By using ioctl, at least on Linux. The various serial port modules take care of that when you use them. I think Windows has something similar, RTFM. In the worst case, you get the OS defaults for the port.
Note that /dev/S01 is usually not your first serial port. That would be named /dev/ttyS0, and the second one /dev/ttyS1. USB converters usually appear as /dev/ttyUSB0, /dev/ttyUSB1, and so on. (/dev/S01 seems to be a typo or a system-specific device.)
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)