[Scons-users] clarification on Depends()

[Bill Deegan]

Jason,

I've been spending a lot of time in the decider/taskmaster logic lately.
Depends do actually matter and are not "generally pointless"
They're added to Node's bdepend list and depend_set which combined with
sources and implicit dependencies are what is used to determine if the
sources are out of date with respect to the target.

-Bill

On Tue, Jun 5, 2018 at 5:47 PM, Jason Kenny <dragon512 at live.com> wrote:

> I think of this more as a make rule.
>
>
>
> I want to say for whatever reason that if the fake.txt is out of date that
> mean hello.c is out of date.
>
>
>
> So something like:
>
>
>
> Fake.txt:
>
>
>
> Hello.c:fake.txt
>
>
>
> Hello.o:hello.c
>
>               cc hello.c
>
>
>
> Here in make land a change to fake.txt will spawn all rules that have
> fake.txt as a source.
>
>
>
> From what I understand the Depend() is generally pointless as the
> execution logic only cares about implicit depends of the source via the
> scanner, and if the source has a builder. If it has a builder then we care
> about depends of the source node, as now it is technically a target in the
> other builder. I can get different behavior via one of three ways.
>
>
>
>    1. Make an empty do nothing builder to make a depends() as a source of
>    the of the node
>    2. Change/add a different decider to say this node is up-to-date if
>    and only if it and any depends are all up-to-date. Currently this only
>    returns True if this node in question is up-to-date, not if anything it
>    depends on is as well.
>    3. Change the executor to care about depends of a source even if it
>    does not have a builder
>
>
>
> Honestly there are special cases in which this is useful. Most of the time
> the scanner is the way to go.
>
>
>
> Jason
>
>
>
> *From:* Bill Deegan <bill at baddogconsulting.com>
> *Sent:* Tuesday, June 5, 2018 7:25 PM
> *To:* SCons users mailing list <scons-users at scons.org>
> *Cc:* Jason Kenny <dragon512 at live.com>
> *Subject:* Re: [Scons-users] clarification on Depends()
>
>
>
>
>
>
>
> On Tue, Jun 5, 2018 at 3:27 PM, Marc Branchaud <marcnarc at xiplink.com>
> wrote:
>
> 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.
>
>
>
> It would rebuild hello, but not necessarily rebuild hello.o
>
>
>
> So not exactly the same thing.
>
>
>
> -Bill
>
>
>
>
>                 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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