Yocto项目中的BitBake使用指南

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"bitbake-user-manual.pdf" 是一份关于嵌入式Linux开发中使用的Yocto项目关键工具——BitBake的用户手册。BitBake是一个构建系统，主要用于生成Linux Board Support Packages (BSP) 和整个嵌入式系统的镜像。这份手册由Richard Purdie、Chris Larson和Phil Blundell编写，涵盖了从2004年至2014年的知识版权，并遵循Creative Commons Attribution License发布。 ### BitBake基础概念 1. **概述**: BitBake提供了一种方法来处理软件构建的元数据，包括配置、依赖关系和构建指令。它在Yocto项目中扮演着核心角色，帮助开发者创建定制化的Linux发行版。 1. **历史与目标**: BitBake的历史可以追溯到嵌入式Linux早期，其目标是简化软件包的构建和管理，特别是在多样化的硬件平台之间。它的设计目标是可扩展性和灵活性，以适应不断变化的软件环境。 1. **概念**: - ** Recipes**: 代表了构建一个特定软件包的步骤和元数据。它们定义了源代码的位置、如何配置、编译以及打包。 - **配置文件**: 包含变量设置，用于调整构建过程的行为和选项。 - **Classes**: 是一组共享的函数或变量，用于扩展或修改Recipe的行为。 - **Layers**: 允许组织和分离不同来源的元数据，比如上游项目、第三方软件和定制化代码。 - **Append Files**: 用于添加或修改已存在Recipe的属性，而无需复制整个Recipe。 1. **获取BitBake**: 提供了获取BitBake源码和构建环境的指导，以便开发者能够开始使用这个工具。 1. **BitBake命令**: 描述了如何运行BitBake命令，包括基本的用法、语法和示例。 ### 执行流程 1. **解析基础配置元数据**: BitBake首先读取并解析基础的配置信息，以了解构建环境的基本设置。 2. **定位和解析Recipes**: BitBake查找所有相关的Recipe文件，并理解它们的结构和内容。 3. **优先级和提供者**: 确定哪个Recipe将被用来构建特定的软件包，可能涉及多个Recipe提供同一服务的情况。 4. **依赖关系**: 分析Recipe间的依赖关系，确保构建顺序正确。 5. **任务列表**: 生成执行任务的顺序列表。 6. **执行任务**: 按照任务列表进行操作，包括下载源码、解压、配置、编译和打包。 7. **校验和（签名）**: 检查文件的完整性以确保安全和一致性。 8. **设置场景**: 为下一次构建准备环境，确保一致性。 ### 语法和运算符 1. **基本语法**: 介绍变量设置和表达式的规则，使开发者能有效地编写和理解BitBake的元数据。 2. **基本变量设置**: 如何定义、赋值和引用变量，包括对变量类型的理解。手册的后续部分可能还会涵盖变量扩展、函数、自定义脚本、调试技巧等更深入的主题，这些对于理解BitBake的工作原理和有效利用其功能至关重要。BitBake的用户需要熟悉这些概念和流程，以成功地在Yocto项目中构建和定制Linux BSP。

Overview

$ bitbake -g foo

$ bitbake -g -I virtual/kernel -I eglibc foo

Chapter2.Execution

The primary purpose for running BitBake is to produce some kind of output such as an image, a kernel,

or a software development kit. Of course, you can execute the bitbake command with options that

cause it to execute single tasks, compile single recipe files, capture or clear data, or simply return

information about the execution environment.

This chapter describes BitBake's execution process from start to finish when you use it to create an

image. The execution process is launched using the following command form:

$ bitbake <target>

For information on the BitBake command and its options, see "The BitBake Command" section.

Note

Prior to executing BitBake, you should take advantage of parallel thread execution by setting

the BB_NUMBER_THREADS variable in your local.conf configuration file.

2.1.Parsing the Base Configuration Metadata

The first thing BitBake does is parse base configuration metadata. Base configuration metadata

consists of the bblayers.conf file to determine what layers BitBake needs to recognize, all necessary

layer.conf files (one from each layer), and bitbake.conf. The data itself is of various types:

• Recipes: Details about particular pieces of software.

