Serial Port Programming Part 2 - Canonical Mode vs Non-Canonical Mode

In Linux, Serial Port input can be configured in two ways:

1. Canonical Mode

2. Non-Canonical Mode

Canonical Mode: In canonical mode, read on the serial port will not return until a new line, EOF or EOL character is received  by the Linux Kernel. In this more, read will always return an entire line, no matter how many bytes are requested.

Non-Canonical Mode: In non-canonical mode, read on the serial port will not wait until the new line. Number of bytes read depends on the MIN and TIME settings.

MIN: Minimum number of bytes that must be available in the input queue in order for read to return

TIME: How long to wait for input before returning, in units of 0.1 seconds.

MIN and TIME	Behavior
MIN = 0, TIME = 0	Read returns immediately with as many characters available in the queue, up to the number requested. If no characters in the queue, read returns 0
MIN = 0, TIME > 0	Read waits for TIME time for input to become available, if we receive a single character in the TIME duration, read returns immediately. It returns as many characters up to the number requested. If timer expires and there is no receive characters, read returns 0
MIN > 0, TIME = 0	Read waits until at least MIN bytes are available in the input queue. At the time, read returns as many characters are available up to the number requested.
MIN > 0, TIME > 0	Read returns until either MIN bytes have arrived in input queue, or TIME elapses with no further input. read always block until the first character  arrives, even if TIME elapses first.

 

Setting of canonical or non-canonical mode is determined by the ICANON flag in the c_lflag of the struct termios 

The MIN and TIME can be set in the c_cc[VMIN], c_cc[VTIME] of the termios structure.

When you open serial port in minicom/cutecom, it configures the input mode to non-canonical mode, you can observe it by running 'stty -F <port> -a' after you opened the port in minicom/cutecom

For this code to work, I connected two PL2303 USB to serial device  (TX -> RX, RX->TX)

Note: '\r' is Carriage Return , '\n' is Line Feed

C Code Demonstrating  Canonical Mode

canonical_write.c:



canonical_read.c:

Output

You can observe when you run the code, only when the new line character is transmitted by the write code, the read function returns.

C Code Demonstrating  Non-Canonical Mode

non_canonical_write.c

non_canonical_read.c

Output:

You can observe when you run the code, for each two characters transmitted, read returns with data