'Problem with monitors in C--' Thread RSS Feed

Problem with monitors in C--

Wed, 27 May 2009 07:59:23 -0700

Hello, I started studying BACI recently and I am having some trouble with monitors.
I know this is not strictly C/C++ but I did not know where to post this. I hope someone can help.

I made a very simple application that shows what my problem is:

monitor monSimple
{
    int variable;
   
    void method1()
    {
        cout << "method1: variable = " << variable << endl;
        variable++;
        cout << "method1: variable = " << variable << endl;
    }
   
    void method2()
    {
        cout << "method2: variable = " << variable << endl;
        variable++;
        cout << "method2: variable = " << variable << endl;
    }
    
    init
    {
        variable = 0;
    }
    
} //monitor


main()
{
    cobegin 
    {
        method1();
        method2();
    }
} // main

I expect the output to be:

method1: variable = 0
method1: variable = 1
method2: variable = 1
method2: variable = 2

or

method2: variable = 0
method2: variable = 1
method1: variable = 1
method1: variable = 2.

However, the real output is either:

method1: variable = 0
method1: variable = 2407
method2: variable = 0
method2: variable = 2607

or:

method2: variable = 0
method2: variable = 2607
method1: variable = 0
method1: variable = 2407.

What am I doing wrong?

Re: Problem with monitors in C--

Wed, 27 May 2009 09:14:43 -0700

This is the first time I hear of this "BACI". While I have no idea of the syntax, my guess is that the variable isn't initialized to zero properly.

Re: Problem with monitors in C--

Wed, 27 May 2009 15:40:14 -0700

This is their website.

In case it helps, if the main is changed from:

main()
{
    cobegin 
    {
        method1();
        method2();
    }
} // main

to:

main()
{
   method1();
   method2();
} // main

Then it outputs the 'expected' result (and the value in the init block initializes the variable properly in this case). I think this proves the monitor's code right. There could be some interference between cobegin and monitor.

A monitor is a block so that functions on it will only execute one at a time, basically. Ideal for mutual exclusion problems.
A cobegin a block whose functions will be executed concurrently. For example, you don't know which of the functions will be executed first.

Calling monitor functions inside a cobegin block, I am just trying to make a program where any of the monitor functions may be called first, but only one is running at a time.

Re: Problem with monitors in C--

Wed, 27 May 2009 16:34:06 -0700

It works now.

monitor monSimple
{
    int variable;
   
    void method1()
    {
        cout << "method1: variable = " << variable << endl;
        variable++;
        cout << "method1: variable = " << variable << endl;
    }
   
    void method2()
    {
        cout << "method2: variable = " << variable << endl;
        variable++;
        cout << "method2: variable = " << variable << endl;
    }
    
    init
    {
        variable = 0;
    }
    
} //monitor


void method1aux()
{
    method1();
}

void method2aux()
{
    method2();
}

main()
{
    cobegin 
    {
        method1aux();
        method2aux();
    }
} // main

Apparently you can't call monitor functions directly from inside a cobegin block (which is something I found by trial and error, it's not in the documentation as far as I can see). So I just enclosed the calls in auxiliary functions and it works.

Re: Problem with monitors in C--

Thu, 28 May 2009 05:42:36 -0700

That language appears to be quite obscure :)