• Class Data: An abstraction of common build information (e.g. how to build a Linux kernel).

• Configuration Data: Machine-specific settings, policy decisions, and so forth. Configuration data

acts as the glue to bind everything together.

The layer.conf files are used to construct key variables such as BBPATH and BBFILES. BBPATH is used

to search for configuration and class files under conf/ and class/ directories, respectively. BBFILES

is used to find recipe files (.bb and .bbappend). If there is no bblayers.conf file, it is assumed the

user has set the BBPATH and BBFILES directly in the environment.

Next, the bitbake.conf file is searched using the BBPATH variable that was just constructed. The

bitbake.conf file may also include other configuration files using the include or require directives.

Prior to parsing configuration files, Bitbake looks at certain variables, including:

• BB_ENV_WHITELIST

• BB_PRESERVE_ENV

• BB_ENV_EXTRAWHITE

• BITBAKE_UI

You can find information on how to pass environment variables into the BitBake execution

environment in the "Passing Information Into the Build Task Environment" section.

The base configuration metadata is global and therefore affects all recipes and tasks that are

executed.

BitBake first searches the current working directory for an optional conf/bblayers.conf

configuration file. This file is expected to contain a BBLAYERS variable that is a space delimited list of

'layer' directories. Recall that if BitBake cannot find a bblayers.conf file then it is assumed the user

has set the BBPATH and BBFILES directly in the environment.

For each directory (layer) in this list, a conf/layer.conf file is searched for and parsed with the

LAYERDIR variable being set to the directory where the layer was found. The idea is these files

automatically setup BBPATH and other variables correctly for a given build directory.

Execution

BitBake then expects to find the conf/bitbake.conf file somewhere in the user-specified BBPATH.

That configuration file generally has include directives to pull in any other metadata such as files

specific to the architecture, the machine, the local environment, and so forth.

Only variable definitions and include directives are allowed in .conf files. Some variables directly

influence BitBake's behavior. These variables might have been set from the environment depending

on the environment variables previously mentioned or set in the configuration files. The "Variables

Glossary" chapter presents a full list of variables.

After parsing configuration files, BitBake uses its rudimentary inheritance mechanism, which is

through class files, to inherit some standard classes. BitBake parses a class when the inherit directive

responsible for getting that class is encountered.

The base.bbclass file is always included. Other classes that are specified in the configuration using

the INHERIT variable are also included. BitBake searches for class files in a "classes" subdirectory

under the paths in BBPATH in the same way as configuration files.

A good way to get an idea of the configuration files and the class files used in your execution

environment is to run the following BitBake command:

$ bitbake -e > mybb.log

Examining the top of the mybb.log shows you the many configuration files and class files used in

your execution environment.

Note

You need to be aware of how BitBake parses curly braces. If a recipe uses a closing curly brace

within the function and the character has no leading spaces, BitBake produces a parsing error.

If you use a pair of curly brace in a shell function, the closing curly brace must not be located

at the start of the line without leading spaces.

Here is an example that causes BitBake to produce a parsing error:

fakeroot create_shar() {

cat << "EOF" > ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh

usage()

{

echo "test"

###### The following "}" at the start of the line causes a parsing error ######

}

EOF

}

Writing the recipe this way avoids the error:

fakeroot create_shar() {

cat << "EOF" > ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh

usage()

{

echo "test"

######The following "}" with a leading space at the start of the line avoids the error ######

}

EOF

}

2.2.Locating and Parsing Recipes

During the configuration phase, BitBake will have set BBFILES. BitBake now uses it to construct a list

of recipes to parse, along with any append files (.bbappend) to apply. BBFILES is a space-separated

list of available files and supports wildcards. An example would be:

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Yocto项目中的BitBake使用指南

openbmc-bitbake-user-manual-pdf

BitBake User Manual-V2.4.pdf

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