bitbake: user-manual-hello.xml: Added new chapter for "Hello World Example"
This file was evidently a "working" file and not included in the manual at the point Bill left off. The wmat branch, however, had a load of commits dedicated to this file. Rather than attempt to replay them all one-by-one, I simply copied the file from the wmat branch and hand-inserted the changes to make it equal to what was there. Note also that I re-formatted the file to have the same formatting standards I use in the YP manuals. (Bitbake rev: 9ddbf31ba7d05a596ca53b8ed78d94221850894b) Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Scott Rifenbark
Richard Purdie
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<chapter id='hello'>
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<title>A BitBake Hello World</title>
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<section id='bitbake-hello-world'>
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<title>BitBake Hello World</title>
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<para>
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The simplest example commonly used to demonstrate any new
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programming language or tool is the
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<ulink url="http://en.wikipedia.org/wiki/Hello_world_program">Hello World</ulink>
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example.
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This chapter demonstrates, in tutorial form, Hello
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World within the context of BitBake.
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This tutorial describes how to create a new Project
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and the applicable metadata files necessary to allow
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BitBake to build it.
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</para>
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</section>
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<section id='obtaining-bitbake'>
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<title>Obtaining BitBake</title>
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<para>
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Please refer to Chapter 1 Section 1.7 for the various methods to
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obtain BitBake.
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Once the source code is on your machine the BitBake directory will
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appear as follows:
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<literallayout class='monospaced'>
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$ ls -al
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total 100
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drwxrwxr-x. 9 wmat wmat 4096 Jan 31 13:44 .
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drwxrwxr-x. 3 wmat wmat 4096 Feb 4 10:45 ..
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-rw-rw-r--. 1 wmat wmat 365 Nov 26 04:55 AUTHORS
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drwxrwxr-x. 2 wmat wmat 4096 Nov 26 04:55 bin
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drwxrwxr-x. 4 wmat wmat 4096 Jan 31 13:44 build
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-rw-rw-r--. 1 wmat wmat 16501 Nov 26 04:55 ChangeLog
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drwxrwxr-x. 2 wmat wmat 4096 Nov 26 04:55 classes
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drwxrwxr-x. 2 wmat wmat 4096 Nov 26 04:55 conf
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drwxrwxr-x. 3 wmat wmat 4096 Nov 26 04:55 contrib
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-rw-rw-r--. 1 wmat wmat 17987 Nov 26 04:55 COPYING
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drwxrwxr-x. 3 wmat wmat 4096 Nov 26 04:55 doc
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-rw-rw-r--. 1 wmat wmat 69 Nov 26 04:55 .gitignore
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-rw-rw-r--. 1 wmat wmat 849 Nov 26 04:55 HEADER
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drwxrwxr-x. 5 wmat wmat 4096 Jan 31 13:44 lib
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-rw-rw-r--. 1 wmat wmat 195 Nov 26 04:55 MANIFEST.in
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-rwxrwxr-x. 1 wmat wmat 3195 Jan 31 11:57 setup.py
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-rw-rw-r--. 1 wmat wmat 2887 Nov 26 04:55 TODO
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</literallayout>
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</para>
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<para>
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At this point you should have BitBake extracted or cloned to
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a directory and it should match the directory tree above.
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Please note that you'll see your username wherever
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"wmat" appears above.
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</para>
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</section>
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<section id='setting-up-the-bitbake-environment'>
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<title>Setting Up the BitBake Environment</title>
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<para>
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The recommended method to run BitBake is from a directory of your
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choice.
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The directory can be within your home directory or in
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<filename>/usr/local</filename>,
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depending on your preference.
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Let's run BitBake now to make sure it's working.
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</para>
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<para>
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From the BitBake source code directory, issue the following command:
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<literallayout class='monospaced'>
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$ ./bin/bitbake --version
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BitBake Build Tool Core version 1.19.0, bitbake version
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1.19.0
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</literallayout>
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You're now ready to use BitBake.
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</para>
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<para>
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A final step to make development easier is to add the executable
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binary to your environment <filename>PATH</filename>.
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First, have a look at your current <filename>PATH</filename> variable.
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If I check mine, I get:
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<literallayout class='monospaced'>
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$ echo $PATH
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/home/wmat/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
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/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
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</literallayout>
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Now add the directory location for the BitBake binary to the <filename>PATH</filename>
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with:
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<literallayout class='monospaced'>
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$ export PATH={path to the bitbake executable}:$PATH
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</literallayout>
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This will add the directory to the beginning of your PATH environment
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variable.
