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C program to interact with frame buffer devices

Screen information are queried by applications using the FBIOGET_FSCREENINFO and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a fb_fix_screeninfo and fb_var_screeninfo structure respectively.

struct fb_fix_screeninfo stores device independent unchangeable information about the frame buffer device and the current format. Those information can’t be directly modified by applications, but can be changed by the driver when an
application modifies the format.

struct fb_fix_screeninfo {
	char id[16];			/* identification string eg "TT Builtin" */
	unsigned long smem_start;	/* Start of frame buffer mem */
					/* (physical address) */
	__u32 smem_len;			/* Length of frame buffer mem */
	__u32 type;			/* see FB_TYPE_*		*/
	__u32 type_aux;			/* Interleave for interleaved Planes */
	__u32 visual;			/* see FB_VISUAL_*		*/
	__u16 xpanstep;			/* zero if no hardware panning  */
	__u16 ypanstep;			/* zero if no hardware panning  */
	__u16 ywrapstep;		/* zero if no hardware ywrap    */
	__u32 line_length;		/* length of a line in bytes    */
	unsigned long mmio_start;	/* Start of Memory Mapped I/O   */
					/* (physical address) */
	__u32 mmio_len;			/* Length of Memory Mapped I/O  */
	__u32 accel;			/* Indicate to driver which	*/
					/*  specific chip/card we have	*/
	__u16 capabilities;		/* see FB_CAP_*			*/
	__u16 reserved[2];		/* Reserved for future compatibility */

struct fb_var_screeninfo stores device independent changeable information about a frame buffer device, its current format and video mode, as well as other miscellaneous parameters.

struct fb_var_screeninfo {
	__u32 xres;			/* visible resolution		*/
	__u32 yres;
	__u32 xres_virtual;		/* virtual resolution		*/
	__u32 yres_virtual;
	__u32 xoffset;			/* offset from virtual to visible */
	__u32 yoffset;			/* resolution			*/

	__u32 bits_per_pixel;		/* guess what			*/
	__u32 grayscale;		/* 0 = color, 1 = grayscale,	*/
					/* >1 = FOURCC			*/
	struct fb_bitfield red;		/* bitfield in fb mem if true color, */
	struct fb_bitfield green;	/* else only length is significant */
	struct fb_bitfield blue;
	struct fb_bitfield transp;	/* transparency			*/

	__u32 nonstd;			/* != 0 Non standard pixel format */

	__u32 activate;			/* see FB_ACTIVATE_*		*/

	__u32 height;			/* height of picture in mm    */
	__u32 width;			/* width of picture in mm     */

	__u32 accel_flags;		/* (OBSOLETE) see fb_info.flags */

	/* Timing: All values in pixclocks, except pixclock (of course) */
	__u32 pixclock;			/* pixel clock in ps (pico seconds) */
	__u32 left_margin;		/* time from sync to picture	*/
	__u32 right_margin;		/* time from picture to sync	*/
	__u32 upper_margin;		/* time from sync to picture	*/
	__u32 lower_margin;
	__u32 hsync_len;		/* length of horizontal sync	*/
	__u32 vsync_len;		/* length of vertical sync	*/
	__u32 sync;			/* see FB_SYNC_*		*/
	__u32 vmode;			/* see FB_VMODE_*		*/
	__u32 rotate;			/* angle we rotate counter clockwise */
	__u32 colorspace;		/* colorspace for FOURCC-based modes */
	__u32 reserved[4];		/* Reserved for future compatibility */

To modify variable information, applications call the FBIOPUT_VSCREENINFO ioctl with a pointer to a fb_var_screeninfo structure. If the call is successful, the driver will update the fixed screen information accordingly.

Instead of filling the complete fb_var_screeninfo structure manually, applications should call the FBIOGET_VSCREENINFO ioctl and modify only the fields they care about.

 $ vim fb_test.c 
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

int main(int argc, char **argv) {
	int fbfd = 0;
	struct fb_var_screeninfo vinfo;
	struct fb_fix_screeninfo finfo;
	long int screensize = 0;
	char *fbp = 0;

	// Open the file for reading and writing
	fbfd = open("/dev/fb0", O_RDWR);
	if (fbfd == -1) {
		perror("Error: cannot open framebuffer device");
	printf("The framebuffer device was opened successfully.\n");

	// Get fixed screen information
	if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {
		perror("Error reading fixed information");

	// Get variable screen information
	if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
		perror("Error reading variable information");

	printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);

	// Figure out the size of the screen in bytes
	screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

	// Map the device to memory
	fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0);
	if ((int)fbp == -1) {
		perror("Error: failed to map framebuffer device to memory");
	printf("The framebuffer device was mapped to memory successfully.\n");

	printf("This code is referenced from QT core https://www.syzdek.net/~syzdek/docs/SDK%20Documentation/qt-4.2.2/qtopiacore-testingframebuffer.html"); 

	munmap(fbp, screensize);
	return 0;

Reference : https://www.kernel.org/doc/Documentation/fb/api.txt

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