Some CDL properties describe the consequences of manipulating configuration options. There are two main types of consequences. Typically enabling a configuration option results in one or more #define's in a configuration header file, and properties that affect this include define , define_proc and no_define . Enabling a configuration option can also affect the build process, primarily determining which files get built and added to the appropriate library. Properties related to the build process include compile and make . This chapter describes the whole build process, including details such as compiler flags and custom build steps.
Part of the overall design of the eCos component framework is that it can interact with a number of different build systems. The most obvious of these is GNU make :the component framework can generate one or more makefiles, and the user can then build the various packages simply by invoking make . However it should also be possible to build eCos by other means: the component framework can be queried about what is involved in building a given configuration, and this information can then be fed into the desired build system. Component writers should be aware of this possibility. Most packages will not be affected because the compile property can be used to provide all the required information, but care has to be taken when writing custom build steps.
It is necessary to create an eCos configuration before anything can be built. With some tools such as the graphical configuration tool this configuration will be created in memory, and it is not essential to produce an ecos.ecc savefile first (although it is still very desirable to generate such a savefile at some point, to allow the configuration to be re-loaded later on). With other tools the savefile is generated first, for example using ecosconfig new , and then a build tree is generated using ecosconfig tree . The savefile contains all the information needed to recreate a configuration.
An eCos build actually involves three separate trees. The component repository acts as the source tree, and for application developers this should be considered a read-only resource. The build tree is where all intermediate files, especially object files, are created. The install tree is where the main library libtarget.a , the exported header files, and similar files end up. Following a successful build it is possible to take just the install tree and use it for developing an application: none of the files in the component repository or the build tree are needed for that. The build tree will be needed again only if the user changes the configuration. However the install tree does not contain copies of all of the documentation for the various packages, instead the documentation is kept only in the component repository.
By default the build tree, the install tree, and the ecos.ecc savefile all reside in the same directory tree. This is not a requirement, both the install tree and the savefile can be anywhere in the file system.
It is worth noting that the component framework does not separate the usual make and make install stages. A build always populates the install tree, and any make install step would be redundant.
The install tree will always begin with two directories, include for the exported header files and lib for the main library libtarget.a and other files such as the linker script. In addition there will be a subdirectory include/pkgconf containing the configuration header files, which are generated or updated at the same time the build tree is created or updated. More details of header file generation are given below. Additional include subdirectories such as sys and cyg/kernel will be created during the first build, when each package's exported header files are copied to the install tree. The install tree may also end up with additional subdirectories during a build, for example as a result of custom build steps.
The component framework does not define the structure of the build tree, and this may vary between build systems. It can be assumed that each package in the configuration will have its own directory in the build tree, and that this directory will be used for storing the package's object files and as the current directory for any build steps for that package. This avoids problems when custom build steps from different packages generate intermediate files which happen to have the same name.
Some build systems may allow application developers to copy a source file from the component repository to the build tree and edit the copy. This allows users to experiment with small changes, for example to add a couple of lines of debugging to a package, without having to modify the master copy in the component repository which could be shared by several projects or several people. Functionality such as this is transparent to component writers, and it is the responsibility of the build system to make sure that the right thing happens.
Note: There are some unresolved issues related to the build tree and install tree. Specifically, when updating an existing build or install tree, what should happen to unexpected files or directories? Suppose the user started with a configuration that included the math library, and the install tree contains header files include/math.h and include/sys/ieeefp.h . The user then removed the math library from the configuration and is updating the build tree. It is now desirable to remove these header files from the install tree, so that if any application code still attempts to use the math library this will fail at compile time rather than at link time. There will also be some object files in the existing libtarget.a library which are no longer appropriate, and there may be other files in the install tree as a result of custom build steps. The build tree will still contain a directory for the math library, which no longer serves any purpose.
However, it is also possible that some of the files in the build tree or the install tree were placed there by the user, in which case removing them automatically would be a bad idea.
At present the component framework does not keep track of exactly what should be present in the build and install trees, so it cannot readily determine which files or library members are obsolete and can safely be removed, and which ones are unexpected and need to be reported to the user. This will be addressed in a future release of the system.