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bitbake: bitbake-user-manual: Correct description of the ??= operator

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Stating that the assignment is done at the end of parsing is misleading.
The weak default value is the value which a variable will expand to if no value
has been assigned to it using any of the assignment operators.

(Bitbake rev: 8189f58d0449d16f162b6e8d98c4e5edc6bff875)

Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Jacob Kroon
2022-06-15 11:23:45 +02:00
committed by Richard Purdie
parent 5aca7a8f10
commit 1c02032564
1 changed files with 36 additions and 13 deletions
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49
bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst
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@@ -195,22 +195,45 @@ value. However, if ``A`` is not set, the variable is set to "aval".
Setting a weak default value (??=)
----------------------------------
It is possible to use a "weaker" assignment than in the previous section
by using the "??=" operator. This assignment behaves identical to "?="
except that the assignment is made at the end of the parsing process
rather than immediately. Consequently, when multiple "??=" assignments
exist, the last one is used. Also, any "=" or "?=" assignment will
override the value set with "??=". Here is an example::
The weak default value of a variable is the value which that variable
will expand to if no value has been assigned to it via any of the other
assignment operators. The "??=" operator takes effect immediately, replacing
any previously defined weak default value. Here is an example::
A ??= "somevalue"
A ??= "someothervalue"
W ??= "x"
A := "${W}" # Immediate variable expansion
W ??= "y"
B := "${W}" # Immediate variable expansion
W ??= "z"
C = "${W}"
W ?= "i"
If ``A`` is set before the above statements are
parsed, the variable retains its value. If ``A`` is not set, the
variable is set to "someothervalue".
After parsing we will have::
Again, this assignment is a "lazy" or "weak" assignment because it does
not occur until the end of the parsing process.
A = "x"
B = "y"
C = "i"
W = "i"
Appending and prepending non-override style will not substitute the weak
default value, which means that after parsing::
W ??= "x"
W += "y"
we will have::
W = " y"
On the other hand, override-style appends/prepends/removes are applied after
any active weak default value has been substituted::
W ??= "x"
W:append = "y"
After parsing we will have::
W = "xy"
Immediate variable expansion (:=)
---------------------------------