Thread: How to read the mbr-sector ( sector zero of /dev/sda ) and print-out the partition table?

Hello ,
As a java-newbee I wonder how todo this simple( in C that-is) task ?
Which IO-class to use for reading a binary record of 512-bytes and how to access the 4 entries of the partition-table ( no pointers in java to handle this ) ?
I know howto todo it in C see under) , but in Java I dont even know howto start and begin ?

// mbr0.c 
#define SECTOR_SIZE 512
#include <stdio.h>
#include <stdlib.h>
typedef struct s_table_entry 
  {  unsigned char boot_status ;   // 0x80 = bootable 0 = non-boot other=?
     unsigned char chs_begin[3];   // cyl,head,sector adres of first sector
     unsigned char type ;   	     // partition type   
     unsigned char chs_end[3];     // cyl,head,sector adres of last sector
     unsigned int  begin ;         // logical block adres first sector
     unsigned int  length ;        // length of partition in sectors 512 bytes 
  } table_entry ;   // size 16 bytes 
 
unsigned char buf512[512];
size_t bytes_readed;
short I;
table_entry* ptable_entry = buf512+446; 
FILE*  fdev ;  
int main(int argc , char* args[] )   
 { fdev = fopen("/dev/sda","rb");
   if(fdev == NULL  )
    { puts("\n OOPS cant open device /dev/sda: ??");
      return 99 ;
    } 
   bytes_readed = fread(buf512,1,512,fdev);
 
   if(bytes_readed  < SECTOR_SIZE ) 
    {  printf("\n OOPS readed only  %d bytes ?? ",bytes_readed) ;
       return bytes_readed ;
    }
   for  (I=0;I<4;I++)
    { printf("\n  entry:%2d boot:%3u start:%12u sectors:%12u ",I ,ptable_entry->boot_status , ptable_entry->begin,ptable_entry->length);
      ptable_entry++;
    }
    fclose(fdev);
    puts(" ");
  return 0 ;  
 
} // -----------------------------------------------------

TIA for any hint !
Erics

Originally Posted by Norm

I don't think it's possible.

Thank You for replying Norm , but I find it hard to believe Java not having a class or method somewhere to handle this ( in Cobol yes impossible , but Java )?
So I suppose my next Java question ( I'll post it to morrow ) will be in the same impossible-to-do category ( the title of a new thread ?

My Thanks for everyone !
I think i solved the problem by using a dataInputStream ( as suggested by copeg ) and skipping the first 446 bytes ( the initial bootcode ) , but alas the printed-out figures were all wrong ??
I was certain I readed the correct MBR-sector as the two last bytes of sector0 of the hard-disk carry a magic number , the 2 bytes were there but in the "wrong" order ??
Sounds like an endian-problem right ?

Made two test-programs : a C-version and a Java-version which wrote some ints to a file and hexdumped the result :
( the ints written were in hex : 0xAB 0xABC 0x ABCD and a short= 1 : )
results:
eric@sysvaio:~$ hexdump IntJtest.bin
0000000 0000 0a00 0000 ab00 0000 bc0a 0000 cdab
0000010 0100
0000012
eric@sysvaio:~$ hexdump IntCtest.bin
0000000 000a 0000 00ab 0000 0abc 0000 abcd 0000
0000010 0001
0000012
This is on a Intel-machine : why do C and Java on the same machine interpret an int on a different manner ?
Life is complicated enough already without this crap !
Hopeless!
Erics

Thank You Copeg and Norm for the replies .
First time I heard about the big-endianness of Java , have a couple of Java-books around , not one of them does mention this fact !
One should expect as Java is designed to run on different platforms , the endianness to be dependent on the target-machine or at least as an command-line option for the Javac-compiler ?

Ok now I know , I' ll have to tackle the beast , I have found there is even a java-class which maybe can be used for the job : " bytebuffer " .

Yes indeed computing is a lot of fun !
Greetz.
Eric

--- Update ---

Hello !
Voila , got it right now .
To tackle the endian-problem it is not even necessary to use the bytebuffer-class but simply the "reversebytes"-method of the Integer-class !
By the way : inspecting the first sector of the hard-disk via reading /dev/sda works only in a linux-box , don't know how todo this on an MS-Windows-platform !
Suppose Windows will not even allow it ?
Thanks everybody for all the hints , being aware I'll have a lot to learn .
erics