Download - All IT eBooks

Chapter 2
Preparing tasklist
BitBake starts looking through the PROVIDES set in recipe files. The PROVIDES set
defaults to the recipe name, and we can define multiple values to it. We can have
multiple recipes providing a similar package. This task is accomplished by setting
PROVIDES in the recipes. While actually making such recipes part of the build, we
have to define PRREFERED_PROVIDER_foo so that our specific recipe foo can be used.
We can do this in multiple locations. In the case of kernels, we use it in the manchin.
conf file. BitBake iterates through the list of targets it has to build and resolves them,
along with their dependencies.
If PRREFERED_PROVIDER is not set and multiple versions of a
package exist, BitBake will choose the highest version.
Each target/recipe has multiple tasks, such as fetch, unpack, configure, and compile.
BitBake considers each of these tasks as independent units to exploit parallelism
in a multicore environment, which you learned to configure in the previous
chapter. Although these tasks are executed sequentially for a single package/
recipe, for multiple packages, they are run in parallel. We may be compiling one
recipe, configuring the second, and unpacking the third in parallel. Or, may be at
the start, eight packages are all fetching their sources. For now, we should know
the dependencies between tasks that are defined using DEPENDS and RDEPENDS. In
DEPENDS, we provide the dependencies that our package needs to build successfully.
So, BitBake takes care of building these dependencies before our package is built.
RDEPENDS are the dependencies that are required for our package to execute/
run successfully on the target system. So, BitBake takes care of providing these
dependencies on the target's root filesystem.
Executing tasks
Tasks can be defined using the shell syntax or Python. In the case of shell tasks, a
shell script is created under a temporary directory as run.do_taskname.pid and
then, it is executed. The generated shell script contains all the exported variables and
the shell functions, with all the variables expanded. Output from the task is saved in
the same directory with log.do_taskname.pid. In the case of errors, BitBake shows
the full path to this logfile. This is helpful for debugging.
[ 25 ]