# Bitbake Note

## Syntax

### ?=

Softer assignment.如果在statement在parse後變數還沒有被定義，則變數就會被賦予該值，否則變數為原本的值。
當有複數"?="作用在同一變數，則採用第一個

Example:

A ?= val

--> If A is set at the time this statement is parsed, the variable retains its value. However, if A is not set, the variable is set to "val".

### ??=

Weaker assignment. Same as "?=" except that the assignment is made at the end of the parsing process rather than immediately.

### :=

Expand immediately.

Example:

T = "123"
A := "${B} ${A} test ${T}"
T = "456"
B = "${T} bval"
C = "cval"
C := "${C}append"

A = test 123 因為 B和A此時為undefined
C = cvalappend

### += and =+

Append and prepend with space

Example:

B = "bval"
B += "additionaldata"
C = "cval"
C =+ "test"

B = bval additionaldata
C = test cval

### .= and =.

Append and prepend without space

B = "bval"
B .= "additionaldata"
C = "cval"
C =. "test"

B = bvaladditionaldata
C = testcval

### Appending and Prepending (Override Style Syntax)

使用override syntax方法來append/prepend，使用此方法不會有空白

B = "bval"
B_append = " additional data"
C = "cval"
C_prepend = "additional data "
D = "dval"
D_append = "additional data"

B = bval additional data
C = additional data cval
D = additional datadval

### Conditional Metadata

如果有OVERRIDES variable是一個colon-character-separated list，如果滿足OVERRIDES定義的字串，則會使用定義字串的變數

Example:

OVERRIDES = "architecture:os:machine"
TEST = "default"
TEST_os = "osspecific"
TEST_nooverride = "othercondvalue"

Select the os-specific version of the TEST variable by appending the "os" override to the variable (i.e.TEST_os).

OVERRIDES也可搭配append/prepend語句

Example:

DEPENDS = "glibc ncurses"
OVERRIDES = "machine:local"
DEPENDS_append_machine = "libmad"

DEPENDS = glibc ncurses libmad

## Sharing Functionality

### inherit Directive

使用inherit directive來繼承.bbclass

### include Directive v.s. require Directive

Bitbake會parse指定路徑下的.inc file，當找不到檔案時，inclue不會報錯，require則會報錯

## Functions

支援 shell function, bitbake style python, python, anonymous python function

### Bitbake Style Python Functions

使用bb.build.exec_func()執行此類function

### Anonymous Python Functions

不同於一般的python function，anonymous Python functions只在parsing時運作

Example:

python __anonymous () {
if d.getVar('SOMEVAR', True) == 'value':
d.setVar('ANOTHERVAR', 'value2')
}

一般 "__anonymous" 也可以忽略

"d" variable指的是entire recipe的database

## Tasks

Bitbake使用"addtask"來定義function執行順序

Example (meta/class/kernel-yocto.bbclass line:320)

addtask kernel_checkout before do_kernel_metadata after do_symlink_kernsrc
addtask kernel_metadata after do_validate_branches do_unpack before do_patch

解讀為:

do_kernel_checkout在do_kernel_metadata之前，但在do_symlink_kernsrc之後
do_kernel_metadata在do_validate_branches之後，但在do_unpack之前

因此全部順序為:
do_symlink_kernsrc --> do_kernel_checkout --> do_validate_branches
--> do_kernel_metadata --> do_patch

## Logging

soure code: meta/classes/logging.bbclass

bbfatal
bbfatal_log
bberror
bbwarn
bbnote
bbplain
bbdebug

Both `bbfatal` and `bbfatal_log` will exit building process

Usage:

bbnote "test"

Log file path (take meta/classes/kernel-yocto.bbclass for example):

build/tmp/work/witherspoon-openbmc-linux-gnueabi/linux-aspeed/5.4.39+gitAUTOINC+30079d65ac-r0/temp/log......