I would like to know wich arguments do I have to give to the devctl function to know how many letters there is in the buffer of the serial port.
Would be better if you would explain why you need this. Maybe there are better alternative. Usually this is a bad idea because by the time you’d get the info it may be obsolete.
That being said I don’t know the answer to your question, sorry. Took a quick look in the doc and didn’t find anything. Well you can read it and then put it back but that’s going to be very expensive.
There must be a header file like devctl_ser.h or something that contains all valid DCMD_* macros that you can possibly use. Many of them have tc*() cover functions.