Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mp035]

@kidandcat Yeah, you are right, it's not an error, just a warning, sorry. I'm 
used to C where a warning usually means I've done something wrong. @mratsim and 
@rayman22201 thanks for explaining that, it seems like the issue has been 
thought through, so there's no point raising it. 

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[kidandcat]

Anyway you stated you got an error accesing the global variable, if you don't 
compile your program with --threads:on you will get only a warning and it will 
compile correctly

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[rayman22201]

As you noted, the stdlib asynchttpserver is not multi-threaded, but httpbeast 
is multithreaded.

But, both libraries use the same core stdlib asyncdispatch (they were written 
by the same author @dom96).

asyncdispatch supports both single threaded and multi-threaded environments. 
This is why the {.gcsafe.} is required.

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mratsim]

Sure, it's better to have a discussion. But I think the constraints are not on 
the asynchttpserver module but directly on asyncdispatch, I may be wrong though

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mp035]

Thank you @mratsim. I did a lot of research and found an old github issue 
discussing implementing threads in asynchttpserver. Araq closed it without 
action noting that httpbeast is available, which is esentially a multithreaded 
server. Do you think I should file an issue requesting the removal of .gcsafe 
seeing as there are now no plans to thread the server? It would remove this 
kludge of adding threadvars to the application.

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mratsim]

Iirc the gcsafe requirement was made so that it would not be breaking in the 
future to make async threadsafe.

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mp035]

Thank you, that explanation makes sense regarding {.threadvar.} however I am 
now confused as to why I need to worry about it at all. I am using **async** 
http server which I am assuming is concurrent (single threaded), so everything 
should be running in the same thread. This would explain why the threadvar 
state is consistent across my application, but does not explain why I need to 
worry about threadvars. Do you know why threadvar is even required?

Re: {.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[kidandcat]

The Threadvar pragma, as its name explains, marks the variable as a thread 
variable, then each thread you create will have its own copy of that variable, 
thus, even if you error has dissapeared, you will find that if you modify that 
variable in your thread A, and you then read it in your thread B, you will find 
that it will be empty in B because A has written in its own copy, not in B's 
copy of the variable.

{.gcsafe.}, What does {.threadvar.} do to make a function gcsafe?

[mp035]

Hi, I'm working with asynchttpserver and I found that I can not access global 
variables from within the server callback.

This seems to be because the server requires a {.gcsafe.} callback, and if a 
global variable (in this case a table) is accessed from within the callback it 
is no longer gc safe.

Adding the {.threadvar.} pragma to the global variable fixed the error, but I 
don't know why. So:

  1. How does accessing global variables make a function not gcsafe?
  2. What does the threadvar pragma do to fix this?



Thanks.