How can I determine the amount of write/output buffer space left on a linux serial port?

4
votes

You can determine how much data is available to read from a serial port under linux using an ioctl. Is it possible to determine how much buffer space remains for the serial port when writing to it? Effectively I want to write a block of data to a serial port, succeeding only if it can all be offloaded in one go, or failing if it would have to be chunked. The writes and reads to the ports are non-blocking. I'm not expecting this to be the UARTs buffer, but the kernels memory buffer ahead of the UARTs buffer (I guess).

3
clinuxserial-portbufferioctl
I suspect it comes down to the hardware tx/rx FIFO size, which is abstracted from you by the kernel... but does the kernel just hold it in an internal buffer when it's too much for the HW? That I do not know. – Mike
That's what I expect, so there must be some upper bound on that kernels internal buffer. A bound I was expecting to be able to retrieve. – Joe
It's likely worth re-evaluating the problem definition and considering alternative design options. Maybe an asynchronous IO scheme would be better. Or open w/O_NONBLOCK, look for EAGAIN. – Brian Cain
Effectively I'm trying to replicate the functionality send() offers for sockets, i.e. it all goes or none goes. I'm abstracting the physical transport away from the rest of the application. The whole system is already non-blocking, so I know that the write won't block, I just don't know if it will offload all the data I want to kernel space. – Joe
Ok, so try O_NONBLOCK. – Brian Cain

3 Answers

5
votes

You can determine the amount of write/output.

For read:

ioctl(device_handler, TIOCINQ, &bytes);

For write:

ioctl(device_handler, TIOCOUTQ, &bytes);

Size of FIFO buffer:

serial_struct serinfo;
memset(&serinfo, 0, sizeof(serinfo));
ioctl(device_handler, TIOCGSERIAL, &serinfo);
serinfo.xmit_fifo_size;

Regards, VA.

0
votes

The serial port is a character device not a block device. It has no buffer. Character devices ( such as Serial port, keyboard, mouse) only writes and reads a character not a word. For exame if you listen to a serial someone writes " have a nice day ", if you do not listen from the time he starts typing, you would not see the entire phrase. You would see only the characters typed when you listen

0
votes

If you access serial port using file descriptor, you can use select for checking whenever the descriptor is ready for non-blocking write. I don't know whenever this works for serial ports. I used this for TCP sockets and it worked.