Open a file in a process already running

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C++

I have a program that I am opening files in via system() calls from c++. When it opens for the first time, it runs about 5 minutes of tests before it can start.

The command line argument I'm using to open these files is "program.exe -lfile.mpt".

The problem I'm having is that this program does not allow two instances open at once, but if I close it and reopen it, all the start up tests would have to be ran again.

So what I'm looking for is a command line argument I can use to open a file in this program on a process that is already running. I'm assuming it will be something along the lines of "(PID) -lfile.mpt".

Thanks in advance,
Pat

EDIT:
What I have tried so far is "net share mpt=c:\MPT" (sharing the directory where programs.exe is). I'm not sure where I should go from here, anyone have any ideas?