Examples of using different system calls for sending/receiving CAN frames under Linux
=====================================================================================

There are two types of programs: candump-* and cansend-*. Candump
receives the messages from a CAN interface and prints their content to
stdout. Cansend sends one or more CAN frames to the specified CAN
interface. Variants of each program differ in which Linux system calls
they use to accomplish their task.

The syntax of candump commands is:

    candump-xxx <CAN interface>

For example:

    candump-read can0

The syntax of cansend commands is:

    cansend-xxx <CAN interface> <CAN frame>...

For example:

    cansend-write can0 123#4567 321#987654

The individual programs are described below/

- candump-read.c - The simplest possible candump implementation using
  read() system call.

- candump-recvmmsg.c - Implementation based on recvmmsg() system call.
  Here, one system call can receive "multiple messages". Therefore, in
  case of highbandwidth CAN traffic and slow CPU, this can
  significantly improve system throughput.

- candump-mmap.c - Implementation based on PF_PACKET protocol family
  and memory mapped ring buffers. The performance is slightly better
  than of recvmmsg. If needed, reception can be happen completely
  without any system call, because the kernel automatically stores
  received CAN frames in memory mapped to the user space.

  Bug: When this is used with a virtual CAN interface (vcan), each
  message is received two times.

- cansend-write.c - The simplest way to send CAN frames.

- cansend-sendmmsg.c - Sends multiple CAN frames with one system call
  and thus reducing the overhead. Note that the sendmmsg() system call
  is only available in Linux 3.0 and later.

- cansend-mmap.c - Implementation based on PF_PACKET protocol family
  and memory mapped ring buffers. In terms of performance it is
  similar to sendmmsg().

  Bug: This method does not work with virtual CAN (vcan) interfaces.

More information about these system calls and their performance in the
context of CAN bus can be found in
https://rtime.felk.cvut.cz/can/can-syscalls.pdf.