Druntime without pthreads?

[Jacob Carlborg]

On Tuesday, 20 October 2020 at 16:58:12 UTC, Severin Teona wrote:
> Hi guys.
>
> I have a curiosity, regarding [1] - I had encountered some 
> "undefined reference" errors when trying to link the druntime 
> (compiled for an embedded architecture) without some 
> implementation of the POSIX thread calls (and other stuff too).

First, keep in mind that druntime and Phobos depends heavily on 
the C standard library, the Posix APIs, Windows APIs and other 
platform specific APIs. There's no list of what features or APIs 
need to be supported by a platform. There's no general concept of 
optional platform features, i.e. druntime will not automatically 
adopt if your platform doesn't support a specific feature (like 
threads). For example, on iOS (and related platforms) 
`std.process` is not supported (because you're not allowed to 
call `fork`). This is just hardcoded: if the platform is iOS, 
don't provide `std.process`.

What I'm saying, if you modify druntime to remove threads and 
locks, and resolve your undefined references, you might end up 
with new undefined references for other symbols.

> My curiosity is what would change if I removed from the 
> druntime everything that has to do with mutexes or threads. 
> Would it be possible for the druntime to run and work properly 
> on a microcontroller - where those concepts are not necessary? 
> Could I just remove everything about synchronisation from the 
> druntime, and classes or Garbage Collector to still work 
> properly?

The garbage collector will probably work (if you remove the 
interaction with threads from it). Classes are a bit more 
complicated, they have a hidden monitor field [1]. There's also 
the concept of synchronized classes [2]. There's also the 
`synchronized` statement [3]. All these depend on 
mutex/monitors/locks in some form (I don't know exactly how the 
implementation works). Hopefully it should be fine as long as 
your code doesn't use the `synchronized` statement or 
synchronized classes. But all classes do come with a hidden 
monitor field.

[1] https://dlang.org/spec/abi.html#classes
[2] https://dlang.org/spec/class.html#synchronized-classes
[3] https://dlang.org/spec/statement.html#synchronized-statement

--
/Jacob Carlborg