[Scons-users] clarification on Depends()

[Marc Branchaud]

On 2018-06-05 12:05 AM, Jason Kenny wrote:
> HI,
> 
> I just want to clarify behavior of Depends() in SCons.
> 
> I have a small sample:
> 
> Depends(["hello.c")],["fake.txt"])
> 
> Program("hello","hello.c")
> 
> If I run this sample “hello” will build. If I change fake.txt, nothing 
> will rebuild.

Right.  This is because you have nothing that builds hello.c, so SCons 
doesn't care that hello.c depends on anything.

But SCons does know how to build hello.o.  That knowledge is implicit in 
the Program builder.  So SCons respects
	Depends("hello.o", "fake.txt")

Put another way, SCons only performs actions to build hello.o (and, 
ultimately, hello), so it only cares about dependencies for the actions 
it undertakes.

You could also use the target returned by the Program builder:
	hello_target = Program("hello", "hello.c")
	Depends(hello_target, "fake.txt")
This achieves the same thing, and lets you avoid knowing the Program 
builder's intermediate artifacts.

		M.


> I have a tree like this:
> 
> +-hello
> 
>    | +-hello.o
> 
>    | | +-hello.c
> 
>    | | +-fake.txt
> 
>    | | +-/usr/bin/gcc
> 
>    | +-/usr/bin/gcc
> 
> If I change the sample to this ( depends() is not hello.o vs hello.c)
> 
> Depends(["hello.c")],["fake.txt"])
> 
> Program("hello","hello.c")
> 
> Now when I change fake.txt hello.o will rebuild. The tree here looks like:
> 
> +-hello
> 
>    | +-hello.o
> 
>    | | +-hello.c
> 
>    | | +-fake.txt
> 
>    | | +-/usr/bin/gcc
> 
>    | +-/usr/bin/gcc
> 
> Is this right… I have to know the target of a builder to use? I cannot 
> use the source of a builder?
> 
> Jason
> 
> 
> 
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