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docs/manual/using-buildroot-development.txt

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+// -*- mode:doc; -*-
+// vim: set syntax=asciidoc:
+
+==== Using Buildroot during development
+
+The normal operation of Buildroot is to download a tarball, extract
+it, configure, compile and install the software component found inside
+this tarball. The source code is extracted in
++output/build/<package>-<version>+, which is a temporary directory:
+whenever +make clean+ is used, this directory is entirely removed, and
+re-created at the next +make+ invocation. Even when a Git or
+Subversion repository is used as the input for the package source
+code, Buildroot creates a tarball out of it, and then behaves as it
+normally does with tarballs.
+
+This behavior is well-suited when Buildroot is used mainly as an
+integration tool, to build and integrate all the components of an
+embedded Linux system. However, if one uses Buildroot during the
+development of certain components of the system, this behavior is not
+very convenient: one would instead like to make a small change to the
+source code of one package, and be able to quickly rebuild the system
+with Buildroot.
+
+Making changes directly in +output/build/<package>-<version>+ is not
+an appropriate solution, because this directory is removed on +make
+clean+.
+
+Therefore, Buildroot provides a specific mechanism for this use case:
+the +<pkg>_OVERRIDE_SRCDIR+ mechanism. Buildroot reads an _override_
+file, which allows the user to tell Buildroot the location of the
+source for certain packages.
+
+The default location of the override file is +$(CONFIG_DIR)/local.mk+,
+as defined by the +BR2_PACKAGE_OVERRIDE_FILE+ configuration option.
++$(CONFIG_DIR)+ is the location of the Buildroot +.config+ file, so
++local.mk+ by default lives side-by-side with the +.config+ file,
+which means:
+
+* In the top-level Buildroot source directory for in-tree builds
+  (i.e., when +O=+ is not used)
+* In the out-of-tree directory for out-of-tree builds (i.e., when
+  +O=+ is used)
+
+If a different location than these defaults is required, it can be
+specified through the +BR2_PACKAGE_OVERRIDE_FILE+ configuration
+option.
+
+In this _override_ file, Buildroot expects to find lines of the form:
+
+------------------
+<pkg1>_OVERRIDE_SRCDIR = /path/to/pkg1/sources
+<pkg2>_OVERRIDE_SRCDIR = /path/to/pkg2/sources
+------------------
+
+For example:
+
+------------------
+LINUX_OVERRIDE_SRCDIR = /home/bob/linux/
+BUSYBOX_OVERRIDE_SRCDIR = /home/bob/busybox/
+------------------
+
+When Buildroot finds that for a given package, an
++<pkg>_OVERRIDE_SRCDIR+ has been defined, it will no longer attempt to
+download, extract and patch the package. Instead, it will directly use
+the source code available in the specified directory and +make clean+
+will not touch this directory. This allows to point Buildroot to your
+own directories, that can be managed by Git, Subversion, or any other
+version control system. To achieve this, Buildroot will use _rsync_ to
+copy the source code of the component from the specified
++<pkg>_OVERRIDE_SRCDIR+ to +output/build/<package>-custom/+.
+
+This mechanism is best used in conjunction with the +make
+<pkg>-rebuild+ and +make <pkg>-reconfigure+ targets. A +make
+<pkg>-rebuild all+ sequence will _rsync_ the source code from
++<pkg>_OVERRIDE_SRCDIR+ to +output/build/<package>-custom+ (thanks to
+_rsync_, only the modified files are copied), and restart the build
+process of just this package.
+
+In the example of the +linux+ package above, the developer can then
+make a source code change in +/home/bob/linux+ and then run:
+
+-----------------------
+make linux-rebuild all
+-----------------------
+
+and in a matter of seconds gets the updated Linux kernel image in
++output/images+. Similarly, a change can be made to the BusyBox source
+code in +/home/bob/busybox+, and after:
+
+-----------------------
+make busybox-rebuild all
+-----------------------
+
+the root filesystem image in +output/images+ contains the updated
+BusyBox.
+
+Source trees for big projects often contain hundreds or thousands of
+files which are not needed for building, but will slow down the process
+of copying the sources with _rsync_. Optionally, it is possible define
++<pkg>_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS+ to skip syncing certain files
+from the source tree. For example, when working on the +webkitgtk+
+package, the following will exclude the tests and in-tree builds from
+a local WebKit source tree:
+
+------------------
+WEBKITGTK_OVERRIDE_SRCDIR = /home/bob/WebKit
+WEBKITGTK_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = \
+	--exclude JSTests --exclude ManualTests --exclude PerformanceTests \
+	--exclude WebDriverTests --exclude WebKitBuild --exclude WebKitLibraries \
+	--exclude WebKit.xcworkspace --exclude Websites --exclude Examples
+------------------
+
+By default, Buildroot skips syncing of VCS artifacts (e.g., the *.git* and
+*.svn* directories). Some packages prefer to have these VCS directories
+available during build, for example for automatically determining a precise
+commit reference for version information. To undo this built-in filtering at a
+cost of a slower speed, add these directories back:
+
+------------------
+LINUX_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = --include .git
+------------------