On Fri, Aug 12, 2005 at 11:48:22PM +0300, Nanakos Chrysostomos wrote:
> Yes,the sector size is 512 bytes,but this is not the beginning of my
> fourth partition.Please check the code below,and if you can please test
> it..
>
> mbr.c
> ------
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
>
>
> struct mbr {
> unsigned char boot_indicator;
> unsigned char s_head;
> unsigned char s_sector;
> unsigned char s_cylinder;
> unsigned char f_desc;
> unsigned char e_head;
> unsigned char e_sector;
> unsigned char e_cylinder;
> unsigned int rs_sector;
> unsigned int n_sectors;
> } __attribute__((packed));
>
>
>
> int main()
> {
> int fd;
> struct mbr s;
>
>
> fd= open("/dev/hdb",O_RDONLY);
> lseek(fd,0x01ee,SEEK_SET); /* This is the 4rth entry,extended for
> me*/
> read(fd,&s,sizeof(struct mbr));
>
> printf("Partition Entry 1:\n");
> printf("Boot Indicator: %#x\n",s.boot_indicator);
> printf("Starting head %u, cylinder %u, sector
> %u.\n",s.s_head,((s.s_sector &
> 0xc0)<<2)+s.s_cylinder,s.s_sector&0x3f);
> printf("Filesystem descriptor: %#x\n",s.f_desc);
> printf("Ending head %u, cylinder %u, sector
> %u.\n",s.e_head,((s.e_sector & 0xc0)<<2)+
> s.e_cylinder,s.e_sector&0x3f);
> printf("Starting sector: %u\n",s.rs_sector);
> printf("Number of sectors in partition: %u\n",s.n_sectors);
>
> fd= open("/dev/hdb4",O_RDONLY); /* Where is this in hdb,offset??*/
It is wherever the partition entry in sector 0 of hdb says partition 4
starts.
Why didn't you just print all the partition table entries above rather
than just #1, especially since in this case it is number 4 you care
about.
Print out the start sector number for partition 4 above and you should
find the offset for the extended partition table, in the cases where
partition 4 is the extended partition.
Seek to that location, read the partition table there. That should
contain either just a primary partition, or a primary partition and
another extended partition.
In case it had another extended partition, repeat until no more extended
partitions found.
> lseek(fd,446L,SEEK_SET);
> read(fd,&s,sizeof(struct mbr));
>
> printf("\nPartition Entry 4: Extended partition\n");
> printf("Boot Indicator: %#x\n",s.boot_indicator);
> printf("Starting head %u, cylinder %u, sector
> %u.\n",s.s_head,((s.s_sector &
> 0xc0)<<2)+s.s_cylinder,s.s_sector&0x3f);
> printf("Filesystem descriptor: %#x\n",s.f_desc);
> printf("Ending head %u, cylinder %u, sector
> %u.\n",s.e_head,((s.e_sector & 0xc0)<<2)+
> s.e_cylinder,s.e_sector&0x3f);
> printf("Starting sector: %u\n",s.rs_sector);
> printf("Number of sectors in partition: %u\n",s.n_sectors);
>
> return 0;
> }
Say you had this partition table:
start sector number of sectors
pri1 1 10 (hda1)
pri2 11 20 (hda2)
pri3 31 10 (hda3)
pri4 41 100 (extended)
then at sector 41 you would find another partition table:
pri1 42 9 (hda5)
pri2 51 90 (extended)
Then sector 51 contains a partition table:
pri1 52 19 (hda6)
pri2 71 70 (extended)
Then sector 71 has a partition table:
pri1 72 69 (hda7)
And no more extended entries, so you are done.
C/H/S entries are entirely ignored (or should be) on modern drives.
Only start sector and total sectors are relevant as far as I understand
things.
Len Sorensen
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