條件賦值有兩種方法

1. += : 追加賦值
2. ?= : 條件賦值，如果沒有賦值則賦值，如果已經賦值則不賦值

測試範例

testA = GOOD
testB = GOOD

test1 = morning!
test1 ?= afternoon!
test2 ?= afternoon!

testA += ${test1}
testB += ${test2}


test:
        @echo "testA = ${testA}"
        @echo "testB = ${testB}"
        @echo "test1 = ${test1}"
        @echo "test2 = ${test2}"

輸出:

testA = GOOD morning!
testB = GOOD afternoon!
test1 = morning!
test2 = afternoon!

我們測試另一個範例，這跟明天要講的有關

我們將 testB 的賦值由 GOOD 改成了 ${test}

testA = GOOD
testB = ${testA}

test1 = morning!
test1 ?= afternoon!
test2 ?= afternoon!

testA += ${test1}
testB += ${test2}


test:
        @echo "testA = ${testA}"
        @echo "testB = ${testB}"

輸出:

testA = GOOD morning!
testB = GOOD morning! afternoon!

輸出的 testB 卻變了 GOOD morning! afternoon!

這跟 Verilog 道理有點像， 明天在講解