Question

I have written a code to calculate the hardisk size, but for some reason it always gives the size lesser than the actual size.

Like, 80GB will show as 74GB, and 160GB will show as 149 GB.
Where is the catch?

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <linux/fs.h>


int main()
{

        long bytes = 0;
        int fd = open("/dev/sdb1", O_RDONLY);
        const unsigned long long a = (1024ULL* 1024ULL * 1024ULL);

        int retval = ioctl(fd, BLKGETSIZE64, &bytes);
        int hdSize = bytes / a;

        printf(" Harddisk = %lld \n",hdSize);

        return EXIT_SUCCESS;
}

Answer 1

Simply different Gigabyte definitions. You use 1 GB = 2³⁰ bytes, many harddrive vendors use 10⁹ bytes.

For instance, an “80 GB” disk from a manufacturer using the latter definition will hold 80,000,000,000 bytes, which equals 78,125,000 KB, 76,294 MB, or (as you verified) 74.51 GB where all my units use power-of-two definitions.