Celebrating 10 years – my top 10 blog articles

It’s difficult to believe we’ve been writing articles for 10 years.  In that time I’ve written over 90 technical articles on C, C++ and embedded system design.

To celebrate I’ve picked my ‘Top 10’ articles, with a little background into why I enjoyed writing them so much, or the story behind them.

So, sit back, cue up At the Sign of the Swinging Cymbal“* and enjoy.


(* This really dates me!)

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Posted in C/C++ Programming, Design Issues, Testing, UML | Tagged , , , , , , , | Leave a comment

Using a Raspberry Pi as a remote headless J-Link Server

Here at Feabhas we tend to favour using Segger J-Link’s as our ‘go-to’ solution for target flashing and debug, as they fall into that category of tools that just work.

As part of our ongoing work around Agile and CI (Continuous Integration), we’re always interested in addressing that challenging step of automating target based test in a cost-effective manner.

The Raspberry Pi (RPi) is a ubiquitous low-cost platform for numerous tasks. One useful tasks that it can be used for is as a network-based conjugate between a client machine and a target board, where the target board is connected to the RPi via a local J-Link.

Setting up the RPi

The Segger J-Link utilities are available for ARM-based systems, which means it should work on any Raspberry Pi. However, we have, so far, only tested it on a RPi3+, but intend to try the PiZero as well.

As we are using the RPi in a headless setup we installed Raspbian Stretch Lite, connected an HDMI screen plus keyboard and booted up the Pi.

We then proceeded through the usual Pi setup

  • ensured all software was updated
  • changed the pi user password (I know many people go further and swap out pi for a different user)
  • enabled SSH (this becomes useful later when remote working from a client):
$ sudo raspi-config
--> 5 Interfacing Options  Configure connections to peripherals  
--> P2 SSH         Enable/Disable remote command line access to your Pi using SSH 

In addition, make a note of the RPi’s IP address (assuming you’re using the WiFi interface), as we’ll need that later to connect:

$ ip addr show wlan0

and make a note of the inet number (e.g. 192.168.0.XXX or similar), or use ifconfig.

Installing the J-Link utilities

Connect your J-Link via USB to the Pi and to your target board via the appropriate header. Assuming SSH was set up we can now work remotely from the client machine.

On our client machine, log in to the RPi via ssh:

 $ ssh pi@192.168.0.XXX
pi@192.168.0.XXX's password: 
Linux raspberrypi 4.19.50-v7+ #1234 SMP Thu Jun 13 11:06:37 BST 2019 armv7l

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Thu Jun 20 10:27:31 2019 from 192.168.0.YYY
pi@raspberrypi:~ $ 

In the pi user home directory, download and un-tar the Segger utilities for the Pi. Then configure the udev rules as per README.txt file in the JLink_Linux_V646g_arm directory.

$ wget --post-data 'accept_license_agreement=accepted&non_emb_ctr=confirmed&submit=Download+software' https://www.segger.com/downloads/jlink/JLink_Linux_arm.tgz
$ tar xvf JLink_Linux_arm.tgz
$ cd JLink_Linux_V646g_arm/
$ more README.txt
$ sudo cp 99-jlink.rules /etc/udev/rules.d/
$ sudo reboot

This will close the ssh connection.

Starting the RPi’s J-Link Server Software

Re-login to the RPi via ssh.

Under the directory JLink_Linux_V646g_arm there are a number of utilities. As we are running from the command line we need to invoke the application JLinkRemoteServerCLExe on the RPi:

$ ./JLink_Linux_V646g_arm/JLinkRemoteServerCLExe -Port 19020
SEGGER J-Link Remote Server V6.46g
Compiled Jun 14 2019 19:38:47

'q' to quit '?' for help

Connected to J-Link with S/N xxxxxxxxx

Waiting for client connections... 

Reading the documentation, the -Port 19020 shouldn’t be required as it’s supposedly the default port, however, at the time of writing without specifying this the client connection always fails.

