[erlang-questions] example of race conditions ?

[David King]

> The problem is that only one process can have a certain name and what
> happens if more then one process want a certain name?

This example is basically identical to mine (and sent a few minutes  
before mine), so I'm glad to see that my concern about it is warranted.

What is considered "the" solution to this? To create these processes  
before calling functions that may use them? What if you're writing a  
module that others may use, and don't have control over their startup  
process? Or what if a given daemon process is rarely used? It seems a  
waste of resources to spawn it if it may never be used. Can the  
process-table be locked? That doesn't seem very Erlang-ish either.


> For example, you want to spawn several processes and have one as  
> master
> and the rest as slaves.
>
> You do this by having this start function for a module.
>
> start() ->
>     % Check if any process is already registered as master
>     case whereis(master) of
>         undefined ->
>             % No master, I register as master and start looping
>             register(master, self()),
>             loop(as_master);
>         MasterPid
>             % Already exist one master, I report in as slave
>             MasterPid ! {newslave, self()},
>             loop(as_slave)
>     end.
>
> This will work as long as one process get to execute both whereis and
> register without any other processes getting time to execute.
>
> If you now spawn let say ten of these processes, you will likely  
> ending up
> with processes checking if master is registered, getting undefined  
> back,
> but then some other process execute and register the name, the next  
> time
> the first process get to execute it try to register the name  
> master, which
> wont work.
>
> A quick and dirty example of a race condition, code is untested and  
> might
> contain syntax errors.
>
> Regards,
> Ulf
>
> On Tue, July 31, 2007 16:59, Saifi Khan wrote:
>> Hello:
>>
>> The last section in Chap 9 (Errors in Concurrent prog) of
>> Programming Erlang book by Dr. Armstrong, says the following:
>>
>> "When you combine the Erlang primitives spawn(), spawn_link(),
>> register() ..., you must think carefully about possible race
>> conditions.
>> Write your code in such a way that race conditions cannot happen"
>>
>> To me, it seemed like an anti-climax of the whole 'lock free prog'
>> model. Please correct me, if I am wrong.
>>
>> Can somebody share sample code that *has* race conditions ?
>>
>> Appreciate a response from the experienced erlangers.
>>
>>
>> thanks
>> Saifi.
>>
>> TWINCLING Society
>> freedom of innovation
>> http://www.twincling.org/
>>
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>
>
> --
> Ulf Eliasson, ulf@REDACTED
> Erlang Training and Consulting
> http://www.erlang-consulting.com/
>
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