RPG (by default) passes parameters by reference. C (by default) passes parameters by value. To pass a parameter by reference from C, you put a pointer on the prototype, and you pass the address of a variable to that pointer.
RPG's *OMIT is equivalent to passing a NULL pointer. So, from C, you simply pass NULL instead of a variable. For example: prototype: void procname(char *param1, char *param2, char *param3, char *param4); code: char val1, val2, val3, val4; // set the "val" variables to useful values here procname( val1, NULL, val3, val4); - or - procname( NULL, val2, val3, val4); etc, etc.