Read Screen Information in C: Example Using struct fb_var_screeninfo and struct fb_fix_screeninfo

Accessing screen information such as screen size and resolution is crucial for various applications and drivers in Linux. Using the struct fb_var_screeninfo and struct fb_fix_screeninfo structures, you can retrieve detailed information about the framebuffer device. This guide provides a detailed C program example to read and display screen information, including screen size and resolution, using these structures.

Understanding Framebuffer Information Structures

• struct fb_var_screeninfo: Contains information about the variable information of the framebuffer, such as screen resolution and color depth.
• struct fb_fix_screeninfo: Provides fixed information about the framebuffer, including physical dimensions and memory layout.

Why Retrieve Screen Information?

• Display Configuration: Adjust application settings based on screen resolution and size.
• Graphics Programming: Ensure compatibility with different display hardware.
• Troubleshooting: Diagnose display issues by verifying screen parameters.

1. Setting Up the Test Environment

To retrieve screen information, you need a Linux system with framebuffer support. Follow these steps:

1. Install Required Packages:
   Ensure you have the necessary development tools and libraries:

   sudo apt-get update
   sudo apt-get install build-essential libdrm-dev

2. Verify Framebuffer Device:
   Confirm that the framebuffer device is available:

   ls /dev/fb*

You should see /dev/fb0 or similar if the framebuffer is active.

2. Writing a C Program to Read Screen Information

Here’s a C program example that demonstrates how to use struct fb_var_screeninfo and struct fb_fix_screeninfo to read and display screen information.

a. Example C Program

Save the following code as fb_info.c:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <errno.h>

int main() {
    int fb = open("/dev/fb0", O_RDONLY);
    if (fb == -1) {
        perror("Error opening framebuffer device");
        return EXIT_FAILURE;
    }

    struct fb_var_screeninfo vinfo;
    if (ioctl(fb, FBIOGET_VSCREENINFO, &vinfo)) {
        perror("Error reading variable information");
        close(fb);
        return EXIT_FAILURE;
    }

    struct fb_fix_screeninfo finfo;
    if (ioctl(fb, FBIOGET_FSCREENINFO, &finfo)) {
        perror("Error reading fixed information");
        close(fb);
        return EXIT_FAILURE;
    }

    printf("Screen Resolution: %d x %d\n", vinfo.xres_virtual, vinfo.yres_virtual);
    printf("Screen Size: %d x %d\n", vinfo.xres, vinfo.yres);
    printf("Bits per Pixel: %d\n", vinfo.bits_per_pixel);
    printf("Framebuffer Memory Size: %d bytes\n", finfo.smem_len);
    printf("Physical Screen Size: %d x %d mm\n", finfo.yres_virtual / (vinfo.yres_virtual / finfo.yres_virtual) * vinfo.yoffset, finfo.xres_virtual / (vinfo.xres_virtual / finfo.xres_virtual) * vinfo.xoffset);

    close(fb);
    return EXIT_SUCCESS;
}

b. Compile and Run the Program

1. Compile the Program:

   gcc -o fb_info fb_info.c

2. Run the Program:

   sudo ./fb_info

This program reads and prints screen resolution, size, bits per pixel, memory size, and physical screen size.

3. Interpreting the Results

The output will provide:

• Screen Resolution: The virtual resolution used by the framebuffer.
• Screen Size: The physical dimensions of the screen.
• Bits per Pixel: Color depth of the framebuffer.
• Framebuffer Memory Size: Total memory allocated for the framebuffer.
• Physical Screen Size: The physical dimensions of the screen in millimeters.

4. Troubleshooting Common Issues

• Framebuffer Device Not Found: Ensure the framebuffer driver is loaded and the device file exists.
• Permission Issues: Run the program with elevated permissions if necessary.
• Invalid Values: Verify that the framebuffer device is correctly configured and operational.

5. Best Practices

• Test on Multiple Devices: Validate results across different hardware to ensure compatibility.
• Document Results: Record screen information and any issues encountered for future reference.
• Update Regularly: Keep your system and drivers updated for accurate information retrieval.

Conclusion

Using struct fb_var_screeninfo and struct fb_fix_screeninfo to read screen information in Linux is a straightforward process that provides valuable details about your display. By following the example C program provided, you can effectively retrieve and utilize this information for various applications and troubleshooting tasks.