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iodrivers_base

This package mostly contains a generic implementation of a packet reassembly logic. It takes byte-oriented I/O and allows you to turn this stream of bytes into a stream of well-defined, validated packages to be interpreted by a higher-level logic (e.g. data demarshalling).

The core problem this aims at solving is the (lot of) misunderstandings that stems from the complexity of low-level I/O. Additionally, it provides an I/O independence layer that is used to build a test harness for C++ libraries. Finally, within the Rock oroGen integration, it allows to monitor and log byte-level I/O.

Within Rock, it is highly encouraged to use this package to build custom device drivers, even for protcols that are naturally packet-based such as e.g. UDP, as the driver packet-extraction logic should do as many sanity checks on the bytestream as possible.

Functionality

The main functionalities that iodrivers_base provide are:

  • infrastructure to convert byte-oriented streams into streams of validated packages
  • library-level test harness
  • generic oroGen integration (for Rock users)
  • built-in support for multiple type of I/O:
    • serial
    • udp server: listens on a UDP port. Sends to the IP of the last received UDP message.
    • udp client: connects to a given IP and port.
    • tcp client: connects to a given IP and port.
    • file-based (e.g. for named pipes or Unix sockets)

Note that iodrivers_base can (and maybe should) be used even for datagram-oriented mediums such as UDP. These mediums do not need the packet-reassembly, but do need packet validation.

Writing a device driver using iodrivers_base::Driver

One starts by subclassing iodrivers_base::Driver and providing the size of the driver's static internal buffer. This buffer should be a multiple of the maximum packet size (twice as big is usually enough):

class Driver : iodrivers_base::Driver
{
    static const int MAX_PACKET_SIZE = 32;
    static const int INTERNAL_BUFFER_SIZE = MAX_PACKET_SIZE * 4;
public:
    Driver();

};
Driver::Driver()
    : iodrivers_base::Driver::Driver(INTERNAL_BUFFER_SIZE);
{
}

The second required step is to implement iodrivers_base::Driver::extractPacket. This is the method that tells iodrivers_base's internal logic what is meaningful data and what isn't.

See below the pseudo-code implementation of most extractPacket methods. Note that the amount of validation you do in extractPacket may vary, but usually "more is better", as it is validation that won't have to be done later. The return value is fully documented within the method documentation. Check it out for a complete specification.

int Driver::extractPacket(uint8_t const* buffer, size_t buffer_size) const {
    if (not enough bytes in buffer to contain a start code) {
        return 0; // wait for new bytes
    }

    if (buffer does not start with packet start code) {
        for (int i = 1; i < buffer_size; ++i) {
            if (packet start code found at i) {
                return -i; // skip i bytes from buffer and call again
            }
        }
        return -buffer_size; // discard the whole buffer
    }

    if (not enough bytes in buffer to validate packet starting at zero) {
        return 0; // wait for new bytes
    }
    else if (packet starting at zero is valid) {
        return packet_size;
    }
    else {
        return -1; // discard first byte and start searching again
    }
}

Supported URIs

All URIs follow the general format scheme://NAME:NUMBER?option1=value1&option2=value2

Which parts of the URI is accepted by which scheme is detailed below

serial://

Open a serial device and configure it. A basic serial URI is serial://${DEVICE_PATH}:${BAUDRATE}. Note that absolute paths lead to having three consecutive slashes (two for the :// and one for the path)

The serial URIs accept the following options:

  • byte_size byte size in bits, from 5 to 8. The default is 8
  • parity parity, either none, even or odd. The default is none
  • stop_bits stop bits (either 1 or 2). The default is 1

Examples:

  • serial:///dev/ttyUSB0:115200
  • serial:///dev/ttyUSB0:115200?parity=even

udp://

Open an UDP socket on a random port which sends to the given host and port.

If the local_port option is given, the socket is bound to the given local port

Examples:

  • udp://localhost:4000
  • udp://localhost:4000?local_port=4001

The Connection Refused error If configured to do so, UDP streams will report a connection refused error if there are no processes listening on the configured remote peer. This is controlled by the ignore_connrefused parameter which has to be set to 0 or 1. For backward compatibility reasons, the default behavior of UDP streams with respect to this option is complex, see below for details.

