访问Frame Buffer，为何导致"Segment fault"错误？

xparmenides

下面的程序用于在屏幕上绘制一个矩形，在Ubuntu中能够编译通过，但运行时提示“Segmentation fault”。用GDB跟踪发现在第54行发生错误。既然已经将Frame Buffer映射到了进程地址空间中，为何不能访问该内存呢？

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

int main()
{
  int fbfd = 0;
  struct fb_var_screeninfo vinfo;
  struct fb_fix_screeninfo finfo;
  struct fb_cmap cmapinfo;
  long int screensize = 0;
  char *fbp = 0;
  int x = 0, y = 0;
  long int location = 0;
  int b, g, r;
  
  fbfd = open("/dev/fb0", O_RDWR);
  if (fbfd < 0){
    printf("Error: can not open framebuffer device: %x\n", fbfd);
    exit(1);
  }
  printf("The framebuffer device was opened successfully.\n");
  if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)){
    printf("Error reading fixed information.\n");
    exit(2);
  }
  printf("%s\n", finfo.id);
  printf("type: 0x%x\n", finfo.type);
  printf("visual: %d\n", finfo.visual);
  printf("line_length: %d\n", finfo.line_length);
  printf("mem_start: 0x%lx, smem_len: %d\n", finfo.smem_start, finfo.smem_len);
  printf("mmio_start: 0x%lx, mmio_len: %d\n", finfo.mmio_start, finfo.mmio_len);

  if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)){
    printf("Error reading variable information. \n");
    exit(3);
  }
  printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);
  screensize = vinfo.xres * vinfo.bits_per_pixel / 8;
  fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0);
  if ((int)fbp == -1){
    printf("Error: failed to map framebuffer device to memory.\n");
    exit(4);
  }
  printf("The framebuffer device was mapped to memory successfully.\n");
  vinfo.xoffset = (vinfo.xres - 420) / 2;
  vinfo.yoffset = (vinfo.yres - 340) / 2;
  b = 10; g = 100; r = 100;
  for (y = 0; y < 340; y++)
    for ( x = 0; x < 420; x++){
      location = (x + vinfo.xoffset) * (vinfo.bits_per_pixel/8 <<===== 这里发生错误
        + (y + vinfo.yoffset) * finfo.line_length;
      if (vinfo.bits_per_pixel == 32){
        *(fbp + location) = b;
        *(fbp + location + 1) = g;
        *(fbp + location + 2) = r;
        *(fbp + location + 3) = 0;
      }
      else{
        unsigned short int t = r << 11 | g << 5 | b;
        *((unsigned short int *)(fbp + location)) = t;
      }
    }
  munmap(fbp, screensize);
  close(fbfd);
  return 0;
}

rheostat

哥们，这行肯定不会有蛋错误