[CMake] Concurrency issues with copy_if_different

[J. Caleb Wherry]

Our case is pretty simplified in that almost every executable uses the
dependent DLLs/SOs I need copied. For C# apps it is log4net, NDesk.Options,
etc. For native apps it’s fmpeg shared libraries.

So when I create the list at configure time, I immediately just use CMake
to copy the files the where I want them in the build tree. This guarantees
that they are copied the one time when CMake is run instead of at build
time.

A few things that make this work for us:

* These libraries are checked into our repo so they aren’t changed by
automatically pulling new binaries or anything like that.

* We only have a handful of these dependent 3rd party libs, like 7.

* Couldn’t think of a better way to do it.

If you did want to move it to build time instead of configure time, you’d
just have to make a custom target that every other target depends on. In
some systems this isn’t easy but if you write wrappers around creating
targets then you can do extra book keeping when a target is create. In this
case, you just add another depends on the custom target that copies the
dependent libs.

-Caleb

On Sat, Dec 8, 2018 at 7:14 PM Olivier Croquette <ocroquette at free.fr> wrote:

> On 2018-7-12 15:16, J. Caleb Wherry wrote:
> >> in one of our projects, we use copy_if_different to copy some DLLs
> >> required by the runtime. It's called as post-build action. The problem
> >> is that several targets want to copy the same DLLs, and when using
> >> parallelized builds, the different "cmake -E copy_if_different" can
> >> conflict, leading the whole build to fail.
> >>
> >> I see two options to fix this:
> >>
> >> 1. don't use copy_if_different, but a custom tool that can deal with the
> >> concurrency
> >>
> >> 2. instead of adding post-build actions to different targets, fill up a
> >> list containing all the files required, and add a single post-build
> >> action to a single target that copies all the required files in one go
> >>
> >> What do you think?
> > Had the same problem and went with #2
>
> Hi Caleb,
>
> could you give some more details how you did this? Filling up the list
> is easy, but I am not sure how to define the final action that will copy
> all the DLLs, and make sure it happens only once. Did you define a new
> custom target for that?
>
>
> --
Sent from my iPhone SE
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