raspberry pi use of statvfs() produce erroneous results

Question

I am using raspistill to acquire images with my PI. after some weeks my SD card is full and the whole thing dies so I need to install again. so I added some code to see what's happening with my available memory. Here is the code to assess my memory status.

//https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/com.ibm.zos.v2r2.bpxbd00/rstatv.htm           
        int r = statvfs(".", &buf);
        if (r < 0) {
            printf(" Error in statsvf"); 
            memAvailable = 4; // exit
        }       
        else {
            memAvailable = (unsigned long)(buf.f_bavail * buf.f_bsize);
            printf(" Memory available is %.0lu\n", memAvailable);
        }

this is the result I'm getting:

1. take a picture state 2
2. Memory available is 6868992
3. Turn: 325 working = 1 up = 1
4. saving image at Pix/005/img014_2020-07-02_10-23-37.jpg
5. Image saved take a picture state 2
6. Memory available is 2777088
7. Turn: 352 working = 1 up = 1
8. saving image at Pix/005/img015_2020-07-02_10-23-46.jpg
9. Image saved take a picture state 2
10. Memory available is 4293648384
11. Turn: 379 working = 1 up = 1
12. saving image at Pix/005/img016_2020-07-02_10-23-55.jpg
13. Image saved take a picture state 2
14. Memory available is 4289556480
15. Turn: 406 working= 1 up = 1

why is the available memory changing in the middle of the run mind you - I did not start or stop any program during this run, nor do I allocate a large size memory.

EDIT: My bad - it seems that 32GB is too long for an unsigned long. so i needed to assess only the f_bavail for my task. NOOB

Answer 1

You have an overflow when you do buf.f_bavail * buf.f_bsize because the computation is done using an unsigned long which not enough for the free size of your partition

Use unsigned long long, for instance :

#include <sys/statvfs.h>
#include <stdio.h>

int main()
{
  struct statvfs buf;
  int r = statvfs(".", &buf);
  
  if (r < 0) {
    printf(" Error in statsvf"); 
  }       
  else {
    unsigned long long ull = buf.f_bavail * (unsigned long long) buf.f_bsize;

    printf("Memory available is %llu\n", ull);
  }
  return 0;
}

You can also use uint64_t to be sure to have a unsigned integer on 64b.

In my case :

pi@raspberrypi:/tmp $ gcc -Wall c.c
pi@raspberrypi:/tmp $ ./a.out
Memory available is 16778645504
pi@raspberrypi:/tmp $ 

and the result is compatible with the indication given by the command df -H .

Note to write memory about a partition on a disk is not very clear and can be confused with the RAM