Introduction to accessing the Raspberry Pi’s GPIO in C++ (sysfs)

Introduction

In this blog entry I will demonstrate how one can access the Raspberry Pi’s GPIO in C++. There are two approaches to accomplish this. The first is to directly manipulate the Raspberry Pi’s GPIO’s registers much like one would do when programming a microcontroller without an operating system (OS) or a memory management unit (approach using mmap). The advantage of this approach is that because the OS (Linux) is being completely bypassed, the GPIO pins can toggle very fast. Bypassing the OS however means that if two processes (instances of a programs) are trying to access the same physical GPIO registers at the same time, a unsafe resource conflict can happen. Examples of this approach to GPIO coding can be found here.

The second approach is the “Linux approach” (sysfs). Linux already has a built-in driver for safely accessing the GPIOs. It basically views each property of each GPIO pin as a file. This is the preferred method of GPIO access. One disadvantage of this approach is that it tends to make for slower (but safer) GPIO pin toggling. Example code for this approach can be found here, and a wiki on how to access GPIO signals using this approach can he found here. This approach will be used in this tutorial. The file accesses will be wrapped in a C++ class to make GPIO access easy and foolproof (sort of).

Setting up the  test hardware

In order to test our GPIO access program we will need to connect the GPIO pins to actual hardware components. I decided to to use GPIO pin 4 as an output pin that turns on an LED and GPIO pin 17 as an input pin that reads the state of a pushbutton. A connection diagram is provided in Figure 1.

Figure 1 Raspberry Pi Connection Diagram

Figure 1 Raspberry Pi Connection Diagram

GPIO4 is connected to an LED via a 330Ohm current limiting resistor. GPIO17 is connected to a 10KOhm pull-up resistor and a push-button. An unpressed pushbutton will cause the voltage on GPIO17 to be 3.3V. When the button is pressed, the voltage of GPIO17 will be 0V.

The connections between the Raspberry Pi and the other parts can be made via Male-to Female jumper wires or via one of Adafruit’s Pi cobbler kits (ver1  or ver2).

Accessing GPIO From the Terminal

  • When using the Linux (sysfs) way to access GPIO, we must ensure that we’re logged in as root:
pi@raspberrypi ~ $ sudo -i
  • we then must “export the pin” in question. For example to use pins GPIO4 & 17 we  must first export them by writing the GPIO pin numbers to the file “/sys/class/gpio/export” as follows:
root@raspberrypi:~# echo "4" > /sys/class/gpio/export
root@raspberrypi:~# echo "17" > /sys/class/gpio/export

The next step is to set pin 4 as output and pin 17 as input. This is achieved by writing “out” into the “/sys/class/gpio/gpio4/direction” file and writing “in” into the “/sys/class/gpio/gpio17/direction” file as follows:

root@raspberrypi:~# echo "out" > /sys/class/gpio/gpio4/direction
root@raspberrypi:~# echo "in" > /sys/class/gpio/gpio17/direction

To set the output GPIO4 pin high and therefore cause the LED to light-up, we need to write a “1” to the “/sys/class/gpio/gpio4/value” file:

root@raspberrypi:~# echo "1" > /sys/class/gpio/gpio4/value

To clear the output GPIO4 pin to low and therefore cause the LED to turn off we need to write a “0” to the “/sys/class/gpio/gpio4/value” file:

root@raspberrypi:~# echo "0" > /sys/class/gpio/gpio4/value

To read the state of the input pin GPIO17, we need to read the “/sys/class/gpio/gpio17/value” file.  This will return either a “0” if the pin is connected to 0V or a “1” if the pin is connected to 3.3V:

root@raspberrypi:~#cat /sys/class/gpio/gpio17/value

And finally, when we’re done with the GPIO pins we must unexport them by writing the GPIO pin numbers to the “/sys/class/gpio/unexport” file!

root@raspberrypi:~#echo "4" > /sys/class/gpio/unexport
root@raspberrypi:~#echo "17" > /sys/class/gpio/unexport

Notice how the sysfs approach reduces the problem of GPIO access to reading and writing files! This is by the way the norm for Linux. All I/O access (serial ports, console, printers e.t.c.) involves reading to and writing from files. The next step will be to perform the same functions in C++!

