sysfs-gpio.h

#pragma once
#include "die.h"
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>


struct Gpio {
	int fd = -1;
	char *path = NULL;

	Gpio() = delete;
	Gpio( Gpio const & ) = delete;

protected:
	Gpio( uint n, int mode )
	{
		if( asprintf( &path, "/sys/class/gpio/gpio%u/value", n ) < 0 ) {
			path = NULL;
			die( "asprintf: %m" );
		}
		fd = open( path, mode | O_CLOEXEC );
		if( fd < 0 )
			die( "open %s: %m", path );
	}

public:
	~Gpio()
	{
		free( path );
		if( fd >= 0 )
			close( fd );
	}

	operator bool () const
	{
		char c;
		ssize_t res = pread( fd, &c, 1, 0 );
		if( res < 0 )
			die( "pread %s: %m", path );
		if( res == 0 )
			die( "pread %s: no data", path );
		return c & 1;
	}
};


struct GpIn : Gpio {
	GpIn( uint n ) : Gpio( n, O_RDONLY ) {}
};

// assumes direction has already been set
struct GpOut : Gpio {
	GpOut( uint n ) : Gpio( n, O_RDWR ) {}

	bool operator = ( bool value )
	{
		char c = '0' + value;
		ssize_t res = pwrite( fd, &c, 1, 0 );
		if( res < 0 )
			die( "pwrite %s: %m", path );
		return value;
	}
};

We actually declare our GPIOs in Device Tree hence don't need to export or initialize them at runtime, which is why those steps are being skipped here. We also use udev to setup ownership/permissions for the gpios and create symlinks in /dev/gpio/ with sensible names, which is why revision 1 of this gist identified the gpios using names instead of gpio numbers.