With the communication protocol clearly defined in \cref{sec:tinyframe,sec:units-overview}, respective \cref{sec:wireless} for the wireless gateway, the implementation of a client software is relatively straightforward. Two proof-of-concept client libraries have been developed, in languages C and Python.
This is the only platform-dependent part of the library. Unix-based systems provide a standardized POSIX API to configure the serial port. A raw access to \gls{USB} endpoints is possible using the libUSB C library. Access to the serial port or \gls{USB} from C on MS Windows has not been investigated, but should be possible using proprietary APIs.
Accessing the serial port or \gls{USB} endpoints from Python is more straightforward thanks to the cross platform libraries \textit{PySerial} and \textit{PyUSB}.
The host side of the communication protocol described in \cref{sec:tinyframe} should be implemented as a part of the library. This includes the reading and writing of configuration files, unit list read-out, command payload building, and asynchronous event parsing.
Additional utilities may be defined on top of this basic protocol support for the command API of different GEX units, as described in \cref{sec:units-overview}. Those unit-specific ``drivers'' are available in the provided Python library.
The Python GEX library it implements both serial port and raw USB endpoint access, and includes support classes for each unit type. Its development has been proritized over the C library because of its potential to integrate with MATLAB, and the general ease-of-use that comes with the Python syntax.
The library is composed of a \textit{transport}, the core class called \textit{client}, and unit classes. Three transport implementations have been developed; the gateway is accessed by wrapping either of the transports in an instance of \mono{DongleAdapter}.
The unit classes wrap the command and event \gls{API} described in \cref{sec:units-overview}; all classes and methods are annotated by documentation comments for easy understanding.
An example Python program displaying a test pattern on a \gls{LED} matrix using the \gls{I2C}-connected driver chip IS31FL3730 is presented in \cref{lst:py_api} as an illustration of the library usage. A photo of the produced \gls{LED} pattern can be seen in \cref{fig:pydemo}.
\begin{listing}[h]
\begin{pythoncode}
#!/bin/env python3
# The I2C unit called 'i2c' is configured to use PB6 and PB7
The Python library can be accessed from MATLAB scripts thanks to the MATLAB's two-way Python integration~\cite{matlabpy}. Controlling GEX from MATLAB may be useful when additional processing is required, e.g., with data from the \gls{ADC}; however, in many cases, an open source alternative native to Python exists that could be used for the same purpose, such as the NumPy and SciPy libraries~\cite{numpyscipy}.
The example in \cref{lst:matlab_api} demonstrates the use of MATLAB to calculate the frequency spectrum of an analog signal captured with GEX. The syntax needed to use the serial port transport (instead of a raw access to USB endpoints) is shown in a comment.
\begin{listing}
\begin{matlabcode}
% The ADC unit called 'adc' is configured to use PA1 as Channel 0
The C library is more simplistic than the Python one; it supports only the serial port transport (\gls{UART} or \gls{CDCACM}) and does not implement asynchronous polling or the unit support drivers. What \textit{is} implement---the transport, a basic protocol handler, and payload building and parsing utilities---is sufficient for most applications, though less convenient than the Python library.
This low-level library is intended for applications where the performance of the Python implementation is insufficient, or where an integration with existing C code is required. The full \gls{API} can be found in the library header files. A C version of the example Python script shown above, controlling a \gls{LED} matrix driver, is presented in \cref{lst:c_api_full}.
\begin{listing}
\begin{ccode}
#include <signal.h>
#include <assert.h>
#include "gex.h"
static GexClient *gex;
/** Signal handler closing the serial port at exit */
\caption{\label{lst:c_api_full} An example C program controlling GEX using the low-level GEX library; this code has the same effect as the Python script shown in \cref{lst:py_api}.}
\end{listing}
The reader might point out that this example is unnecessarily obtuse, and that the payloads could be constructed in a more readable way. Indeed, two better methods are available.
\caption{\label{lst:c_api_struct} The variable-length struct approach to payload building}
\end{listing}
The structure-based method utilizes C structs to access individual fields in the payload. Simple payloads can be represented by a struct without problems, but payloads of a dynamic length pose a challenge; we can either define a new struct for each required length, or, when the variable-length array is located at the end of the payload, a struct with the largest needed payload size is defined and the real length is then specified when sending the message. The latter approach is illustrated in \cref{lst:c_api_struct}.
\subsection{Using the Payload Builder Utility}
\begin{listing}
\begin{ccode}
uint8_t buff[20];
PayloadBuilder pb;
pb = pb_init(&buff, 20, NULL);
pb_u16(&pb, 0x61);
pb_u8(&pb, 0x00);
pb_u8(&pb, 0x18);
GEX_Send(bus, 2, buff, pb_length(&pb));
pb_rewind(&pb); // reset the builder for a new frame
\caption{\label{lst:c_api_pb}Building and sending payloads using the PayloadBuilder utility}
\end{listing}
The Payload Builder utility, which comes with the TinyFrame library, may be used to construct message payloads; its usage is shown in \cref{lst:c_api_pb}. The third parameter of \mono{pb\_init()} is optional: a pointer to a function called when the buffer overflows; this callback is used to flush the buffer and rewind it, or to throw an error.
Payload Builder is accompanied by Payload Parser, a tool doing exactly the opposite, parsing a binary payload. That is not needed in our example, but we will find Payload Parser useful when processing the response to a query command, or an asynchronous event payload. Where a variable is written to a payload with \mono{pb\_u8(\&pb, var)}, it is retrieved with \mono{pp\_u8(\&pp)}. The full API of those payload utilities can be found in their well-commented header files.