Writing a GPIO C++ Class

The goal of the GPIOClass class is to enable the control of a single GPIO  pin. The class was designed so that the GPIO pin number must be passed to the GPIOClass object when its created via an overloaded constructor. The “GPIOClass.h” header file is provided below

#ifndef GPIO_CLASS_H
#define GPIO_CLASS_H

#include <string>
using namespace std;
/* GPIO Class
 * Purpose: Each object instantiated from this class will control a GPIO pin
 * The GPIO pin number must be passed to the overloaded class constructor
 */
class GPIOClass
{
public:
    GPIOClass();  // create a GPIO object that controls GPIO4 (default
    GPIOClass(string x); // create a GPIO object that controls GPIOx, where x is passed to this constructor
    int export_gpio(); // exports GPIO
    int unexport_gpio(); // unexport GPIO
    int setdir_gpio(string dir); // Set GPIO Direction
    int setval_gpio(string val); // Set GPIO Value (putput pins)
    int getval_gpio(string& val); // Get GPIO Value (input/ output pins)
    string get_gpionum(); // return the GPIO number associated with the instance of an object
private:
    string gpionum; // GPIO number associated with the instance of an object
};

#endif

Each GPIOClass object has member functions that enable us to export/unexport GPIO pins, set the direction of the GPIO pins as well as set and get the value of the GPIO pins. The GPIOClass object has one private variable which is the GPIO pin number. The implementation of these member functions  are provided below “GPIOClass.cpp”:

#include <fstream>
#include <string>
#include <iostream>
#include <sstream>
#include "GPIOClass.h"

using namespace std;

GPIOClass::GPIOClass()
{
	this->gpionum = "4"; //GPIO4 is default
}

GPIOClass::GPIOClass(string gnum)
{
	this->gpionum = gnum;  //Instatiate GPIOClass object for GPIO pin number "gnum"
}

int GPIOClass::export_gpio()
{
	string export_str = "/sys/class/gpio/export";
	ofstream exportgpio(export_str.c_str()); // Open "export" file. Convert C++ string to C string. Required for all Linux pathnames
	if (exportgpio < 0){
		cout << " OPERATION FAILED: Unable to export GPIO"<< this->gpionum <<" ."<< endl;
		return -1;
	}

	exportgpio << this->gpionum ; //write GPIO number to export
    exportgpio.close(); //close export file
    return 0;
}

int GPIOClass::unexport_gpio()
{
	string unexport_str = "/sys/class/gpio/unexport";
	ofstream unexportgpio(unexport_str.c_str()); //Open unexport file
	if (unexportgpio < 0){
		cout << " OPERATION FAILED: Unable to unexport GPIO"<< this->gpionum <<" ."<< endl;
		return -1;
	}

	unexportgpio << this->gpionum ; //write GPIO number to unexport
    unexportgpio.close(); //close unexport file
    return 0;
}

int GPIOClass::setdir_gpio(string dir)
{

	string setdir_str ="/sys/class/gpio/gpio" + this->gpionum + "/direction";
	ofstream setdirgpio(setdir_str.c_str()); // open direction file for gpio
		if (setdirgpio < 0){
			cout << " OPERATION FAILED: Unable to set direction of GPIO"<< this->gpionum <<" ."<< endl;
			return -1;
		}

		setdirgpio << dir; //write direction to direction file
		setdirgpio.close(); // close direction file
	    return 0;
}

int GPIOClass::setval_gpio(string val)
{

	string setval_str = "/sys/class/gpio/gpio" + this->gpionum + "/value";
	ofstream setvalgpio(setval_str.c_str()); // open value file for gpio
		if (setvalgpio < 0){
			cout << " OPERATION FAILED: Unable to set the value of GPIO"<< this->gpionum <<" ."<< endl;
			return -1;
		}

		setvalgpio << val ;//write value to value file
		setvalgpio.close();// close value file 
	    return 0;
}

int GPIOClass::getval_gpio(string& val){

	string getval_str = "/sys/class/gpio/gpio" + this->gpionum + "/value";
	ifstream getvalgpio(getval_str.c_str());// open value file for gpio
	if (getvalgpio < 0){
		cout << " OPERATION FAILED: Unable to get value of GPIO"<< this->gpionum <<" ."<< endl;
		return -1;
			}

	getvalgpio >> val ;  //read gpio value

	if(val != "0")
		val = "1";
	else
		val = "0";

	getvalgpio.close(); //close the value file
    return 0;
}

string GPIOClass::get_gpionum(){

return this->gpionum;

}

In order to open and close files in C++, the ifstream/ofstream  classes where used. Even though the C++ string class was used, the C++ strings had to be converted into C strings since pathnames in Linux can only be interpreted as C strings.  The GPIOClass is pretty self explanatory. It is also very basic I intend to clean in up a bit in the future, but for now it will have to do.

