How do I convert s.st_dev to /sys/block/<name> - c++

Question

I want to determine whether a file is on an HDD or an SDD.
I found out that I could check the type of drive using the /sys/block info:
prompt$ cat /sys/block/sdc/queue/rotational
1
This has 1 if it is rotational or unknown. It is 0 when the disk is an SSD.
Now I have a file and what to know whether it is on an HDD or an SDD. I can stat() the file to get the device number:
struct stat s;
stat(filename, &s);
// what do I do with s.st_dev now?
I'd like to convert s.st_dev to a drive name as I have in my /sys/block directory, in C.
What functions do I have to use to get that info? Or is it available in some /proc file?

Answer 1

First of all for the input file we need to file on which partition the file exists
you can use the following command for that
df -P <file name> | tail -1 | cut -d ' ' -f 1
Which will give you output something like this : /dev/sda3
Now you can apply following command to determine HDD , SDD
cat /sys/block/sdc/queue/rotational
You can use popen in your program to get output of these system commands

Answer 2

Okay, I really found it!
So my first solution, reading the partitions, wouldn't work. It would give me sbc1 instead of sbc. I also found the /proc/mounts which includes some info about what's mounted where, but it would still not help me convert the value to sbc.
Instead, I found another solution, which is to look at the block devices and more specifically this softlink:
/sys/dev/block/<major>:<minor>
The <major> and <minor> numbers can be extracted using the functions of the same name in C (I use C++, but the basic functions are all in C):
#include <sys/types.h>
...
std::string dev_path("/sys/dev/block/");
dev_path += std::to_string(major(s.st_dev));
dev_path += ":";
dev_path += std::to_string(minor(s.st_dev));
That path is a soft link and I want to get the real path of the destination:
char device_path[PATH_MAX + 1];
if(realpath(dev_path.c_str(), device_path) == nullptr)
{
return true;
}
From that real path, I then break up the path in segments and search for a directory with a sub-directory named queue and a file named rotational.
advgetopt::string_list_t segments;
advgetopt::split_string(device_path, segments, { "/" });
while(segments.size() > 3)
{
std::string path("/"
+ boost::algorithm::join(segments, "/")
+ "/queue/rotational");
std::ifstream in;
in.open(path);
if(in.is_open())
{
char line[32];
in.getline(line, sizeof(line));
return std::atoi(line) != 0;
}
segments.pop_back();
}
The in.getline() is what reads the .../queue/rotational file. If the value is not 0 then I consider that this is an HDD. If something fails, I also consider that the drive is an HDD drive. The only way my function returns false is if the rotational file exists and is set to 0.
My function can be found here. The line number may change over time, search for tool::is_hdd.
Old "Solution"
The file /proc/partition includes the major & minor device numbers, a size, and a name. So I just have to parse that one and return the name I need. Voilà.
$ cat /proc/partitions
major minor #blocks name
8 16 1953514584 sdb
8 17 248832 sdb1
8 18 1 sdb2
8 21 1953263616 sdb5
8 0 1953514584 sda
8 1 248832 sda1
8 2 1 sda2
8 5 1953263616 sda5
11 0 1048575 sr0
8 32 976764928 sdc
8 33 976763904 sdc1
252 0 4096 dm-0
252 1 1936375808 dm-1
252 2 1936375808 dm-2
252 3 1936375808 dm-3
252 4 16744448 dm-4
As you can see in this example, the first two lines represent the column names and an empty.The Name column is what I was looking for.