On Sunday, 96/06/16, Francois Thunus wrote to All about "semaphore" as
follows:

FT> I don't want 2 copies of my program to run at the same time.
FT> I use DosCreateMutEx to create a semaphore when starting 
FT> the program, and DosDeleteMutEx when I'm done. or do I ?. 

Hi Francois,

For a single resource, in this case your program, a mutex semphore is
correct. A semaphore is destroyed when the last handle using it is
closed.

So your strategy is to attempt to create the semaphore, with the
obtain option coded. If the create fails, open the semaphore to obtain
a handle and then wait on it (this will be for the 2nd and subsequent
active instances of the program). At the end of the program close the
semaphore, which will unblock the next instance of the program if there
is one. They should queue in a FIFO manner. The last closure will
destroy the semaphore.

If you've done any assembler programming on IBM mainframes then the
ENQ and DEQ macros of MVS will provide you with a very good background
in how mutex semaphores work.

FT> Should I use DosCreateMuxWaitSem
FT> instead ?

No. Muxwait semaphores are for multiple resources and/or events.

Regards

Dave


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