Writing a Test Main

A main program “GPIOtest1.cpp that tests this class is provided below:

#include <iostream>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include "GPIOClass.h"

using namespace std;

int main (void)
{

	string inputstate;
	GPIOClass* gpio4 = new GPIOClass("4"); //create new GPIO object to be attached to  GPIO4
	GPIOClass* gpio17 = new GPIOClass("17"); //create new GPIO object to be attached to  GPIO17

	gpio4->export_gpio(); //export GPIO4
	gpio17->export_gpio(); //export GPIO17

	cout << " GPIO pins exported" << endl;

	gpio17->setdir_gpio("in"); //GPIO4 set to output
	gpio4->setdir_gpio("out"); // GPIO17 set to input

	cout << " Set GPIO pin directions" << endl;

	while(1)
	{
		usleep(500000);  // wait for 0.5 seconds
		gpio17->getval_gpio(inputstate); //read state of GPIO17 input pin
		cout << "Current input pin state is " << inputstate  <<endl;
		if(inputstate == "0") // if input pin is at state "0" i.e. button pressed
		{
		    cout << "input pin state is "Pressed ".n Will check input pin state again in 20ms "<<endl;
	            usleep(20000);
                    cout << "Checking again ....." << endl;
                    gpio17->getval_gpio(inputstate); // checking again to ensure that state "0" is due to button press and not noise
		    if(inputstate == "0")
		    {
		    	cout << "input pin state is definitely "Pressed". Turning LED ON" <<endl;
		    	gpio4->setval_gpio("1"); // turn LED ON

		    	cout << " Waiting until pin is unpressed....." << endl;
		    	while (inputstate == "0"){ 
		    	gpio17->getval_gpio(inputstate);
		    	};
		    	cout << "pin is unpressed" << endl;

		    }
		    else
		    	cout << "input pin state is definitely "UnPressed". That was just noise." <<endl;

		}
		gpio4->setval_gpio("0");

	}
	cout << "Exiting....." << endl;
	return 0;
}

The GPIOtest1.cpp program is pretty straightforward. It first instantiates two GPIOClass objects attached to GPIO4 & GPIO17 (gpio4 and gpio17) on the heap. Note that the object names was chosen to reflect the GPIO pin number but could’ve been called anything else.

The GPIOs are then exported, GPIO4 set to output and GPIO17 set to input. At this point code execution enters into an infinite loop that reads the status of the input pin GPIO17. If the pin has a state of “0”, this means that the button could be pressed. The state of the GPIO17 pin is checked again to ensure that this is indeed the case. If so the GPIO4 pin is set to high, causing the LED to turn ON. Another while loop ensures that the LED is ON, so long as the button continues to be pressed. Note that line before “return 0;” , “cout << “Exiting…..” << endl;” never executes.

Using Signals

There is a major flaw in this program and that is, the only way to exit the program is to press ctrl-C. Terminating the program in this way means that the program is unable to unexport the GPIO pins. More importantly it means that the program is unable to free the heap memory allocated for GPIOClass objects gpio4 & 17.

One way of fixing this problem is to write a signal handler (software interrupt handler) and attach it to the SIGINT (ctrl-C) signal (event). This enables the program to jump to the signal handler whenever ctrl-C is pressed, instead of directly exiting the program. The signal handler can then initiate a graceful exit of the program i.e. deallocate heap memory and unexport pins before exiting. First we have to include:

#include <signal.h>

The next step is then instantiate a sigaction struct and to fill it  as follows:

struct sigaction sig_struct;
sig_struct.sa_handler = sig_handler;
sig_struct.sa_flags = 0;
sigemptyset(&sig_struct.sa_mask);

The first line above instantiates a sigaction struct called sig_struct. The second line causes the signal handler field (sa_handler) of sig_struct to point to a function called “sig_handler”. The “sa_flags” in the sigaction structure can be used to trigger certain behaviors that modify the signal event. The sa_flags field was chosen to be zero because the functionality it provides is not needed.  Finally the last line of code ensures that the sa_mask field of the sigaction structure is empty. The sa_mask field enables certain additional set of signals to be blocked during execution of signal handler function.