Connecting the client via CLI

On the client machine, download and install the Segger J-Link Software and Documentation Pack specific for your host OS (I’m using macOS).

Once installed, simply type the following (substituting the IP address as appropriate):

$ JLinkExe ip 192.168.0.XXX
SEGGER J-Link Commander V6.46g (Compiled Jun 14 2019 19:36:04)
DLL version V6.46g, compiled Jun 14 2019 19:35:53

Connecting to J-Link via IP...O.K.
Firmware: J-Link V10 compiled Jun 14 2019 19:25:26
Hardware version: V10.10
S/N: xxxxxxxxx
License(s): FlashBP, GDB

Type "connect" to establish a target connection, '?' for help

On the RPi you should see:

Conn. 8: Client connected.
Connected to J-Link with S/N xxxxxxxxx

To disconnect the client, simply type qc at the J-Link command line and the RPi should go back to:

Waiting for client connections... 

Connecting a client debugger

If the J-Link remotely connects correctly, then it is also possible to use tools such as Segger’s Ozone to do remote target debugging. For example, in Ozone simply set the IP address of the remote RPi in the J-Link Settings under the Tools menu.

You can then download and debug an image as if the target was local to the client machine:

Remote-remote connection!

Back in February if this year (2019), Segger announced a new feature call tunnelling. This enables remote connection to a J-Link Server via the internet.

To use this utility, then on the RPi, type:

$ ./JLink_Linux_V646g_arm/JLinkRemoteServerCLExe -UseTunnel -TunnelByName <some_name> -TunnelPW <some_password>


$ ./JLink_Linux_V646g_arm/JLinkRemoteServerCLExe -UseTunnel -TunnelByName feabhas_jlink_42 -TunnelPW monkey

And before you ask, no we don’t use monkey as our password 😉

On the client connect via:

$ JLinkExe ip tunnel:feabhas_jlink_42:monkey

Full details can be found at https://www.segger.com/products/debug-probes/j-link/tools/j-link-remote-server/.

Note: I was unsuccessful getting this to work with Ozone


It is important to note, that for the Linux ARM utilities, Segger clearly state that the : This package comes without any support.

I have found that quite regularly once I have quit the client I am then unable to reconnect, with the following error:

$ JLinkExe ip 192.168.0.XXX
SEGGER J-Link Commander V6.46g (Compiled Jun 14 2019 19:36:04)
DLL version V6.46g, compiled Jun 14 2019 19:35:53

Connecting to J-Link via IP...FAILED: Can not connect to J-Link via TCP/IP (192.168.0.XXX, port 19020)

And the Server application still thinks the client is connected. The only option I’ve found is to type q in the RPi session which quits JLinkRemoteServerCLExe and then to restart it from the command line. This is the major reason for using SSH as it allows JLinkRemoteServerCLExe can be restarted remotely.

The same issue was persistent when using tunnelling and, obviously, having no SSH connection to the RPi it proved pretty useless (if I use our VPN to connect to the RPi it negates the benefit of tunnelling).

However, others have indicated they have had no such problems (it may be something LAN specific?).


The use of a Raspberry Pi acting as a network-connected conduit to a target board is very attractive. RPi’s are easy to source and very inexpensive. Combined with Segger’s J-Link utilities, including applications to handle GDB and SWO, then it may provide a good platform for taking many CI pipelines to their next step.

Unfortunately, the issues of failed reconnection do seem to only be happening with the Linux Arm utilities. Running the same Remote Server commands a Linux laptop proved reliable, both on the local LAN and using tunnelling.

As Raspberry Pi’s support Docker, then my next angle of attack is to see if there is a way I can run the server software in a docker container, which would allow for simpler restarts without needing the SSH connection.

For now, we’ll live with the reconnection issue and continue to use SSH, but watch this space…


Posted in Agile, ARM, C/C++ Programming, Testing | Tagged , , | 11 Comments

Python 3 File Paths

If you’ve used Python for a while you will probably be familiar with the os module for working with files and directories; often called pathnames by Linux users. In moving to Python 3 you may continue to use the same os and os.path functions from Python 2.7, however a new pathlib module provides an alternative object-oriented (OO) approach.

