www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Best practices for multithread global flags

reply Vladimirs Nordholm <v vladde.net> writes:
Hello people from D-land.

To summarise my problem: I have a program in the terminal (Posix) 
with two threads: one which my main program is run on, and a 
second one which polls input via `poll(...)` and `read(...)`.

Let's call main thread T1, and a semi-blocking input-thread T2.

Every second T2 checks if T1 is terminated by checking if the a 
global flag is set to true. This is done with a 
`while(global_flag)`.

This feels to me like iffy code. So my question becomes: what is 
the best way to do this check with?

Also: I do not know that much about multithreading. __gshared is 
super global in my mind, but unsafe. Also bumped into the word 
mutex, but I also do not know what it means.


// i have tried Thread.isDaemon(true), but never managed to get 
the thread to terminate. now i use `spawn(...)` and don't see a 
clear way to use that. is it worth looking into even more?
Nov 15 2017
next sibling parent rikki cattermole <rikki cattermole.co.nz> writes:
On 15/11/2017 11:57 AM, Vladimirs Nordholm wrote:

 Hello people from D-land.
 
 To summarise my problem: I have a program in the terminal (Posix) with 
 two threads: one which my main program is run on, and a second one which 
 polls input via `poll(...)` and `read(...)`.
 
 Let's call main thread T1, and a semi-blocking input-thread T2.
 
 Every second T2 checks if T1 is terminated by checking if the a global 
 flag is set to true. This is done with a `while(global_flag)`.
 
 This feels to me like iffy code. So my question becomes: what is the 
 best way to do this check with?


You're fine, a __gshared variable will do this nicely.
Nov 15 2017
prev sibling next sibling parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Wednesday, November 15, 2017 11:57:25 Vladimirs Nordholm via Digitalmars-
d-learn wrote:

 Hello people from D-land.

 To summarise my problem: I have a program in the terminal (Posix)
 with two threads: one which my main program is run on, and a
 second one which polls input via `poll(...)` and `read(...)`.

 Let's call main thread T1, and a semi-blocking input-thread T2.

 Every second T2 checks if T1 is terminated by checking if the a
 global flag is set to true. This is done with a
 `while(global_flag)`.

 This feels to me like iffy code. So my question becomes: what is
 the best way to do this check with?

 Also: I do not know that much about multithreading. __gshared is
 super global in my mind, but unsafe. Also bumped into the word
 mutex, but I also do not know what it means.


 // i have tried Thread.isDaemon(true), but never managed to get
 the thread to terminate. now i use `spawn(...)` and don't see a
 clear way to use that. is it worth looking into even more?


If you don't know about mutexes, then I would strongly advise that you go
read up on them before you do much of anything with multi-threaded code.

In this case, I would suggest that you use std.concurrency with send and
receive (or probably receiveTimeout) to send a message to the thread that
you want to shut down.

I would suggest that you read http://ddili.org/ders/d.en/concurrency.html
for info on using std.concurrency. And in general, reading the entire book
would likely benefit you:

http://ddili.org/ders/d.en/index.html

- Jonathan M Davis
Nov 15 2017
prev sibling next sibling parent Guillaume Piolat <first.last gmail.com> writes:
On Wednesday, 15 November 2017 at 11:57:25 UTC, Vladimirs 
Nordholm wrote:

 Hello people from D-land.

 To summarise my problem: I have a program in the terminal 
 (Posix) with two threads: one which my main program is run on, 
 and a second one which polls input via `poll(...)` and 
 `read(...)`.

 Let's call main thread T1, and a semi-blocking input-thread T2.

 Every second T2 checks if T1 is terminated by checking if the a 
 global flag is set to true. This is done with a 
 `while(global_flag)`.

 This feels to me like iffy code. So my question becomes: what 
 is the best way to do this check with?



An easy way out is to have a shared(bool) and to use atomicLoad 
in T2, and atomicStore in the other thread T1.

For memory model reasons in x86, your code will work though 
because it's equivalent to atomicLoad(MemoryOrder.raw, x) but 
this isn't valid otherwise.
Nov 15 2017
prev sibling parent Vladimirs Nordholm <v vladde.net> writes:
On Wednesday, 15 November 2017 at 11:57:25 UTC, Vladimirs 
Nordholm wrote:

 Hello people from D-land.

 To summarise my problem: I have a program in the terminal 
 (Posix) with two threads: one which my main program is run on, 
 and a second one which polls input via `poll(...)` and 
 `read(...)`.

 [...]


Thanks for the replies given! Will look into the options given :)
Nov 15 2017