Yes, for example if I want to set the serial device baud rate(TCSETS),
should I open it with O_WRONLY?
Linux has the concept of read IOCTLs and write IOCTLs as determined by
the IOCTL macros. My understanding is that you much provide O_RDONLY or
O_RDWR to use the read IOCTLs and O_WRONLY or O_RDWR to use the write
IOCTLs.
But I am not sure of that and there are a lot of things I don't fully
understand. I have seen references to some mysterious Bit 3 in the open
flags that can be used open IOCTL-only drivers. Like
https://github.com/NoHomey/open-ioctl:
"Opens device file in non-blocking ioctl mode. File is opend with
flags 3 | O_NONBLOCK.
"Flag 3 means that only ioctl calls can be made for comunication
with the device driver (remember read/write operations are expensive
this is why open-ioctl was made in first place to make it easer for
performance and command oriented device drivers)."
My gut feeling is that these things are too non-standard and poorly
thought out to be useful.
