framebuffer.c

// https://cmcenroe.me/2018/01/30/fbclock.html

// gcc -std=gnu11 -Wall -Wextra -O0 -g framebuffer.c

// fbgrab -d /dev/fb0 -s 5 -v -z 9 framebuffer.png

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

int main(){

  int fb=open("/dev/fb0",O_RDWR);
  assert(fb>0);

  // https://www.kernel.org/doc/Documentation/fb/api.txt
  // https://www.kernel.org/doc/html/latest/driver-api/frame-buffer.html?highlight=fb_var_screeninfo
  struct fb_var_screeninfo info=(struct fb_var_screeninfo){};
  assert(ioctl(fb,FBIOGET_VSCREENINFO,&info)==0);
  size_t len=4*info.xres*info.yres;

  uint32_t *buf=mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_SHARED, fb, 0);
  assert(buf!=MAP_FAILED);


  printf("\033[999E\n"); // Move cursor to the bottom
  printf("\033[2K\n");
  printf("\033[3J\n");

  #define buf(r,c,color) buf[r*info.xres+c]=color & 0x00FFFFFF
  #define vline(c,color,rBeg,rEnd) {for(uint32_t r=rBeg;r<=rEnd;++r)buf(r,c,color);}
  #define hline(r,color,cBeg,cEnd) {for(uint32_t c=cBeg;c<=cEnd;++c)buf(r,c,color);}
  #define area(color,rBeg,rEnd,cBeg,cEnd) {for(uint32_t r=rBeg;r<=rEnd;++r)for(uint32_t c=cBeg;c<=cEnd;++c)buf(r,c,color);}

  // vline(10,0xFFFFFF,10,100);

  const uint32_t rBeg=10;
  const uint32_t rEnd=110;
  uint32_t cBeg=0;
  uint32_t cEnd=0;
  #define next {cBeg=cEnd+10;cEnd=cBeg+100;}

  next;area(0xFFFFFF,rBeg,rEnd,cBeg,cEnd);

  next;area(0x00FFFF,rBeg,rEnd,cBeg,cEnd);
  next;area(0xFF00FF,rBeg,rEnd,cBeg,cEnd);
  next;area(0xFFFF00,rBeg,rEnd,cBeg,cEnd);

  next;area(0xFF0000,rBeg,rEnd,cBeg,cEnd);
  next;area(0x00FF00,rBeg,rEnd,cBeg,cEnd);
  next;area(0x0000FF,rBeg,rEnd,cBeg,cEnd);

  // free(buf); // crash
  buf=NULL;

  return 0;
}

framebuffer.png

free(buf) crashes because this operation is invalid.
You must call munmap(buf, len) then close(fb) as well.