Reading block device sizes in python - More in the blog post: https://blog.lnx.cx/?p=678

linux-compile-run-request-code.sh

$ gcc ./linux-request-code.c
$ ./a.out 
Block count request code: 0x80081272

linux-request-code.c

#include <linux/fs.h>
#include<stdio.h>

int main()
{
  printf("Block count request code: 0x%lx\n", BLKGETSIZE64);
}

osx-calclulate-disk-size.c

#include <sys/disk.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <stdio.h>

/* Originally posted in http://stackoverflow.com/questions/9073614/open-raw-disk-and-get-size-os-x/9764508#9764508 */

int main()
{
    // Open disk
    uint32_t dev = open("/dev/disk1",O_RDONLY);
    if (!dev || dev == -1)
        return 1;
    uint64_t sectors = 0;
    // Query the number of sectors on the disk
    ioctl(dev, DKIOCGETBLOCKCOUNT, &sectors);

    uint32_t sectorSize = 0;
    // Query the size of each sector
    ioctl(dev, DKIOCGETBLOCKSIZE, &sectorSize);

    uint64_t diskSize = sectors * sectorSize;
    printf("Disk dize: %llu", diskSize);
    return 0;
}

osx-compile-run-request-code.sh

$ gcc ./osx-request-code.c
$ ./a.out
Block size request code: 0x40046418
Block count request code: 0x40086419

osx-request-code.c

#include <sys/disk.h>
#include<stdio.h>

int main()
{
  printf("Block size request code: 0x%lx\n", DKIOCGETBLOCKSIZE);
  printf("Block count request code: 0x%lx\n", DKIOCGETBLOCKCOUNT);
}

readsize.py

import fcntl
import struct

device_path = '/dev/sda'

req = 0x80081272 # BLKGETSIZE64, result is bytes as unsigned 64-bit integer (uint64)
buf = ' ' * 8
fmt = 'L'

with open(device_path) as dev:
    buf = fcntl.ioctl(dev.fileno(), req, buf)
bytes = struct.unpack(fmt, buf)[0]

print device_path, 'is about', bytes / (1024 ** 2), 'megabytes'