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C Program

Created Wednesday 11 January 2017

Interacting with GPIOs within a C / C++ program:

Using a your favorite editor (I recommend "Geany", already installed on Raspberry Pi),
enter the following C / C++ program:
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// File: gpiotest1.cpp

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>


#define GPIO_DIRECTORY "/sys/class/gpio"
#define GPIO_EXPORT "/sys/class/gpio/export"
#define GPIO_UNEXPORT "/sys/class/gpio/unexport"

#define GREENLED 87 // GPIO value for the green LED

#define DIRECTION_INPUT 0
#define DIRECTION_OUTPUT 1

void msleep(int imsleepy)
{

usleep(imsleepy*1000);
}

// Return negative value for error, non-negative value for success
int exportGpio(int gpio)
{

char buf[50];
// Before we continue, let's see if the GPIO is already exported
struct stat exportstat;
sprintf(buf,"%s/gpio%d",GPIO_DIRECTORY,gpio);
if(!stat(buf,&exportstat)) {
printf("%s: %d already exported\n",__FUNCTION__,gpio);
return 0; // already exported, return 0
}

int fd = open(GPIO_EXPORT,O_WRONLY); // Request write only access to "export" file
if(fd < 0) {
printf("%s: Error - open(%s)\n",__FUNCTION__,GPIO_EXPORT);
return -1;
}
// Writing to the open file descriptor, export the GPIO
// Return value is number of bytes written
sprintf(buf,"%d",gpio);
int bytesWritten = write(fd,buf,strlen(buf));
//printf("%s: Wrote %d bytes to %s\n",__FUNCTION__,bytesWritten,GPIO_EXPORT);
close(fd);
return bytesWritten;
}

// Return negative value for error, non-negative value for success
// Use 0: for input, non-zero for output
int setGpioDirection(int gpio,int direction)
{

char buf[50];
int bytesWritten;
sprintf(buf,"%s/gpio%d/direction",GPIO_DIRECTORY,gpio);
int fd = open(buf,O_RDWR); // Request access to gpio's "direction" file
if(fd < 0) {
printf("%s: Error - open(%s)\n",__FUNCTION__,buf);
return -1;
}
// Send the appropriate string to the direction "file"
if(direction == DIRECTION_INPUT)
bytesWritten = write(fd,"in",2);
else
bytesWritten = write(fd,"out",3);

//printf("%s: Wrote %d bytes to %s\n",__FUNCTION__,bytesWritten,buf);
close(fd);
return bytesWritten;
}

// Return negative value for error, non-negative value for success
// LOW: 0, HIGH: 1
int setGpioValue(int gpio,int value)
{

char buf[50];
int bytesWritten;
sprintf(buf,"%s/gpio%d/value",GPIO_DIRECTORY,gpio);
int fd = open(buf,O_RDWR); // Request access to gpio's "value" file
if(fd < 0) {
printf("%s: Error - open(%s)\n",__FUNCTION__,buf);
return -1;
}
// Send the appropriate string to the value "file"
if(value == 0)
bytesWritten = write(fd,"0",1);
else
bytesWritten = write(fd,"1",1);

//printf("%s: Wrote %d bytes to %s\n",__FUNCTION__,bytesWritten,buf);
close(fd);
return bytesWritten;
}

// Return negative value for error, non-negative value for success
int unexportGpio(int gpio)
{

char buf[50];
int fd = open(GPIO_UNEXPORT,O_WRONLY); // Request write only access to "export" file
if(fd < 0) {
printf("%s: Error - open(%s)\n",__FUNCTION__,GPIO_UNEXPORT);
return -1;
}
// Writing to the open file descriptor, export the GPIO
// Return value is number of bytes written
sprintf(buf,"%d",gpio);
int bytesWritten = write(fd,buf,strlen(buf));
//printf("%s: Wrote %d bytes to %s\n",__FUNCTION__,bytesWritten,GPIO_UNEXPORT);
close(fd);
return bytesWritten;
}

int main(int argc, char * argv[])
{

char buf[50];
exportGpio(GREENLED);
setGpioDirection(GREENLED,DIRECTION_OUTPUT);

// If we have our own file descriptor to the "value" file, the code is more efficient
sprintf(buf,"%s/gpio%d/value",GPIO_DIRECTORY,GREENLED);
int fd = open(buf,O_RDWR); // Request access to gpio's "value" file
if(fd < 0) {
printf("%s: Error - open(%s)\n",__FUNCTION__,buf);
return -1;
}
// Flash the LED 20 times
for(int i=0;i<20;i++)
{
write(fd,"1",1); // Raise gpio
msleep(200);
write(fd,"0",1); // Lower gpio
msleep(200);
}
unexportGpio(GREENLED);
close(fd);
return 0;
}

Compile and link the above program, using gcc:
$ gcc -o gpiotest1 gpiotest1.cpp

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