ttyUSB0 access from user code: Debian

I’m working with embedded boards that run Debian. They have an RS232 port (ttyS0) plus a USB port. I use USB for common FTDI serial adaptors (RS232), they appear as /dev/ttyUSB0 as you’d expect, and from picocom they work fine. However, when I access them from my own coded (C) applications they don’t. I know that my code is OK because it works fine with the ttyS0 port, the problem appears to be an access block in the OS between the USB serial and my application.

ttyUSB0 opens fine in my code, there are no error messages. Just no apparent connection to the serial lines.

Is there something I need to do to get ttyUSB0 working with my programs? Thanks for any advice

What are the differences in attributes of ttyS0 and ttyUSB0?
(baud, parity, start-stop bits, etc.).
Ownership and permissions (and SELinux) also matter.

I can use both since my regular user is a member of the dialout group.

$ ls -l /dev/ttyS0 /dev/ttyUSB0
crw-rw----. 1 root  dialout   4, 64 Dec 13 11:07 /dev/ttyS0
crw-rw----+ 1 USER  dialout 188,  0 Dec 16 09:50 /dev/ttyUSB0

Does your program run either as root or as a user which is a member of the dialout group? The program may need to execute 'suid'.

Thanks a lot, your reply was very useful.