In this posting, we examine the common file handling situations; comparing the OO approach of pathlib against the procedural approach of os functions.

Current Directory

To obtain the current working directory with os functions we use getcwd():

import os
cwd = os.getcwd()

The cwd variable is a standard Python string object containing the directory path:


On Windows this would be, for example:


In contrast, the OO approach provided by pathlib is to create a Path object with no arguments:

from pathlib import Path
cwd = Path()

Path objects encapsulate the concept of a path (the name of a file or directory on disk) but do not necessarily imply that the path exists. A Path object can be queried and used to manipulate pathnames. Converting a Path to a string for printing returns the simplest form of the path which for the current directory is always:


To get the full, or absolute, pathname use:


which will display the same string value as returned by os.getcwd().

Directory Listing

The names in the current directory are returned by os.listdir() as a list of strings:

names = os.listdir()

Normally you’d want to examine these files. In this example, we’ll simply get the file size and modification date. To start with we need to convert each name into a pathname using os.path.join():

for name in os.listdir(cwd):
    path = os.path.join(cwd, name)

We can now use this path variable to access the file on disk. If you’ve read the wrong tutorial you may, in the past, have written:

path = cwd + '/' + name

Which works but isn’t very Pythonic If you’re a Windows user and write:

path = cwd + '\\' + name

then you have missed the part that says Python treats both forward and backward slash characters as directory separators on both Linux and Windows. In other words, it doesn’t matter which form of slash character you use – it’s all the same to Python. Of course, you may have got around the forward/backward slash issue by using the host separator character from the os module:

path = cwd + os.sep + name

None of these concatenation solutions is as clean or maintainable as the os.path.join() function.

Returning to the directory listing example code, we can read the file size and modification timestamp using two separate os.path function calls, e.g.:

import os
From datetime import datetime
for name in os.listdir(cwd):
    path = os.path.join(cwd, name)
    size = os.path.getsize(path)
    mtime = datetime.fromtimestamp(os.path.getmtime(path))
    print(f'{name} {size} bytes, modified {mtime}')

For the pathlib.Path code example we can use iterdir() which is an iterator function, which also fits nicely with our for loop approach. As the iterator yields up Path objects there is no need to join the file name with the parent directory:

for path in cwd.iterdir():

Accessing file information from a Path object is a single method call. This returns all statistics in a single object which is likely to be more efficient than the multiple os.path function calls:

stats = path.stat()

The revised OO code looks thus:

from pathlib import Path
for path in cwd.iterdir():
    stats = path.stat()
    size = stats.st_size
    mtime = datetime.fromtimestamp(stats.st_mtime)
    print(f'{path} {size} bytes, modified {mtime}')

Building Pathnames

We’ve already looked at how os.path.join() is used to build filenames. With Path objects there is a similar joinpath() method used to access files from a directory path.

Consider where we want to access a file called ‘example.settings’ in a ‘conf’ sub-directory. Using os.path.join() we would write:

settings = os.path.join(cwd, 'conf', 'example.settings')

Using a Path object we now write:

settings = cwd.joinpath('conf').joinpath('example.settings')

Alternatively, we could use the / operator on a Path object and a string (or anther Path object):

settings = cwd / 'config' / 'example.settings'

Path objects also support wildcard filename expansion, e.g.:

py_files = list(cwd.glob('*.py'))

The glob() method returns an iterator; which in this case we’ve used to initialise a list containing Path objects.

The traditional approach for expanding wildcards is to use the glob module to get back a list of strings:

import glob
py_files = glob.glob('*.py')

Both versions of the glob() method/function also support recursive wildcards using ‘**’ (but the glob.glob() method requires a recursive=True argument to enable this feature):

py_files = list(cwd.glob('**/*.py'))
py_files = glob.glob('**/*.py', recursive=True)

This capability means that you no longer need to use the cumbersome os.walk() method if all you want to do is find files that match a wildcard pattern.