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For example, on my machine:
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<literallayout class='monospaced'>
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$ export PATH=/media/wmat/Backups/dev/bitbake/bin:$PATH
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/media/wmat/Backups/dev/bitbake/bin:/home/wmat/bin:
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/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
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/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
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</literallayout>
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Now, you should be able to simply enter the
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<filename>bitbake</filename>
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command at the command line to run bitbake.
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For a more permanent solution and assuming you are running the BASH
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shell, edit <filename>~/.bashrc</filename> and add the following to the end
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of that file:
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<literallayout class='monospaced'>
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PATH={path to the bitbake executable}:$PATH
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</literallayout>
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</para>
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<para>
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Note that if you're a Vim user, you will find useful
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Vim configuration contributions in the
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<filename>contrib/vim</filename> directory.
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Copy the files from that directory to your
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<filename>/home/yourusername/.vim</filename>
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directory.
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If it doesn't exist, create it, and restart Vim.
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</para>
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</section>
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<section id='the-hello-world-example'>
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<title>The Hello World Example</title>
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<para>
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The following example leaps directly into how BitBake
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works.
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Every attempt is made to explain what is happening,
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however, further information can be found in the
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Metadata chapter.
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</para>
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<para>
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The overall goal of this exercise is to create a Hello
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World example utilizing concepts used to
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build and construct a complete example application
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including Tasks and Layers.
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This is how modern projects such as OpenEmbedded and
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the Yocto Project utilize BitBake, therefore it
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provides an excellent starting point for understanding
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BitBake.
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</para>
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<para>
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It should be noted that this chapter was inspired by
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and draws heavily from several sources:
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<itemizedlist>
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<listitem><para>
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<ulink href="http://www.mail-archive.com/yocto@yoctoproject.org/msg09379.html">Mailing List post - The BitBake equivalent of "Hello, World!"</ulink>
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</para></listitem>
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<listitem><para>
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<ulink href="http://hambedded.org/blog/2012/11/24/from-bitbake-hello-world-to-an-image/">Hambedded Linux blog post - From Bitbake Hello World to an Image</ulink>
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</para></listitem>
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</itemizedlist>
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</para>
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<section id='a-reverse-walkthrough'>
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<title>A Reverse Walkthrough</title>
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<para>
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One of the best means to understand anything is to walk
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through the steps to where we want to be by observing first
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principles.
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BitBake allows us to do this through the -D or Debug command
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line parameter.
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We know we want to eventually compile a HelloWorld example, but
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we don't know what we need to do that.
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Remember that BitBake utilizes three types of metadata files:
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Configuration Files, Classes, and Recipes.
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But where do they go, how does BitBake find them, etc. etc.?
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Hopefully we can use BitBake's error messaging to figure this
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out and better understand exactly what's going on.
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</para>
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<para>
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First, let's begin by setting up a directory for our HelloWorld
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project.
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I'll do this in my home directory and change into that
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directory:
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<literallayout class='monospaced'>
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$ mkdir ~/dev/hello && cd ~/dev/hello
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</literallayout>
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Within this new, empty directory, let's run BitBake with
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Debugging output and see what happens:
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<literallayout class='monospaced'>
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$ bitbake -DDD
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The BBPATH variable is not set
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DEBUG: Removed the following variables from the environment:
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GNOME_DESKTOP_SESSION_ID, LESSOPEN, WINDOWID,
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GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG,
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XDG_SESSION_PATH, XAUTHORITY, LANGUAGE, SESSION_MANAGER,
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SHLVL, MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, TEXTDOMAIN,
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GPG_AGENT_INFO, SSH_AUTH_SOCK, XDG_RUNTIME_DIR,
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COMPIZ_BIN_PATH, GDMSESSION, DEFAULTS_PATH, TEXTDOMAINDIR,
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XDG_SEAT_PATH, XDG_CONFIG_DIRS, XDG_CURRENT_DESKTOP,
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DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE,
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DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID,
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UBUNTU_MENUPROXY, OLDPWD, GTK_MODULES, XDG_DATA_DIRS,
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COLORTERM, LS_COLORS
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</literallayout>
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The majority of this output is specific to environment variables
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that are not directly relevant to BitBake.
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However, the very
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first message <filename>The BBPATH variable is not set</filename>
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is and needs to be rectified.
