int MLMain(int argc, char **argv) sets up communication between an external program started using Install and Mathematica.
The code for MLMain() is generated automatically by mprep or mcc.
MLMain() opens a MathLink connection using the parameters specified in argv, then goes into a loop waiting for CallPacket objects to arrive from Mathematica.
MLMain() internally calls MLOpenArgv().
See Section 2.13.3.