Override Strategies

Far too often you’ll find yourself needing to override a particular behaviour that was done in another recipe in some OE layer included in your project. There are many ways to override the contents of a recipe. We’ll discuss a few methodologies in this section.

Recipe Overrides

Typically recipe overrides happen via:

  • The use of a bbappend file.

    Typically two forms:

    • <recipe_name>_%.bbappend

      Means no matter the version override behaviour of recipe.

    • <recipe_name>_<recipe_version>.bbappend

      Means given a specific version override behaviour of recipe.

  • Including the same file name with different version in your layer. Then set the PREFERRED_VERSION_<recipe> variable accordingly to choose which version of the recipe you want to utilize.

If you choose the bbappend route. Bellow sections talk a bit about common override strategies.

Variable Overrides

There are many ways to override variables, but in order to override we must first talk about variable assignment.

  • MYVAR ??= “some value”
    Very weakly assign variable. Making extremely easy to override.
  • MYVAR ?= “some value”
    Weakly assign variable. Making it easy to override.
  • MYVAR = “some value”
    Strongly assign variable. Making it almost impossible to override.

Techinally speaking overrides for variables.

  • MYVAR += “some value”
    Append with spaces.
  • MYVAR .= “ some value”
    Append without spaces.
  • MYVAR:append = “ some value”
    Append without spaces.
  • MYVAR =+ “some value”
    Prepend with spaces.
  • MYVAR =. “ some value”
    Prepend without spaces.
  • MYVAR:prepend = “ some value”
    Prepend without spaces.

If you have a variable weakly assigned in one layer. You can easily override it in another layer.

Typically strongly type variable overrides happen via:

  • CLASSOVERRIDE
    Internal to OE build system. This is a variable that allows you to
    set a variable or function for a specific type of build. So, lets
    say you only want to assign a variable for target builds.
    Then you would

    • MYVAR:class-target = “some value”


    Values of CLASSOVERRIDE are as such

    • class-target

    • class-native

    • class-nativesdk

  • MACHINEOVERRIDES
    Typically set in machine configs. Allows for overriding
    and typing variables to a specific machine.
  • DISTROOVERRIDES
    Typically set in distro configs. Allows for overriding
    and typing variables to a specific distro.

After setting these two variables if a variable is strongly set in a layer other than your custom layer:

  • MYVAR:my-machine-override = “Some more important value”

  • MYVAR:my-distro-override = “Some more important value”

Will override the strongly set value ensuring your variable takes priority.

Function Overrides

Typically to override a function all one has to do is re-implement the function in a bbappend or just a normal bb file.

Just like variables functions in Yocto also allow you to append or prepend to them.

do_install:prepend:class-target() {
        install -d ${D}/my/custom/location
        install ${S}/my-custom-file.txt ${D}/my/custom/location
}

do_install:prepend:class-native() {
        install -d ${D}/my/custom/location
        install ${S}/my-custom-file.txt ${D}/my/custom/location
}

do_install:append:class-target() {
        install -d ${D}/my/custom/location
        install ${S}/my-custom-file.txt ${D}/my/custom/location
}

do_install:append:class-native() {
        install -d ${D}/my/custom/location
        install ${S}/my-custom-file.txt ${D}/my/custom/location
}

Yocto also allows you to inject functions or delete them entirely via the commands

  • addtask

  • deltask

BitBake Class Overrides

You can’t override a BitBake class the same way you override a recipe. To override a class you have to utilize some of the above aforementioned methods.