>In my experienced, I never had trouble with CreateProcess. Just be careful if you set "No Window" in Startup_Info, your process must quit it's process correctly without user intervention, or else you have to kill it with Ctrl-Alt-Del or programmaticaly.
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>I'm not really sure what you mean with "pass data". Can you just pass it as a string (by refference) in a parameter ?

No, you need to use a mechanism to marshall the memvar across a process boundary in both directions; both to pass the inbound parameter value not as a value, but as a shared memory reference, and again to return any alterations through shared memory. The simplest mechanism for such direct transfers is memory-mapped file I/O.