Difference between ?= and ??= in Yocto

= operator assigns a value to the variable. Values must be surrounded by double quotes.
E.g. FOO = "hello"

The value of FOO variable is "hello"

?= operator allows you to have a default value for a variable.

When the statement with ?= is parsed

  • If the variable is undefined at the statement, the default value is assigned
  • If the variable is assigned at this statement, then it will keep the original value and will not assign this value.
When there are multiple ?= assignments to a single variable, first of these is used.

To check the value of any environmental variable:
"bitbake -e <image_name> | grep ^VAR_NAME="

E.g. bitbake -e core-image-sato | grep ^DISTRO=




??= operator allows you to achieve a "weaker" assignment for a variable. This assignment is similar to ?= except the value is assigned at the end of the parsing process.

When multiple assignments are present, the value of the last assignment is taken.



Comments

Popular posts from this blog

bb.utils.contains yocto

Difference between RDEPENDS and DEPENDS in Yocto

make config vs oldconfig vs defconfig vs menuconfig vs savedefconfig