fb/fb.h

#include <linux/fb.h>

// Represents a colour on the screen.
// For each RGB value, accepts a value between 0-255, where
// 0 is all the way off and 255 is all the way on.
typedef struct Colour {
	char red;
	char green;
	char blue;
	char alpha;
} Colour;

// Represents a pixel on the screen.
typedef struct Pixel {
	int x;
	int y;
} Pixel;

// Represents the framebuffer that the user has opened.
// Use newFramebuffer() to construct this struct.
// Get x and y size of screen with .vinfo.xres and .vinfo.yres
typedef struct Framebuffer {
	int fd;
	struct fb_var_screeninfo vinfo;
	long int screensize;
	char* data;
} Framebuffer;

// Constructs a Framebuffer struct.
// Asserts whether device is able to be opened. If unable, program is aborted.
Framebuffer newFramebuffer(const char* device);

// Writes a colour to the screen.
// fb: Pointer to Framebuffer struct (get one with newFramebuffer("/dev/fb0") )
// pixel: Which pixel on the screen to apply to
// colour: The colour of the pixel
void writeColourToFramebuffer(Framebuffer* fb, Pixel pixel, Colour colour);

// Destroys a framebuffer.
void destroyFramebuffer(Framebuffer* fb);

// Example library usage (puts a red pixel at 100, 100)

/*

int main() {

	Framebuffer fb = newFramebuffer("/dev/fb0");

	writeColourToFramebuffer(&fb, (Pixel){100, 100}, (Colour){0, 0, 255, 0});

	destroyFramebuffer(&fb);
	return 0;
}

*/

// To use as single-header library, define FB_IMPLEMENTATION before including this header.
#ifdef FB_IMPLEMENTATION

#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>

Framebuffer newFramebuffer(const char* device) {
	Framebuffer fb;
	
	// Open the framebuffer file descriptor
	fb.fd = open(device, O_RDWR);
	assert(fb.fd >= 0 && "Invalid framebuffer device, please ensure your user is in the 'video' group.");

	// Get information about framebuffer
	ioctl(fb.fd, FBIOGET_VSCREENINFO, &fb.vinfo);

	// Get size of screen for writing pixels in bounds
	fb.screensize = fb.vinfo.xres * fb.vinfo.yres * fb.vinfo.bits_per_pixel / 8;

	fb.data = mmap(0, fb.screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fb.fd, 0);

	return fb;
} 

void writeColourToFramebuffer(Framebuffer* fb, Pixel pixel, Colour colour) {
	int offset = (pixel.y * fb->vinfo.xres + pixel.x) * (fb->vinfo.bits_per_pixel / 8);

	fb->data[offset + 0] = colour.red;
	fb->data[offset + 1] = colour.green;
	fb->data[offset + 2] = colour.blue;
	fb->data[offset + 3] = colour.alpha;

	
	munmap(fb->data, fb->screensize);
}

void destroyFramebuffer(Framebuffer* fb) {
	close(fb->fd);

	// NOTE: Do not free fb->data, this is not memory owned by us.
	// Will cause a segfault
}
#endif