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So how do we set the BBPATH
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variable?
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</para>
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<para>
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When BitBake is run it begins looking for metadata files.
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The BBPATH variable is what tells BitBake where to look.
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It is possible to set BBPATH as an environment variable as you
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did above for the BitBake exexcutable's PATH.
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However, it's much more flexible to set the BBPATH variable for
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each project, as this allows for greater flexibility.
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</para>
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<para>
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Without BBPATH Bitbake will not find any <filename>.conf</filename>
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files or recipe files at all.
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It will also not find <filename>bitbake.conf</filename>.
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Note the reference to <filename>conf/</filename>.
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It is standard practice to organize the project's directory tree
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to include a <filename>conf/</filename> and a
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<filename>classes/</filename> directory.
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Add those now to your project directory:
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<literallayout class='monospaced'>
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$ mkdir conf classes
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</literallayout>
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Now let's copy the sample configuration files provided in the
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BitBake source tree to their appropriate conf and classes
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directory.
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Change to the BitBake source tree directory and:
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<literallayout class='monospaced'>
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cp conf/bitbake.conf ~/dev/hello/conf/
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cp classes/base.bbclass ~/dev/hello/classes/
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</literallayout>
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At this point your project directory structure should look like
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the following:
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<literallayout class='monospaced'>
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~/dev/hello$ tree
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.
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├── classes
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│ └── base.bbclass
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└── conf
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└── bitbake.conf
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</literallayout>
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</para>
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<para>
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But what about BBPATH, we still haven't set it?
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</para>
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<para>
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The first configuration file that BitBake looks for is always
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<filename>bblayers.conf</filename>.
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With this knowledge we know that to resolve our BBPATH error we
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can add a <filename>conf/bblayers.conf</filename> file to our
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project source tree and populate it with the BBPATH variable
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declaration.
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From your project source tree:
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<literallayout class='monospaced'>
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$ vim conf/bblayers.conf
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</literallayout>
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Add the following to the empty bblayers.conf file:
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<literallayout class='monospaced'>
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BBPATH := "${TOPDIR}"
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</literallayout>
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</para>
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<para>
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Now from the root of our project directory, let's run BitBake
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again and see what happens:
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<literallayout class='monospaced'>
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$ bitbake -DDD
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Nothing to do. Use 'bitbake world' to build everything, or run
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'bitbake --help' for usage information.
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DEBUG: Removed the following variables from the environment:
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GNOME_DESKTOP_SESSION_ID, LESSOPEN, WINDOWID,
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GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG,
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XDG_SESSION_PATH, XAUTHORITY, LANGUAGE, SESSION_MANAGER,
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SHLVL, MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, TEXTDOMAIN,
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GPG_AGENT_INFO, SSH_AUTH_SOCK, XDG_RUNTIME_DIR,
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COMPIZ_BIN_PATH, GDMSESSION, DEFAULTS_PATH, TEXTDOMAINDIR,
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XDG_SEAT_PATH, XDG_CONFIG_DIRS, XDG_CURRENT_DESKTOP,
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DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE,
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DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID, UBUNTU_MENUPROXY,
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OLDPWD, GTK_MODULES, XDG_DATA_DIRS, COLORTERM, LS_COLORS
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DEBUG: Found bblayers.conf (/home/wmat/dev/hello/conf/
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bblayers.conf)
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DEBUG: LOAD /home/wmat/dev/hello/conf/bblayers.conf
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DEBUG: LOAD /home/wmat/dev/hello/conf/bitbake.conf
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DEBUG: BB configuration INHERITs:0: inheriting /home/wmat/dev/
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hello/classes/base.bbclass
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DEBUG: BB /home/wmat/dev/hello/classes/base.bbclass: handle
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(data, include)
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DEBUG: LOAD /home/wmat/dev/hello/classes/base.bbclass
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DEBUG: Clearing SRCREV cache due to cache policy of: clear
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DEBUG: Using cache in '/home/wmat/dev/hello/tmp/cache/
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local_file_checksum_cache.dat'
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DEBUG: Using cache in '/home/wmat/dev/hello/tmp/cache/
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bb_codeparser.dat'
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</literallayout>
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<note>
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From this point forward, the environment variable
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removal messages will be ignored and omitted.
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Let's examine the relevant DEBUG messages:
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</note>
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</para>
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</section>
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</section>
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</chapter>
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