how to get hdd's free space

Question

I want to get free hdd's size in a remote system with ssh.
I found this ways:
1-do it via command line: hdd's totalsize - sum of partitions size on that hdd + sum of free spaces on all partitions on that hdd;
There are some problems with this way.
first, I prefer not to use commands, because it will get way complicated , it will be full of converting types to each other. like string to float and... second is that with running command, I cannot have unmounted partitions free space.
third problem is that maybe the hdd itself be unmounted! the other way is:
2-using statvfs structure.
again, with this structure, you can have mounted hdd's free space. the code I am using :

#include <stdio.h>
#include <sys/statvfs.h>
int main(int argc, const char *argv[])
{

    const unsigned int GB = (1024 * 1024) * 1024;
    struct statvfs buffer;
    int ret = statvfs(argv[1], &buffer);

    if (!ret) {
        const double total = (double)(buffer.f_blocks * buffer.f_bsize) / GB;
        const double available = (double)(buffer.f_bfree * buffer.f_bsize) / GB;
        const double used = total - available;
        const double usedPercentage = (double)(used / total) * (double)100;
        printf("Total: %f --> %.0f\n", total, total);
        printf("Available: %f --> %.0f\n", available, available);
        printf("Used: %f --> %.1f\n", used, used);
        printf("Used Percentage: %f --> %.0f\n", usedPercentage, usedPercentage);
    }
    return ret;
}

I've seen this link: Get free space of HDD in linux
but the outputs of stat, are not similar to numbers that I see in df command's output.
I am using Qt and my os is ubuntu 18.04.
when Irun this code I get this :

heydari.f@swkb-dev2:~/projects/cpptest/build-cpptest-Desktop-Debug$ ./cpptest .
Total: 31.570432 --> 32
Available: 15.594684 --> 16
Used: 15.975748 --> 16.0
Used Percentage: 50.603513 --> 51

and my df's output is like this:

heydari.f@swkb-dev2:~$ df -H ~/projects/cpptest/build-cpptest-Desktop-Debug
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        32G   16G   14G  54% /

EDIT:

heydari.f@swkb-dev2:~/projects/cpptest/build-cpptest-Desktop-Debug$ ./cpptest .
Total: 31.570432 --> 32
Available: 13.967569 --> 14
Used: 17.602863 --> 17.6
Used Percentage: 55.757435 --> 56

Answer 1

you do not give the results you have, so I can see two reasons :

First may be you have an overflow when doing buffer.f_blocks * buffer.f_bsize and buffer.f_bfree * buffer.f_bsize because your unsigned long are on 32b, so do for instance :

const double total = ((double) buffer.f_blocks) * buffer.f_bsize / GB;
const double available = ((double) buffer.f_bfree) * buffer.f_bsize / GB;

Second for df 1Gb is 1000*1000*1000 rather than 1024*1024*1024

If I execute your version on my 2Gb PI4 I get :

pi@raspberrypi:/tmp $ ./a.out .
Total: 3.338009 --> 3
Available: 0.439247 --> 0
Used: 2.898762 --> 2.9
Used Percentage: 86.841044 --> 87
pi@raspberrypi:/tmp $ 

after the modification to avoid overflow I get

pi@raspberrypi:/tmp $ ./a.out .
Total: 27.338009 --> 27
Available: 16.439247 --> 16
Used: 10.898762 --> 10.9
Used Percentage: 39.866699 --> 40
pi@raspberrypi:/tmp $ 

while df -H . gives :

pi@raspberrypi:/tmp $ df -H .
Sys. de fichiers Taille Utilisé Dispo Uti% Monté sur
/dev/root           30G     12G   17G  42% /

but

pi@raspberrypi:/tmp $ bc
30*1000*1000*1000/1024/1024/1024
27
pi@raspberrypi:/tmp $