Acess variable that was set by thread

ag0aep6g anonymous at
Mon Aug 8 22:01:43 UTC 2022

On Monday, 8 August 2022 at 20:36:34 UTC, Steven Schveighoffer 
> shared gives you a sense that the language is helping you 
> prevent problems. Again, if C isn't playing ball, this is a lie.

The C side is playing ball, by whatever rules the C library 

`shared` (with `-preview=nosharedaccess`) prevents you from going 
on the field. Can't unwittingly commit a foul. Can't hurt 
yourself. You can tell the compiler with a cast that (1) you're 
sure you want to play, and (2) you're going to play by the rules 
of the C side (whatever they are).

`__gshared` just lets you run on the field. Don't know the rules? 
The compiler doesn't care. Have fun breaking your legs.

> Consider if the proper way to use such a variable is to call 
> `properlyUse(int *)`, it won't accept a `shared int *`. Now you 
> are doubly-sure to mess up using it specifically because it's 
> marked `shared`.

With `__gshared`:

extern(C) extern __gshared int x;
void fun() { x = 42; } /* compiles, race condition */

I never even realize that I'm doing something dangerous, because 
my first naive attempt passes compilation and seems to work fine.

With `shared` (and `-preview=nosharedaccess`):

extern(C) extern shared int x;
void fun() { x = 42; } /* error */

If I remember to check the documentation, I might find out about 
`properlyUse`. As you suggest, I come up with this:

extern(C) extern shared int x;
void fun() { properlyUse(&x, 42); } /* still error because 
`shared` */

I'm forced to think more about thread-safety. I figure that it's 
ok to cast away `shared` in this case, because I'm calling the 
thread-safe `properlyUse` function. So:

extern(C) extern shared int x;
void fun() { properlyUse(cast(int*) &x, 42); } /* compiles, is 
correct */

I don't believe that people are more likely to get that right 
with `__gshared`. The call to `properlyUse` might look nicer 
without the cast, but I'm not buying that people remember to use 
the function without the compiler yelling at them.

Even if they get it right the first time, they're bound to slip 
up as time progresses. When simple, incorrect code compiles, it 
will surely make its way into the source files.

Thread-safety is hard to get right. We need every help we can get 
from the compiler. `__gshared` provides zero help. `shared` at 
least highlights the interesting spots.

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