If I start it with system(), I don't see the output.

Really? If that is true, it's not really the fault of system - if you use system to call a program that writes to STDERR or STDOUT, than it will be connected to the programs STDERR or STDOUT. Could it be that your program is redirecting them? Or perhaps the called program is quiet if it isn't ran interactively? Or are you suffering from buffering?

Without knowing why you aren't seeing the output, I cannot tell you what to do to see it.