multithreaded hell, help!

[Jonathan M Davis via Digitalmars-d-learn]

On Thursday, July 21, 2016 21:47:09 rikki cattermole via Digitalmars-d-learn 
wrote:
> On 21/07/2016 9:44 PM, Brons wrote:
> > Im trying to access data in a global between threads.
> > So ive set my global to shared, however the object trying to access it
> > needs to also then be shared, but it cant be because its part of a
> > library...
> >
> > Is there any simple way to transfer data from one thread to another?
> >
> > 1. My situation is im reading from a serial port in a thread. Which
> > writes to my global.
> > 2. In a draw thread i need to get that data and draw my output.
> >
> > Cheers
>
> __gshared or cast away shared.

Be _very_ careful of __gshared. It's intended for C-style global variables
only - really, with the idea of using them when linking against C code. The
type system still treats them as thread-local, not shared, so it can do
optimizations based on thread-local which are incompatible with the object
being on multiple threads. You may not have much choice in this case, but
I'd strongly argue that you avoid using __gshared if possible.

An alternative would be marking the object as shared and then casting away
shared while the variable is protected by a mutex (or synchronized block) so
that you can then call non-shared, member functions on the shared object. If
you can do that instead of __gshared, I'd strongly recommend it.

However, the preferred way to share information across threads if possible
is to use std.concurrency to send messages across threads. It's somewhat
limited if you're trying to pass user-defined types across threads if
they're not value types (since they have to be immutable or shared), but if
you need to, you can cast an object to immutable on one side and back to
mutable on the other - so long as you _know_ that there are no other
instances of that object on the original thread. Value types are easy
though, since they can be copied when passed across.

__gshared _might_ be the best choice in your case. However, usually, it's
just the quick and easy way out that gets things working but which risks
subtle bugs down the line. Personally, I would only ever use it when
integrating with a C or C++ library.

- Jonathan M Davis