Null-modem emulator is described as 'The Null-modem emulator (com0com) is a kernel-mode virtual serial port driver for Windows. You can create an unlimited number of virtual COM port pairs and use any pair to connect one COM port based application to another'. There are four alternatives to Null-modem emulator for Windows and Linux. The best alternative is Virtual Serial Port Driver. It's not free, so if you're looking for a free alternative, you could try Free Virtual Serial Ports or Virtual Serial Ports Emulator.
Virtual Serial Port Driver is a powerful technology designed specifically for those who develop, test, or debug serial port software and hardware. This solution will provide your system with as many virtual COM interfaces as you need.