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In Linux I can write and read data from an USB device by calling C's fopen('/dev/ttyUSB0', 'rw')

Specifically what is the equivalent of the directory "/dev/ttyUSB0" in windows I'd like to do the same in windows for COM3.

Marco Ramirez Castro
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If you are using a runtime environment like Cygwin or msys-2.0.dll which provides a POSIX compatibility, you can run ls /dev/tty* in the shell provided by the environment to see what kind of entries you get. It looks like COM3 would correspond to /dev/ttyS2, at least with msys-2.0.dll.

If you are writing a native Windows program, you should be able to open "COM3" with fopen or CreateFile. Using CreateFile is probably better than fopen because it returns a native Windows handle which allows you to use the SetCommTimeouts and SetCommState API functions. COM ports higher than COM9 need a prefix of \\.\, which is written as "\\\\.\\" in C because we need to escape the backslashes.

David Grayson
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    Please also note that using `"\\\\.\\"` is always possible for devices (also for COM1-COM9): `CreateFile("\\\\.\\COM1", ...)` – Martin Rosenau Apr 09 '20 at 11:37