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To allow us access to the GPIO pins of the Raspberry Pi in C++ code, we will use a library known as “WiringPi”. What is nice about this library is that the functions we call are similar to those found in the usual Python examples. This should hopefully make it easier for those coming from Python programming on the Raspberry Pi.
As stated in my overview of C++, third party code usually comes in the form of a shared/static library which you link into your projects code. This is the case for WiringPi!
To acquire the use of a third party library, you normally get it in the form of precompiled binaries or downloading and compiling the source code yourself. What “precompiled binaries” mean, is that someone else has compiled the source code on their Pi/machine and distributed the .a/.so (static/shared library) file via a download medium. Precompiled binaries are usually the go-to way of using libraries on Windows machines. In this example however, we will be download the WiringPi source code and compile it ourselves.
The first thing you want to do is download the WiringPi source code. To retrieve the latest working copy of Wiring Pi, we will use git to clone the repository. If you are unsure if git is installed, you can run the following:
> sudo apt-get install git
Once git is installed, clone the repository by entering the following
> git clone git://git.drogon.net/wiringPi
After a few seconds git will have finished downloading. If you list the contents of the current directory (“ls” command) you will notice a folder named “wiringPi”. Simply change into this directory.
> cd wiringPi
If you list the contents of this directory, there will be a lot of files. The one that we are most interested in is the “build” script. Upon running this script, the compilation of the library will begin and its binaries and header files will be automatically installed into the appropriate /usr/local folders on our system.
You should see the words “all done.” at the end of the build process once the binaries are compiled and installed.
When linking a third party library into your C++ project, you usually have to tell the compiler where to find the libraries header and binary files. Thankfully, the build script executed in the compilation process installed these files into our /usr/local/include and /usr/local/lib folders. What is special about these folders is that they are on the system PATH variable, this means that compilers will look for files here by default!
Although we don’t have to tell the compiler where to look, we still have to tell it what to link. Luckily, this is only a one liner with CMake! I have commented each command and highlighted the line which links the wiringPi library. Here is the contents of the complete CMake file (notice I have changed the executable and project name)
And finally, the code to run to see if the build system will link, compile and run our code:
In the above code, we simply include the wiringPi’s declared functions using the include statement and we are calling the wiringPiSetup function.
To generate the make files, make sure you have a subdirectory called build and make it the working direcotry
> mkdir Build && cd Build
Genreate the makefiles using CMake
> cmake ..
Compile the code
If all goes well, you should be able to execute the program via its executable name and see the line “wiringPi is working!” appear in your terminal.
If you see this, then you are ready to get started with programming in C++ for the Raspberry Pi!
Now that we have our program successfully linking and compiling WiringPi, we can now look at preparing the GPIO pins for use in our projects.
If you have come across some of the Python GPIO examples for the Raspberry Pi, the usual procedure is to:
- Initialise the GPIO Library
- Setup the I/O mode on each GPIO pin you intend to use
- Set/Read the pins state dependant on the mode you initiated it as (i.e. input / output)
Luckily, WiringPi is no different! And we’ve already seen from the code above how we initialise the GPIO library.
Just as an example of the functions we will be using in future posts, here is some code (commented on each line) which will set the output & read input from the GPIO pins.
If you copy the above into your main.cpp file from earlier, and recompile the program you should see the line “pin input = 0” when executing. The status will be 0 until there is an input going into the pin… which we will look at when I get to push buttons 🙂
As a quick overall reminder, the full steps to generate your makefiles and compile your code is as follows (don’t execute the contents in brackets):
> cd Build (If not already in Build directory) > cmake .. (If you have not yet generated makefile / added new files) > make > ./MyEXE
In the next post I will cover how to setup and make an LED flash using C++. If you want to give this a shot in the meantime though… Use the appropriate GPIO header table for your Pi and setup a GPIO pin in output mode to light up an LED.
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