Floppies use the FAT12 filesystem, which is good at not wasting space, but very hard to use. Its FAT entries are, as the name suggests, twelve bits in size. That's one and a half bytes, like this:

Code:

00 10 15 05 80 90

You can't make an array of that. You'd have to tease the data out the hard way, figuring out where the entry starts, which whole bytes and which half bytes should be retrieved, etc, etc.

You don't need to use the loop device at all to make the DOS filesystem, just to mount it. mkfs.msdos will warn you a file's not a device but write to it anyway.

Be sure to run losetup -d /dev/loop0 when you're done with that tutorial, or you'll find the loop device still busy when you try to use it for something else.

The instructions I posted in the post before yours should work fine for creating a FAT16 filesystem on a file 500 megabytes or smaller. FAT16 entries fit nicely in an array of 16-bit integers.




You should start making data structures to hold information so you don't have to find absolutely everything by byte offsets all the time:

Code:

#include <stdint.h> // standard sized integer types
// to make sure the compiler doesn't insert extra space in the structure
#pragma pack(push,1)
typedef struct bootsector
{
        uint8_t jump[3];
        uint8_t oemname[8];
        uint16_t bytes_per_sector;
        uint8_t bytes_per_cluster;
        uint16_t reserved;
        uint8_t fats;
        uint16_t root_entries;
        uint16_t total_sectors;
        uint8_t media_descriptor;
        uint16_t sectors_per_fat;
        uint16_t sectors_per_track;
        uint16_t heads;
        uint32_t hidden;
        uint32_t total;
} bootsector;
// reset structure definitions to normal
#pragma pack(pop);

...

bootsector bs;
read(fd, &bs, sizeof(bs));

fprintf(stderr, "%d bytes per sector\n", bs.bytes_per_sector);

All that's gleaned from information on the wiki, and it appears to match the dump I posted above(the OEM name is indeed filled with garbage for some reason!)

Look if the values you get in that structure seem normal. Try to use the information in it to find other structures. See if you can find the information you need to piece the partition together. You have to understand what's going on in those structures before you can alter them properly.

I hope this gives you the idea as I don't have the time to do absolutely everything for you.