ cd $HOME/hello $ mkdir classes
Move to the
classes
directory and then create thebase.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, thebase.bbclass
can have much more depending on which build environments BitBake is supporting.Run BitBake: After making sure that the
classes/base.bbclass
file exists, you can run thebitbake
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.
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 theconf
directory:$ cd $HOME $ mkdir mylayer $ cd mylayer $ mkdir conf
Move to the
conf
directory and create alayer.conf
file that has the following:BBPATH .= ":${LAYERDIR}" BBFILES += "${LAYERDIR}/*.bb" BBFILE_COLLECTIONS += "mylayer" BBFILE_PATTERN_mylayer := "^${LAYERDIR_RE}/" LAYERSERIES_CORENAMES = "hello_world_example" LAYERSERIES_COMPAT_mylayer = "hello_world_example"
For information on these variables, click on BBFILES, LAYERDIR, BBFILE_COLLECTIONS, BBFILE_PATTERN_mylayer or LAYERSERIES_COMPAT to go to the definitions in the glossary.
Note
We are setting both
LAYERSERIES_CORENAMES
and LAYERSERIES_COMPAT in this particular case, because we are using bitbake without OpenEmbedded. You should usually just use LAYERSERIES_COMPAT to specify the OE-Core versions for which your layer is compatible, and add the meta-openembedded layer to your project.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.- 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.
- Creating conf/bblayers.conf: BitBake uses the
conf/bblayers.conf
file to locate layers needed for the project. This file must reside in theconf
directory of the project (i.e.hello/conf
for this example).Set your working directory to the
hello/conf
directory and then create thebblayers.conf
file so that it contains the following:BBLAYERS ?= " \ /home/<you>/mylayer \ "
You need to provide your own information for
you
in the file.
- Run BitBake With a Target: Now that you have supplied the
bblayers.conf
file, run thebitbake
command and provide the target:$ bitbake printhello Loading cache: 100% | Loaded 0 entries from dependency cache. Parsing recipes: 100% |##################################################################################| 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 Initialising tasks: 100% |###############################################################################| NOTE: No setscene tasks NOTE: Executing 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.