Linux serial port reading - can I change size of input buffer?

Question

I am writing an application on Ubuntu Linux in C++ to read data from a serial port. It is working successfully by my code calling select() and then ioctl(fd,FIONREAD,&bytes_avail) to find out how many bytes are available before finally obtaining the data using read().

My question is this: Every time select returns with data, the number of bytes available is reported as 8. I am guessing that this is a buffer size set somewhere and that select returns notification to the user when this buffer is full.

I am new to Linux as a developer (but not new to C++) and I have tried to research (without success) if it is possible to change the size of this buffer, or indeed if my assumptions are even true. In my application timing is critical and I need to be alerted whenever there is a new byte on the read buffer. Is this possible, without delving into kernel code?

Answer 1

You can try to play with the VMIN and VTIME values of the c_cc member of the termios struct. Some info here, especially in the section 3.2.

Answer 2

You want to use the serial IOCTL TIOCSSERIAL which allows changing both receive buffer depth and send buffer depth (among other things). The maximums depend on your hardware, but if a 16550A is in play, the max buffer depth is 14.

You can find code that does something similar to what you want to do here

The original link went bad: http://www.groupsrv.com/linux/about57282.html The new one will have to do until I write another or find a better example.