Once the sigaction struct “sig_struct” is completed, the “sigaction” function is used to map the filled “sig_struct” structure with the SIGINT signal  as follows:

if (sigaction(SIGINT, &sig_struct, NULL) == -1) {
		cout << "Problem with sigaction" << endl;
		return -1;
	}

We will also need to write the signal handler function “sig_handler”. This function’s entire purpose is to set a global boolean variable (a flag). When ctrl-C is pressed, program execution immediately jumps to the signal handler function and sets this boolean variable (flag) to “true”. In the infinite while loop in main this flag is polled. If it is asserted (by the signal handler), the program then exits gracefully ensuring all heap memory is deallocated and GPIO pins unexported properly.

void sig_handler(int sig)
{
	write(0,"nCtrl^C pressed in sig handlern",32);
	ctrl_c_pressed = true;
}

It is important to note here that functions called in signal handler functions must be re-entrant.  This is why the re-entrant “write” function was used instead of the non-re-entrant printf  or cout. The flag variable used in this example is “ctrl_c_pressed”.

The additional code added to the main program’s infinite while loop allows the program to exit gracefully and is provided below:

if(ctrl_c_pressed)
{
	cout << "Ctrl^C Pressed" << endl;
	cout << "unexporting pins" << endl;
	gpio4->unexport_gpio();
	gpio17->unexport_gpio();
	cout << "deallocating GPIO Objects" << endl;
	delete gpio4;
	gpio4 = 0;
	delete gpio17;
	gpio17 =0;
	break;

}

The complete code is provided below “GPIOTest2.cpp”

#include <iostream>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include "GPIOClass.h"
using namespace std;

void sig_handler(int sig);

bool ctrl_c_pressed = false;

int main (void)
{

	struct sigaction sig_struct;
	sig_struct.sa_handler = sig_handler;
	sig_struct.sa_flags = 0;
	sigemptyset(&sig_struct.sa_mask);

	if (sigaction(SIGINT, &sig_struct, NULL) == -1) {
		cout << "Problem with sigaction" << endl;
		exit(1);
	}

	string inputstate;
	GPIOClass* gpio4 = new GPIOClass("4");
	GPIOClass* gpio17 = new GPIOClass("17");

	gpio4->export_gpio();
	gpio17->export_gpio();

	cout << " GPIO pins exported" << endl;

	gpio17->setdir_gpio("in");
	gpio4->setdir_gpio("out");

	cout << " Set GPIO pin directions" << endl;

	while(1)
	{
		usleep(500000);
		gpio17->getval_gpio(inputstate);
		cout << "Current input pin state is " << inputstate  <<endl;
		if(inputstate == "0")
		{
		   cout << "input pin state is "Pressed ".n Will check input pin state again in 20ms "<<endl;
		   usleep(20000);
                   cout << "Checking again ....." << endl;
                   gpio17->getval_gpio(inputstate);
		    if(inputstate == "0")
		    {
		    	cout << "input pin state is definitely "Pressed". Turning LED ON" <<endl;
		    	gpio4->setval_gpio("1");

		    	cout << " Waiting until pin is unpressed....." << endl;
		    	while (inputstate == "0"){
		    		gpio17->getval_gpio(inputstate);
		    	};
		    	cout << "pin is unpressed" << endl;

		    }
		    else
		    	cout << "input pin state is definitely "UnPressed". That was just noise." <<endl;

		}
		gpio4->setval_gpio("0");

		 if(ctrl_c_pressed)
				    {
				    	cout << "Ctrl^C Pressed" << endl;
				    	cout << "unexporting pins" << endl;
				    	gpio4->unexport_gpio();
				    	gpio17->unexport_gpio();
				    	cout << "deallocating GPIO Objects" << endl;
				    	delete gpio4;
				    	gpio4 = 0;
				    	delete gpio17;
				    	gpio17 =0;
				    	break;

				    }

	}
	cout << "Exiting....." << endl;
	return 0;
}

void sig_handler(int sig)
{
	write(0,"nCtrl^C pressed in sig handlern",32);
	ctrl_c_pressed = true;
}

It is important to note that in order for any of these programs to execute properly you need to run them as root either via the sudo command (i.e. “sudo ./GPIOtest) or preferably login into to your root account with “sudo -i” before running the GPIO based program(s).

For those of you interesting in learning more about Linux system programming, I highly recommend Beej’s Guide to Unix Interprocess Communication. For those that would like to learn even more, check out the following books:

This concludes our introduction to GPIO access in C++!!!  The source code discussed in this blog entry can be found (git)here.

UPDATE (26-08-2013). I’ve added a slightly modified version of the GPIOClass C++ class discussed above (git)here . This version makes the export & unexport functions private and calls them in the constructor & destructor respectively. This way the GPIO pins are automatically exported on object creation and unexported just before object destruction. I’ve also used the C system calls to do the File I/O as opposed to the C++ fstream (File I/O) libraries.

 

 

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