allocated space of a directory

Dave31836 · 10-03-2004, 03:09 PM

In C++, what is the best way to find the total storage allocated in a directory? I am familiar with stat and statfs which returns allocations of a file and total disk, respectively, but how to you get the allocated space of the contents of a directory?

CroMagnon · 10-03-2004, 06:52 PM

You add up all the files in that directory and its subdirectories

Dave31836 · 10-04-2004, 10:01 AM

As I'm adding up the directory entries, I do:

struct stat st;
stat( szPath, &st );
long lBytes = st.st_blocks * st.st_blksize;

...

This gives an answer 8 X too large. While st.st_blksize is 4096,
the actual block size is 512 (as derives from the size shown by
the file File Manager.

Any ideas what is wrong?

CroMagnon · 10-04-2004, 03:20 PM

Why are you calculating by blocks? Does st.st_size not contain the right filesize, or do you want to count the total size taken up on the disk?

Apparently st_blksize is the 'preferred blocksize' for talking to the device but st_blocks is always measured in 512-byte blocks, so just do st_blocks << 9.

Dave31836 · 10-04-2004, 03:37 PM

thanks... I want the actual allocation on disk, not just the file size.

I guiess I'll use 512 (i think you meant st_blocks << 3) and hope it
is portable across other file systems.

thnx again

CroMagnon · 10-04-2004, 03:49 PM

No, I meant << 9, as that multiplies by 2^9, or 512. << 3 only multiplies by 8 (which is fine if you want to know how big your file is in terms of 64-byte blocks)

jlliagre · 10-04-2004, 04:08 PM

Beware that either methods can be innacurate.

Summing file size can lead to very different results than the real size the directory is using on the disk:
Files smaller that the allocation unit would lead you to think less space is used that the real one.
Sparse files will make you think the opposite (puzzle people by creating a 2 Gb file on a 1Gb partition).

Finally, pay attention not to count multiple times files sharing the same inode (hard-links).

CroMagnon · 10-04-2004, 05:14 PM

Good points jlliagre, I forgot about sparse files. Of course, if he only wants the physical disk space used, then this method won't cause him trouble...

If you check the st_nlink member to make sure it's 1, you won't have a hardlink issue (if it is >1, add st_ino to an array, and check any future files with nlink > 1 against this array, that should be enough to avoid hard link issues. Also, you may want to use lstat instead of stat to correctly count space used by symbolic links).

Dave31836 · 10-04-2004, 06:07 PM

re No, I meant << 9.... of course -- i misunderstood your point.

thanks again