Note that the connection refused error depends on the reception of a ICMP message, sent by the remote host. The error might not appear at all if this message is not sent by the remote peer, or if the ICMP message is blocked. Moreover, the error reporting in this case is asynchronous and will be reported on follow-up writePacket or readPacket after the ICMP message is received, that is after the actuall writePacket call that generated the error in the first place.

Connected UDP sockets If configured to do so, UDP streams are connected, that is will only accept packets from the configured remote host. If unconnected, they will receive from any host (but still send to the configured host). In addition, sending a lot of packets to the same host will have a better performance with connected sockets. This is controlled by the connected parameter which has to be set to 0 or 1. For backward compatibility reasons, the default behavior of UDP streams with respect to this option is complex, see below for details.

Default connected and ignore_connrefused parameters

For backward compatibility reasons, the default values for connected and ignore_connrefused depends on whether the UDP stream was created with or without a local port. The behavior described below is deprecated. In the future, the default will be set to connected=1 and ignore_connrefused=1. Warnings are currently issued when the current defaults are used. Explicitely set these parameters to shut the warnings and ensure your code will continue working as-is when the defaults change.

  • ignore_connrefused may be set to zero (to report connrefused) only on connected sockets. Attempting to set ignore_connrefused=0&connected=0 will throw in openURI

  • setting connected=0 automatically sets ignore_connrefused=1 even if the default (as described below) would be 0

  • UDP sockets for which the local port is unspecified (without the local_port option) are configured with connected=1 and ignore_connrefused=0 by default.

  • UDP sockets for which the local port is given (with the local_port option) are configured with connected=0 and ignore_connrefused=1 by default.

Host and Network Unreachable errors If configured to do so, UDP streams will either report or ignore host and/or network unreachable errors, as reported by the underlying socket implementation This is controlled respectively by the ignore_hostunreach and ignore_netunreach parameters which have to be set to 0 or 1. The default is to not ignore the error.

Note that the reliability of this error reporting is complex. They may be reported locally if the localhost routing tables do not provide a link to the target, but may also depend on the reception of a ICMP message, sent by routers on the path to the remote host. In this latter case, the error might not appear at all if this message is not sent by the remote peer, or if the ICMP message is blocked. Moreover, the error reporting in this case is asynchronous and will be reported on follow-up writePacket or readPacket after the ICMP message is received, that is after the actuall writePacket call that generated the error in the first place.

The recommendation is to keep the default in situations where the path to the remote host should always be available - i.e. wired scenarios. Disable both of them when using wireless links, as some wireless routers report these errors when the link is down.

udpserver://

Passively listens to UDP packets on a given port. Writing to the driver will send data back to the last UDP client whose packet was received (and does nothing if nothing has been received yet).

Examples:

  • udpserver://5000

tcp://

Open a TCP connection to the given remote host and port

Examples:

  • tcp://localhost:5000

file://

Open a file. Note that absolute paths lead to having three consecutive slashes (two for the :// and one for the file)

Examples:

  • `file:///path/to/file

Test harness

This package provides a testing harness that allows you to write integration tests drivers based on iodrivers_base::Driver.

From a design perspective, one should start by separating the protocol implementation (including the extractPacket logic by iodrivers_base::Driver) into a separate set of stateless functions. This ensures they will be fully and easily testable.

The test harness, then, allows you to check the Driver class logic from the perspective of the device, that is by checking what is being sent by the driver, and sending data to it. The goal is to verify the Driver's logic, not the protocol parsing since this one has been implemented in separation (and separately tested).

Check the documentation of iodrivers_base/Fixture.hpp for more information on how to use the harness.

Command-line tools

This package provides two command-line utilities:

  • iodrivers_base_forward forwards one data stream to another. Both streams are defined by iodrivers_base's URIs
  • iodrivers_base_cat outputs the data from a stream to stdout, in hex and ascii formats

For anything more complicated, we recommend usage of socat

Gotchas

  • do not overload openURI. This will make testing harder (you have to "mock" whathever is being done in openURI for each test), and will definitely reduce the usefulness of your driver. Finally, it is incompatible with Rock's oroGen integration for iodrivers_base.

Design Guidelines

See this document

License

This software is licensed under the GNU LGPL version 2 or later

Copyright 2008-2017 DFKI Robotics Innovation Center 2014-2017 SENAI-CIMATEC 2017-2019 13 Robotics 2019 TideWise

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Common implementation for packet-based I/O on POSIX systems

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