Experimental Physics and
Industrial Control System

Eric Norum <[email protected]> · Thu, 28 Oct 2010 14:44:24 -0700

Well, the vxWorks and RTEMS implementations of epicsEvent use simple binary semaphores.  The POSIX implementation also looks like it will also provide the semantics of a simple binary semaphore.   The WIN32 version I have no idea about.  
Assuming that the WIN32 does provide the desired semantics maybe we should update the documentation to state that this is how epicsEvents are to work.

On Oct 28, 2010, at 2:30 PM, Ben Franksen wrote:

> The documentation of libCom/osi/epicsEvent in the Developer's Guide 
> carefully avoids saying anything about what happens if an event gets 
> signalled and more than one thread waits for the event to happen.
> 
> The name "event" and most of what's written in the docs suggest that 
> *any* thread waiting for an event will be able to continue as soon as 
> the event gets signalled.
> 
> If this is true, how do I get the effect of a (binary) semaphore, i.e. 
> only one thread waiting on the semaphore will continue, the others have 
> to wait until the semaphore is given again?
> 
> Or, if it is false, i.e. epicsEvent really acts like a binary semaphore, 
> I suggest that this be more clearly stated in the docs. It should also 
> be stated how the library or system choses the thread to run; in 
> VxWorks, this can be influenced when creating the semaphore (FIFO or 
> priority based), but if other systems do not allow such a distinction 
> then it should be explicitly stated in the documentation that no 
> assumptions should be made about which thread is chosen. Note that this 
> severely complicates things if you want e.g. FIFO semantics, because 
> you'd have to implement all the queueing yourself.
> 
> Thanks
> Ben

-- 
Eric Norum
[email protected]

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System