[RESOLVED] Problem with reading bmp files' headers

Code:

---

"
struct magic{
 char windowsheader[2];
}magicnumbers;

struct bmp {


unsigned int sizeofthefile;



 short int applicationspecific1;
  short int applicationspecific2;
 unsigned int offset;

 unsigned int sizeheader;
  int width;
  int height;
 short int  colorplanes;
short int btsperpixel;
 unsigned int compressionmethod;
 unsigned int imagesize;
  int horizontalresolution;
  int verticalresolution;
 unsigned int numberofcolors;
 unsigned int numberofimportantcolors;
}bitmap_header;






int main()
{
   

  int windowsheader1[2];
    FILE* fp;
    if ((fp=fopen("wikipedia.bmp","r"))==NULL)

        printf("File error!");
        exit(1);
    }

 
   
 
   
    fread(&magicnumbers,sizeof(magicnumbers),1,fp);

    fread(&bitmap_header,sizeof(bitmap_header),1,fp);
   
   
    printf("\nwindowsheader:%d %d\n",magicnumbers.windowsheader

[0],magicnumbers.windowsheader[1]);
    printf("\napplication specific1:

%d",bitmap_header.applicationspecific1);
    printf("\napplication specific2:

%d",bitmap_header.applicationspecific2);
    printf("\noffset:%d",bitmap_header.offset);
    printf("\nsizeheader:%d",bitmap_header.sizeheader);
    printf("\nwidth:%d",bitmap_header.width);
    printf("\nheight:%d",bitmap_header.height);

    printf("Size of bmp file:%d",bitmap_header.sizeofthefile);
    fclose(fp);
"

---

When I run this program, seems like it is working,at least I am sure that it gives the right file size.

But if I don't use struct magic and instead use:

Code:

---

struct bmp {

char windowsheader[2];
unsigned int sizeofthefile;
short int applicationspecific1;
short int applicationspecific2;
...//same lines with above
 
}bitmap_header;

---

and if I don't use first fread:

Code:

---


fread(&magicnumbers,sizeof(magicnumbers),1,fp);

---

then the values which structure "bmp"'s variables get become wrong.

Of course I change the related printf to:

Code:

---

printf("\nwindowsheader:%d %d\n",bitmap_header.windowsheader[0],bitmap_header.windowsheader[1]);

---

Why does the problem happen,what is the difference between them?Why do I have to use seperate structs?

Your code is faulty in two ways:

1) You must guarantee that the struct is byte aligned. You did not do that or didn't show that in your code (usually a #pragma or compiler option sets the alignment).

2) You must open the file in binary mode, not text mode ("rb", not "r").

Regards,

Paul McKenzie

Thank you for your help :)