Question

I have a makefile such as :

A : B
    echo "made A"
B : D
    echo "made B"

Here, B exists but D doesn’t (sometimes, it does, sometimes it doesn’t)

Now if I execute make, it shouts at me :

make: *** No rule to make target 'D', needed by 'B'.  Stop.

Is there any way to behave correctly i.e. :

• B and D don’t exist : fail
• B exists, D does and is newer than B , apply recipe
• B exists, D exists but is older : don’t do anything
• if B exists and D doesn’t : don’t rebuild B (you can’t) but you can use B to build A

Using an empty line with D as target would always execute recipe to build B, but without D it cannot, so it’s not wanted !

Is there any way ?

I found the solution of using an empty rule

D : 

at the end, so that missing D will not fail. So now all is OK except B is redone if D is missing, which can be tested in B recipe :

B : D
    if [ D ] ; then echo "doing B with D"; fi

But it really seems hackish …

Answer 1

You can get around the re-generation of B from an older D by marking B as intermediate:

 .INTERMEDIATE : B

You can then remove the empty rule for D, and your four criteria for “correct” behaviour are matched.