Get sector size, etc using ioctl()

ioctl_demo.c

/* Relevant:

http://lists.gnu.org/archive/html/bug-parted/2012-10/msg00018.html
http://stackoverflow.com/questions/8416241/block-device-information-without-mounting-in-linux

*/

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

#ifndef BLKGETSIZE
#define BLKGETSIZE _IO(0x12,96)                   /* return device size */
#endif

#ifndef BLKPBSZGET
#define BLKSSZGET  _IO(0x12,104)/* get block device sector size */
#endif


int main(int argc, char **argv)
{
  int size;
  int logicalsectsize = 0;
  int retval;

  int fd = open("/dev/sdb", O_RDONLY | O_NONBLOCK);

  if (ioctl(fd, BLKGETSIZE, &size) != 0)
    size=0;

  /*retval=ioctl(fd, BLKSSZGET, &logicalsectsize);*/
  if (ioctl(fd, BLKSSZGET, &logicalsectsize) < 0)
    {
      printf("Can't get logical sector size. Return code: %d\n",retval);
      logicalsectsize = 0;
    }

  printf("%d %d\n",size, logicalsectsize);


  return 0;
}

logicalsectsize must be long.

If you are doing this in a loop, you will reach the maximum open file limit, and your system can no longer open files.
Make sure to close(fd).