Manipulating Files

The os module has many functions for manipulating files. The following example checks for a file called example.log in the current directory and then deletes it.

Using the os module:
logfile = './example.log'
if os.path.exists(logfile):

The same solution using the pathlib module

logfile = Path('./example.log')
if logfile.exists():

Directories can be created using the os.mkdir() function or Path.mkdir() method and empty directories removed with the rmdir() function or method.

Opening Files

Finally, Python 3.6 upgraded the os and os.path functions as well as file open() to work with both strings and Path objects: this is formalised as a Path Like Object.

The procedural approach for opening all Python files in the current directory looks like:

for path in glob.iglob('*.py'):
    with open(path) as fp:

The object oriented approach looks like:

for path in Path().glob('*.py'):
    with open(path) as fp:

The Path class has a lot more capabilities in addition to those shown here. It is a more or less complete replacement for all of the os, os.path and glob functions and is described in the Python 3 pathlib documentation.


If you like OO languages such as C++ or Java you’ll probably prefer the Python 3 object-oriented approach using Path object. Whereas if you’re used to the procedural approach of C you may be be more comfortable using the os, os.path and glob functions.

As long as you’re aware that both approaches are available with Python 3 you can make an informed decision over which one to use, rather than continue to use the procedural approach without being aware that there is an alternative.

Posted in Python, Python3 | Leave a comment

Brace initialization of user-defined types

Uniform initialization syntax is one of my favourite features of Modern C++.  I think it’s important, in good quality code, to clearly distinguish between initialization and assignment.

When it comes to user-defined types – structures and classes – brace initialization can throw up a few unexpected issues, and some counter-intuitive results (and errors!).

In this article, I want to have a look at some of the issues with brace initialization of user-defined types – specifically, brace elision and initializer_lists.

Read on for more…

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Posted in C/C++ Programming, General | Tagged , , , , , , , | 1 Comment

Thanks for the memory (allocator)

One of the design goals of Modern C++ is to find new ways – better, more effective – of doing things we could already do in C++.  Some might argue this is one of the more frustrating aspects of Modern C++ – if it works, don’t fix it (alternatively: why use lightbulbs when we have perfectly good candles?!)

This time we’ll look at a new aspect of Modern C++:  the Allocator model for dynamic containers.  This is currently experimental, but has been accepted into C++20.

The Allocator model allows programmers to provide their own memory management strategy in place of their library’s default implementation.  Although it is not specified by the C++ standard, many implementations use malloc/free.

Understanding this feature is important if you work on a high-integrity, or safety-critical, project where your project standards say ‘no’ to malloc.

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Posted in C/C++ Programming, General | Tagged , , , , , , , , , , , | 9 Comments

Python 3 Unicode and Byte Strings

A notable difference between Python 2 and Python 3 is that character data is stored using Unicode instead of bytes. It is quite likely that when migrating existing code and writing new code you may be unaware of this change as most string algorithms will work with either type of representation; but you cannot intermix the two.

If you are working with web service libraries such as urllib (formerly urllib2) and requests, network sockets, binary files, or serial I/O with pySerial  you will find that data is now stored as byte strings.

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Posted in Python, Python3 | 2 Comments

Python 3 Type Hints

The expected end of support for Python 2.7 is 1st January 2020, at least according to Guido van Rossum’s blog post. Starting now, you should consider developing all new Python applications in Python 3, and migrating existing code to Python 3 as and when time and workload permit.

Moving to Python 3

If you are unaware of the changes introduced in Python 3 that broke backward compatibility with Python 2 then there is a good summary on this What’s New In Python 3.0 web page.

The biggest difference you will notice moving to Python 3 is that the print statement is now a print function. But there are plenty of other changes that you should be aware of. This and subsequent blogs will look at aspects of Python has been added or improved in Python 3.

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Peripheral register access using C Struct’s – part 1

When working with peripherals, we need to be able to read and write to the device’s internal registers. How we achieve this in C depends on whether we’re working with memory-mapped IO or port-mapped IO. Port-mapped IO typically requires compiler/language extensions, whereas memory-mapped IO can be accommodated with the standard C syntax.

