mbr.bin

spank · 12-24-2005, 06:05 AM

I have a school homework to read the partition table from a binary file mbr.bin
root@marlboro ~/fdisk # file mbr.bin
mbr.bin: x86 boot sector, code offset 0x48
http://cs.pub.ro/~programare/Teme_de_casa/tema6/mbr.bin

how can i do that?

i've tryed

char a[512];
f=fopen("mbr.bin","rb");
fread(a,1,512,f);
but i don't know how to interpret the data from the char array a.
Please pretty please help!

Mara · 12-24-2005, 04:40 PM

You need to know how the MBR structure looks like. You can find the info here: http://www.ata-atapi.com/hiwmbr.htm Then just extract what you want.

spank · 12-25-2005, 06:15 AM

i've read some docs. now i don't understand why the first byte from mbr.bin is displayed "eb" in a hex editor and if i read it in c

fread(a,1,512,f);
x=a[0];
printf("%x ",x);

it shows me "ffffffeb". This happens not only with the first byte. But there are some that are displayed the same like in the hex editor
Help!

Mara · 12-25-2005, 02:06 PM

In you case it's all probably about types. You have a buffer of chars (or unsigned chars), but x is probably int, right? Char is one byte, int four (ia386 architecture). There are casts here and they cause the mess.

paulsm4 · 12-25-2005, 02:13 PM

In other words:
ORIGINAL VERSION (any byte >= 128 will be treated as a negative number):

Code:

  char a[512];
  char x;
  f=fopen("mbr.bin","rb");
  fread(a,1,512,f);
  x=a[0];
  printf("%x ",x);

<= THE BYTE VALUE "0xeb" WILL DISPLAY AS "ffffffeb"

ALTERNATIVE VERSION #1:

Code:

  unsigned char a[512];
  unsigned char x;
  f=fopen("mbr.bin","rb");
  fread(a,1,512,f);
  x=a[0];
  printf("0x%x ",x);

<= THE BYTE VALUE "0xeb" WILL DISPLAY AS "0xeb"

ALTERNATIVE VERSION #2:

Code:

  char a[512];
  char x;
  f=fopen("mbr.bin","rb");
  fread(a,1,512,f);
  x=a[0];
  printf("0x%x ", (unsigned char)x);

<= THE BYTE VALUE "0xeb" WILL *PROBABLY* DISPLAY AS "0xeb"

spank · 12-25-2005, 03:31 PM

great!! really helps!

now i have to read the HCS, this is a serie of 3 bytes. i dont know how to separe every byte into bits. how can i do that ?

paulsm4 · 12-25-2005, 04:09 PM

You get to write a function. For example:

Code:

  unsigned char a[80]
  char s[80;

  // Zero out string
  s[0] = '\0';

  // For each byte...
  for (int ibyte = 0; ibyte < 3; ibyte++)
  {
    // For each bit...
    for (int ibit = 0; ibit < 8; ibit++)
    {
      if ( ((a[ibyte] >> ibit) & 0x1)
      {
        strcat (s, "1");
      }
      else
      {
        strcat (s, "0");
      }
    }
  }

Just a quick'n'dirty example (I'm not even sure if it'll compile, much less certain it's correct or not): you'll undoubtedly come up with something better/more applicable yourself. Good luck!

And Merry Christmas!

spank · 12-26-2005, 10:28 AM

Code:

char *byte2bit(unsigned char a)
{
        char *s=(char*)calloc(9,sizeof(char));
        int i;
        for(i=0;i<CHAR_BIT;i++) *(s+i)=(a>>i&1)+'0';
        return s;
}

i've done this for every byte... but your code works very well. thank you!
happy holidays to you to!

spank · 12-26-2005, 02:46 PM

next problem... i have a 4 byte entry that is an unsigned int, the problem is that i used a char to read the binary file. how can i obtain the 4 byte int ?

spank · 12-27-2005, 03:43 AM

solved it

Code:

#include <stdio.h>

int main(void)
{
   unsigned char b[] = {0x12,0x34,0x56,0x78};
   unsigned long value = (b[3] << 24) | (b[2] << 16)  | (b[1] << 8) | b[0];
   printf("value = 0x%08lX\n", value);
   return 0;
}

paulsm4 · 12-27-2005, 10:15 AM

Cool - good work!

Sincerely .. PSM