A little program to analyse the mbr of any drive on your system, and provide information about the partitions

analyse_mbr.c

/* analyse_mbr.c by glem */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#define MBR_SIZE 512
#define NUM_PARTITIONS 4

/* offsets within the MBR */
#define SEC_PARTITION_START	0x1BE
#define SEC_MAGICBYTES_START	0x1FE

/* offsets within a partition record */
#define OFF_START_CHS	0x1
#define OFF_PART_DESC	0x4
#define OFF_END_CHS	0x5
#define OFF_START_SEC	0x8
#define OFF_PART_SZ	0xC

/* global mbr buffer */
static uint8_t mbr[512];

static void	check_boot_indicator(uint32_t);
static void	check_chs_values(char *, uint32_t);
static uint32_t check_partition_size(uint32_t);
static void	check_starting_sector(uint32_t);
static void	check_partition_fs(uint32_t);
static char *	sector_to_size_str(uint32_t);

int 
main(int argc, char **argv){
	/* sanity checking, need to provide drive name */
	if (argc < 2) {
		printf("usage: %s /dev/<drive>\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	/* open drive and read first sector into mbr */
	printf("** Opening drive %s..\n", argv[1]);
	FILE *root_drive = fopen(argv[1], "r");
	if (!root_drive) {
		printf("Could not open %s, try running as root?\n", argv[1]);
		exit(EXIT_FAILURE);
	}
	fread(&mbr, MBR_SIZE, 1, root_drive);

	/* ensure magic bytes are in the sector read in */
	if (mbr[SEC_MAGICBYTES_START+1]==0xaa && mbr[SEC_MAGICBYTES_START]==0x55)
		puts("** MBR magic bytes in place!");
	else {
		puts("** Error - could not read magic bytes");
		exit(EXIT_FAILURE);
	}

	/* loop through the partitions and print information */
	for (uint8_t i = 0; i < NUM_PARTITIONS; i++) {
		uint32_t start = SEC_PARTITION_START + i*0x10;
		printf("** Analyzing Partition %u\n", i);
		check_boot_indicator(start);
		if(!check_partition_size(start+OFF_PART_SZ))
			continue;
		check_chs_values("Starting", start+OFF_START_CHS);
		check_chs_values("Ending  ", start+OFF_END_CHS);
		check_starting_sector(start+OFF_START_SEC);
		check_partition_fs(start+OFF_PART_DESC);
	}
}

/*
 * prints the boot flag, indicating if this partition is bootable or not
 */
static void
check_boot_indicator(uint32_t start) {
	if (mbr[start]==0x00)
		puts("\tThis is not bootable");
	else if(mbr[start]==0x80)
		puts("\tThis is bootable");
	else
		puts("\tError reading bootable flag");
}

/*
 * retrieve the CHS value of the start/end of the partition for the drive
 */
static void
check_chs_values(char *str, uint32_t start) {
	uint8_t c,h,s;
	c = mbr[start];
	h = mbr[start+0x1];
	s = mbr[start+0x2];
	printf("\t%s CHS values C=%u, H=%u, S=%u\n", str, c, h, s);
}


/*
 * retrieve the number of sectors into the disk drive the partition exists
 */
static void
check_starting_sector(uint32_t start) {
	uint8_t *start_loc = &mbr[start];
	uint32_t start_sec = *(uint32_t *)start_loc;
	char *sz_str = sector_to_size_str(start_sec);
	printf("\tPartition starts at sector number %u (%s in)\n", start_sec, sz_str);
}

/*
 * retrieve the number of sectors the partition occupies
 */
static uint32_t
check_partition_size(uint32_t start) {
	uint8_t *size_loc = &mbr[start];
	uint32_t size = *(uint32_t *)size_loc;
	char *sz_str = sector_to_size_str(size);
	if (size) {
		sz_str = sector_to_size_str(size);
		printf("\tPartition is %u sectors (%s)\n", size, sz_str);
	} else
		puts("\tIs an empty partition");
	return size;
}

/*
 * retrieve the ID indicating the type of filesystem the drive uses
 */
static void
check_partition_fs(uint32_t start) {
	uint8_t fs_id = mbr[start];
	printf("\tPartition filesystem id is %u\n", fs_id);
}

/*
 * static function to convert a number of sectors into a useable size
 */
static char *
sector_to_size_str(uint32_t size) {
	double sz_kb, sz_mb, sz_gb;
	char *sz_str = malloc(10);
	sz_kb = size / 2;
	sz_mb = sz_kb / 1024;
	sz_gb = sz_mb / 1024;
	if (sz_gb>=1)
		sprintf(sz_str, "%.2fGB", sz_gb);
	else if (sz_mb>=1)
		sprintf(sz_str, "%.2fMB", sz_mb);
	else 
		sprintf(sz_str, "%.2fKB", sz_kb);
	return sz_str;
}

output

** Opening drive /dev/sdb..
** MBR magic bytes in place!
** Analyzing Partition 0
        This is not bootable
        Partition is 1953521664 sectors (931.51GB)
        Starting CHS values C=32, H=33, S=0
        Ending   CHS values C=254, H=255, S=255
        Partition starts at sector number 2048 (1.00MB in)
        Partition filesystem id is 7
** Analyzing Partition 1
        This is not bootable
        Is an empty partition
** Analyzing Partition 2
        This is not bootable
        Is an empty partition
** Analyzing Partition 3
        This is not bootable
        Is an empty partition