Signal and wait concept in multithreading

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I am trying to understand concurrency concepts in this program. I am trying to understand it but I am not able to understand the main logic. I am not sure what signal and wait are actually doing here.

the core logic is here

export operation Hi
			if ! flip then
				stdout.putstring["Waiting Hi\n"]
				wait c
			end if
			stdout.putstring["Hi\n"]
			flip <- false
			signal c
		end hi
		export operation Ho
			if flip then
				stdout.putstring["Waiting Ho\n"]
				wait c
			end if
			stdout.putstring["Ho\n"]
			flip <- true
			signal c
		end ho

What I have tried:

The output is

Hi
Waiting Hi
Ho
Hi
Ho
Hi
Ho
Hi
Ho
Hi
Ho
Hi
Ho
Waiting Ho
Hi
Ho
Hi
Ho
Hi
Ho
Hi
Ho

Can anyone explain how this output is generated how signal and wait are executing the output

Posted 23-Feb-17 2:20am

codegeekalpha

Updated 23-Feb-17 6:42am

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Comments

Jochen Arndt 23-Feb-17 9:01am

This looks like Emerald.
I think there are not so many (if any) here knowing that language (I don't).

As a first step I suggest to lookup the documentation for the wait and signal functions.

[no name] 23-Feb-17 9:05am

I can't tell you what it is either. Never saw it before. I can tell that it is not C.

Albert Holguin 23-Feb-17 12:01pm

That's definitely not C. If you need help with whatever language this is, tag appropriately. If you just want help with understanding concurrency in general, don't use a language specific tag.

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Richard MacCutchan · Accepted Answer · 2017-02-23T06:42:00

No idea what that language is, but it is not C. It looks to me like there are two independent activities, hi and ho. Each one tests a flag and either waits, or continues processing. The waiting activity does nothing until the other activity sends a signal which restarts it. They then continue on opposite tracks. The output shows the order of execution,