How can I determine the amount of write/output buf

Question

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).

Answer 1

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.

Answer 2

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

Answer 3

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.