Question

I am trying to print a partition table using C programming language, everything seems to work fine: Opening and reading, but I don’t understand why it is printing garbage values.

Here is the code:

struct partition
{
    unsigned char drive;
    unsigned char chs_begin[3];
    unsigned char sys_type;
    unsigned char chs_end[3];
    unsigned char start_sector[4];
    unsigned char nr_sector[4];
};

int main()
{
    int gc = 0, i = 1, nr = 0, pos = -1, nw = 0;
    int fd =0;
    char  buf[512] ;
    struct partition *sp;
    printf("Ok ");

    if ( (fd = open("/dev/sda", O_RDONLY | O_SYNC )) == -1)
    {
         perror("Open");
         exit(1);
    }
    printf("fd is %d \n", fd);

    pos = lseek (fd, 0, SEEK_CUR);

    printf("Position of pointer is :%d\n", pos);
    if ((nr = read(fd, buf, sizeof(buf))) == -1)
    {
        perror("Read");
        exit(1);
    }

    close(fd);

    printf("Size of buf = %d\n and number of bytes read are %d ", sizeof(buf), nr);


    if ((nw = write(1, buf, 64)) == -1)
    {
        printf("Write: Error");
        exit(1);
    }

    printf("\n\n %d bytes are just been written on stdout\n", nw,"this can also be printed\n");

    printf("\n\t\t*************Partition Table****************\n\n");

    for (i=0 ; i<4 ; i++)
    {
        sp = (struct partition *)(buf + 446 + (16 * i));
        putchar(sp -> drive);
    }


    return 0;
}

It is printing garbage instead of partition table.

I might have some basic understanding issues but I searched with Google for a long time but it did not really help. I also saw the source code of fdisk but it is beyond my understanding at this point. Could anyone please guide me? I am not expecting someone to clear my mistake and give me the working code. Just a sentence or two – or any link.

Answer 1

Try this:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

struct partition
{
        unsigned char boot_flag;        /* 0 = Not active, 0x80 = Active */
        unsigned char chs_begin[3];
        unsigned char sys_type;         /* For example : 82 --> Linux swap, 83 --> Linux native partition, ... */
        unsigned char chs_end[3];
        unsigned char start_sector[4];
        unsigned char nr_sector[4];
};

void string_in_hex(void *in_string, int in_string_size);
void dump_partition(struct partition *part, int partition_number);

void dump_partition(struct partition *part, int partition_number)
{
        printf("Partition /dev/sda%d\n", partition_number + 1);
        printf("boot_flag = %02X\n", part->boot_flag);
        printf("chs_begin = ");
        string_in_hex(part->chs_begin, 3);
        printf("sys_type = %02X\n", part->sys_type);
        printf("chs_end = ");
        string_in_hex(part->chs_end, 3);
        printf("start_sector = ");
        string_in_hex(part->start_sector, 4);
        printf("nr_sector = ");
        string_in_hex(part->nr_sector, 4);
}

void string_in_hex(void *in_string, int in_string_size)
{
        int i;
        int k = 0;

        for (i = 0; i < in_string_size; i++)
        {
                printf("%02x ", ((char *)in_string)[i]& 0xFF);
                k = k + 1;
                if (k == 16)
                {
                        printf("\n");
                        k = 0;
                }
        }
        printf("\n");
}

int main(int argc, char **argv)
{
        int /*gc = 0,*/ i = 1, nr = 0, pos = -1/*, nw = 0*/;
        int fd = 0;
        char  buf[512] ;
        struct partition *sp;
        int ret = 0;

        printf("Ok ");

        if ((fd = open("/dev/sda", O_RDONLY | O_SYNC)) == -1)
        {
                perror("Open");
                exit(1);
        }
        printf("fd is %d\n", fd);

        pos = lseek (fd, 0, SEEK_CUR);

        printf("Position of pointer is :%d\n", pos);
        if ((nr = read(fd, buf, sizeof(buf))) == -1)
        {
                perror("Read");
                exit(1);
        }

        ret = close(fd);
        if (ret == -1)
        {
                perror("close");
                exit(1);
        }

        /* Dump the MBR buffer, you can compare it on your system with the output of the command:
         * hexdump -n 512 -C /dev/sda
         */
        string_in_hex(buf, 512);

        printf("Size of buf = %d - and number of bytes read are %d\n", sizeof(buf), nr);
        /*if ((nw = write(1, buf, 64)) == -1)
        {
                printf("Write: Error");
                exit(1);
        }

        printf("\n\n%d bytes are just been written on stdout\nthis can also be printed\n", nw);
        */
        //printf("\n\t\t*************Partition Table****************\n\n");
        printf("\n\t\t*************THE 4 MAIN PARTITIONS****************\n\n");

        /* Dump main partitions (4 partitions) */
        /* Note : the 4 partitions you are trying to dump are not necessarily existing! */
        for (i = 0 ; i < 4 ; i++)
        {
                sp = (struct partition *)(buf + 446 + (16 * i));
                //putchar(sp->boot_flag);
                dump_partition(sp, i);
        }

        return 0;
}