Embedded “Hello, World!”

We all know the embedded equivalent of the “Hello, world!” program is flashing the LED, so true to form I’m going to use that as an example.

The examples are based on a STM32F407 chip using the GNU Arm Embedded Toolchain .

The STM32F4 uses a port-based GPIO (General Purpose Input Output) model, where each port can manage 16 physical pins. The LEDS are mapped to external pins 55-58 which maps internally onto GPIO Port D pins 8-11.

Flashing the LEDs

Flashing the LEDs is fairly straightforward, at the port level there are only two registers we are interested in.

  • Mode Register – this defines, on a pin-by-pin basis what its function is, e.g. we want this pin to behave as an output pin.
  • Output Data Register – Writing a ‘1‘ to the appropriate pin will generate voltage and writing a ‘0‘ will ground the pin.

Mode Register (MODER)

Each port pin has four modes of operation, thus requiring two configuration bits per pin (pin 0 is configured using mode bits 0-1, pin 2 uses mode bits 2-3, and so on):

  • 00 Input
  • 01 Output
  • 10 Alternative function (details configured via other registers)
  • 11 Analogue

So, for example, to configure pin 8 for output, we must write the value 01 into bits 16 and 17 in the MODER register (that is, bit 16 => 1, bit 17 => 0).

Output Data Register (ODR)

In the Output Data Register (ODR) each bit represents an I/O pin on the port. The bit number matches the pin number.

If a pin is set to output (in the MODER register) then writing a 1 into the appropriate bit will drive the I/O pin high. Writing 0 into the appropriate bit will drive the I/O pin low.

There are 16 IO pins, but the register is 32bits wide. Reserved bits are read as ‘0’.

Port D Addresses

The absolute addresses for the MODER and ODR of Port D are:

  • MODER – 0x40020C00
  • ODR – 0x40020C14

Pointer access to registers

Typically when we access registers in C based on memory-mapped IO we use a pointer notation to ‘trick’ the compiler into generating the correct load/store operations at the absolute address needed. Continue reading

Posted in ARM, C/C++ Programming, CMSIS, Cortex | Tagged , , | 16 Comments

A brief introduction to Concepts – Part 2

In part 1 of this article we looked at adding requirements to parameters in template code to improve the diagnostic ability of the compiler.  (I’d recommend reading this article first, if you haven’t already)

Previously, we looked at a simple example of adding a small number of requirements on a template parameter to introduce the syntax and semantics.  In reality, the constraints imposed on a template parameter could consist of any combination of

  • Type traits
  • Required type aliases
  • Required member attributes
  • Required member functions

Explicitly listing all of this requirements for each template parameter, and every template function / class gets onerous very quickly.

To simplify the specification of these constraints we have Concepts.

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A brief introduction to Concepts – Part 1

Templates are an extremely powerful – and terrifying – element of C++ programs.  I say “terrifying” – not because templates are particularly hard to use (normally), or even particularly complex to write (normally) – but because when things go wrong the compiler’s output is a tsunami of techno-word-salad that can overwhelm even the experienced programmer.

The problem with generic code is that it isn’t completely generic.  That is, generic code cannot be expected to work on every possible type we could substitute.  The generic code typically places constraints on the substituted type, which may be in the form of type characteristics, type semantics or behaviours.  Unfortunately, there is no way to find out what those constraints are until you fail to meet them; and that usually happens at instantiation time, far away from your code and deep inside someone else’s hard-to-decipher library code.

The idea of Concepts has been around for many years; and arguably they trace their roots right back to the very earliest days of C++.  Now in C++17 we are able to use and exploit their power in code.

Concepts allow us to express constraints on template types with the goals of making generic code

  • Easier to use
  • Easier to debug
  • Easier to write

In this pair of articles we’ll look at the basics of Concepts, their syntax and usage.  To be open up-front:  this article is designed to get you started, not to make you an expert on Concepts or generic code.

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