You can use a .bbappend file in your layer to make additions or changes to the content of another layer’s recipe without having to copy the other layer’s recipe into your layer. Your .bbappend file resides in your layer, while the main .bb recipe file to which you are appending Metadata resides in a different layer.
Being able to append information to an existing recipe not only avoids duplication, but also automatically applies recipe changes from a different layer into your layer. If you were copying recipes, you would have to manually merge changes as they occur.
When you create an append file, you must use the same root name as the corresponding recipe file. For example, the append file someapp_2.4.bbappend must apply to someapp_2.4.bb. This means the original recipe and append file names are version number-specific. If the corresponding recipe is renamed to update to a newer version, you must also rename and possibly update the corresponding .bbappend as well. During the build process, BitBake displays an error on starting if it detects a .bbappend file that does not have a corresponding recipe with a matching name. See the BB_DANGLINGAPPENDS_WARNONLY variable for information on how to handle this error.
As an example, consider the main formfactor recipe and a corresponding formfactor append file both from the Source Directory. Here is the main formfactor recipe, which is named formfactor_0.0.bb and located in the “meta” layer at meta/recipes-bsp/formfactor:
SUMMARY = "Device formfactor information"SECTION = "base"LICENSE = "MIT"LIC_FILES_CHKSUM = "file://${COREBASE}/meta/COPYING.MIT;md5=3da9cfbcb788c80a0384361b4de20420"PR = "r45"SRC_URI = "file://config file://machconfig"S = "${WORKDIR}"PACKAGE_ARCH = "${MACHINE_ARCH}"INHIBIT_DEFAULT_DEPS = "1"do_install() { # Install file only if it has contents install -d ${D}${sysconfdir}/formfactor/ install -m 0644 ${S}/config ${D}${sysconfdir}/formfactor/ if [ -s "${S}/machconfig" ]; then install -m 0644 ${S}/machconfig ${D}${sysconfdir}/formfactor/ fi}
In the main recipe, note the SRC_URI variable, which tells the OpenEmbedded build system where to find files during the build.Following is the append file, which is named formfactor_0.0.bbappend and is from the Raspberry Pi BSP Layer named meta-raspberrypi. The file is in the layer at recipes-bsp/formfactor:
FILESEXTRAPATHS_prepend := “${THISDIR}/${PN}:”
By default, the build system uses the FILESPATH variable to locate files. This append file extends the locations by setting the FILESEXTRAPATHS variable. Setting this variable in the .bbappend file is the most reliable and recommended method for adding directories to the search path used by the build system to find files.
The statement in this example extends the directories to include ${THISDIR}/${PN}, which resolves to a directory named formfactor in the same directory in which the append file resides (i.e. meta-raspberrypi/recipes-bsp/formfactor. This implies that you must have the supporting directory structure set up that will contain any files or patches you will be including from the layer.
Using the immediate expansion assignment operator := is important because of the reference to THISDIR. The trailing colon character is important as it ensures that items in the list remain colon-separated.
Note
BitBake automatically defines the THISDIR variable. You should never set this variable yourself. Using “_prepend” as part of the FILESEXTRAPATHS ensures your path will be searched prior to other paths in the final list.
Also, not all append files add extra files. Many append files simply exist to add build options (e.g. systemd). For these cases, your append file would not even use the FILESEXTRAPATHS statement.
Reference –
http://www.lynxbee.com/yocto-understanding-bbappend-file-and-writing-the-bbappend-bitbake-recipe/
http://www.yoctoproject.org/docs/latest/mega-manual/mega-manual.html
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