Block device information without mounting in Linux

Question

I am trying to get some information (specifically block size) of block device in linux, in C++. Is it possible to get block size of a device without mounting it and possibly without looking into dynamic files (like the ones in /sys), but with a system call only.

I was trying with stat, but it returns data about /dev filesystem if I ask about /dev/sdb2.

If it's impossible with system call, where should i look in dynamic files (haven't been able to locate it either.)

Answer 1

You want to use ioctl, in particular BLKSSZGET.

Quoting linux/fs.h:

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

Untested example:

#include <sys/ioctl.h>
#include <linux/fs.h>

int fd = open("/dev/sda");
size_t blockSize;
int rc = ioctl(fd, BLKSSZGET, &blockSize);

Answer 2

I think the ioctl value should rather be unsigned long than size_t (the latest is more memory related), I would also initialize it to 0 (just in case BLKSSZGET returns unsigned int instead).

#include <sys/ioctl.h>
#include <linux/fs.h>

int fd = open("/dev/sda");
unsigned long blockSize = 0;
int rc = ioctl(fd, BLKSSZGET, &blockSize);