C++ writing to Framebuffer, wrong screensize - c++

Question

i try to write to the Linux framebuffer with c++, my problem is, that my program cant write to the whole screen, because the screensize i get is wrong. Although this code worked on an older VM, but i lost it.
#include <fcntl.h>
#include <unistd.h>
#include <linux/fb.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <cerrno>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
int main() {
fb_var_screeninfo vinfo;
fb_fix_screeninfo finfo;
size_t screensize = 0;
long int location = 0;
/* Open the file for reading and writing */
int fbfd = open("/dev/fb0", O_RDWR);
if(fbfd == -1) {
perror("Error: cannot open framebuffer device");
exit(1);
}
printf("The framebuffer device was opened successfully.\n");
/* Get fixed screen information */
if(ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {
perror("Error reading fixed information");
exit(2);
}
/* Get variable screen information */
if(ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
perror("Error reading variable information");
exit(3);
}
/* Figure out the size of the screen in bytes */
screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
printf("Screen size is %ld\n", screensize);
printf("Vinfo.bpp = %d\n", vinfo.bits_per_pixel);
/* Map the device to memory */
auto fbp = static_cast<unsigned char*>(
mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0));
if(fbp == MAP_FAILED) {
perror("Error: failed to map framebuffer device to memory");
exit(4);
}
printf("The framebuffer device was mapped to memory successfully.\n");
unsigned x = 0;
unsigned y = 0; /* Where we are going to put the pixel */
/* Figure out where in memory to put the pixel */
location = (x + vinfo.xoffset) * (vinfo.bits_per_pixel / 8) +
(y + vinfo.yoffset) * finfo.line_length;
for(uint32_t i = 0; i < vinfo.yres; i++) {
for(uint32_t count = 1; count < vinfo.xres; count++) {
fbp[location++] = 255; /* Some blue */
fbp[location++] = 0; /* A little green */
fbp[location++] = 0; /* A lot of red */
fbp[location++] = 0; /* No transparency */
}
}
munmap(fbp, screensize);
close(fbfd);
return 0;
}
I tried increasing the screensize by multiplying it by 2 and it wrote to the full screen, but then my coordinates are wrong.