Get Real Free Usable Space

I've got an issue with a program that should send me back the free disk space usable by any user.

My goal is to get all the free disk space of every partitions of my hard drive that is usable by anyone who doesn't have sudo rights.

So I tryed this :

int main() {
    struct statvfs diskData;
    statvfs("/", &diskData);
    unsigned long long available = (diskData.f_favail + diskData.f_bavail) * diskData.f_frsize) / (1024 * 1024)
    std::cout << "Free Space : " << available << std::endl;
}

This gives me a total of 2810 ...

However, when I output df -h, I can read that the available space is 25G for sda3 and 30G for sda1

This seems completely inaccurate.

I've been running on the posts on Stackoverflow, mixing solutions I saw, but none is satisfactory. How can I get a correct value in Megabytes of my available free space ?

EDIT : Full statvfs and df / output

statvfs :

Block Size            : 4 096
Fragment Size         : 4 096
Blocks                : 9 612 197
Free Blocks           : 7 009 166
Non Root Free Blocks  : 6 520 885
Inodes                : 2 444 624
Free Inodes Space     : 2 137 054
Non Root Free Inodes  : 2 137 054
File System ID        : 4 224 884 198
Mount Flags           : 4 096
Max Filename Length   : 255

df / :

Filesystem     1K-Blocks      Used           Available      Use%     Mounted on
/dev/sda3      38 448 788     10 412 112     26 083 556     29%      /

Solution

It seems that the right value to use is the fragment size, not the block size (i.e. f_frsize)

Have you tried with

diskData.f_bavail * diskData.f_frsize

instead ?