TTY Output space left on serial
 

I and using a serial port and I would like to find how much bytes can still be sent to the output queue. I can find how much bytes are in the output queue with the ioctl TIOCOUTQ. I need to know what is the serial output buffer size to find how much bytes can still be used (memory left) in the queue at all time. Any ideas?

Output queue left = Output queue size - Qty of data in queue

Yves plasse.