[gameprogrammer] Re: variables and multithreading

Simply setting a variable won't work.  For example:

Thread 1:
if (variable==false)
{
// Do my updates
}

Thread 2:
// Need to update my records
variable==true
// Do my stuff
variable==false

What can happen is that variable is set to true while thread 1 is inside the block (where I have // Do my updates )

Checking for the start of a critical section lock doesn't work - however checking at the end does if you only have two threads.

For example

bool done=false;

void myfunc(void)
{
// Do something
done=true;
}

if (done==true)
{
// I know whatever was used in // Do something is done and is not currently in use.
done=false;
}

Therefore, it is safe to access a critical section in myfunc, before done==true, and it is safe to access it in the if case that I posted after that.

This is the same principle that is used in the single producer / consumer queue. I use this in my network library RakNet (http://www.rakkarsoft.com) in the file SingleProducerConsumer.cpp . You have an array with a head and a tail. The head cannot read past the tail, and the tail cannot write past the head. You don't increment either until you are done with writing to a block. This is threadsafe as long as one thread is the producer and the other is the consumer (and there are only two threads)


Roger D Vargas wrote:

Im writing a multithreaded server and I have reached the point where I have to access the array containing the players data from the main loop. Should I use some variable to mark if a record is being modified by network thread?



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