ucos-II semaphore doubt

[ajrox]

In ucos-II semaphore can be used as an event notifying flag/signalling event.

It's declaration is OSSemCreate(0) as per the book MicroC/OS-II: The Real Time Kernel by Jean J. Labrosse.

How can a semaphore have an initial count of 0?

Also can anyone provide an example code for semaphore as event signalling flag?

 

[bhadanisandip]

Suppose we have two running processes and both of them access same interface. Both porcesses are independent of each other. So it is possible that both the process starts driving the interface at the same time. If both processes are driving interface at the same time we will get wrong data on interface until and unless both processes are driving same data on same time !! But that is not going to happen anyway. This type of situations can be managed by keeping some flag that indicates that interface is free or not and our process starts if and only if interface is free. But what will happen if you have say m number of processes that is driving n (n < m) number of interfaces. For this kind of senariaos it will be dificult to syncronize between processes.
For such application semaphore is the best options.


Lest assume that in some BFM two process write_data () & read_data() this both tasks are getting called randomly and they are using same interface to complete read or write operation. For such type of situation we can implement our read and write progreamn following way.
class semaphore_example;

  semaphore smphr = new(1);

    task read();
       smphr.get(1);       

       // Code to read data from interface

       smphr.put(1);
    endtask : read


task write();
       smphr.get(1);
       
       // Code to write data from interface

       smphr.put(1);
 endtask : write

      
endclass : semaphore_example

In the above example whenever the read or write task is getting called it first gets key form semaphore using smphr.get(1) task and than starts driving the interface and once done with the interface puts the key to the semaphore. This way whenever any task/process gets key from semaphore it is certain that interface is not driven by other process.