Disk Block Size in a C program

[dumb_terminal]

Hi all, i need to find out how many blocks a file is being allocated (in actual) in a c program. I tried this code:

lstat(filepath, &info) ;
printf("block size = %d\n", (int)(info.st_blksize)) ;

Which gives me 4096, but the command sfdisk -l gives 1024 which seems to agree with ls -lsi command and therefore correct. So the question is how do i get the actual (correct)block size in a c program ?? Plz help. Thanx in advance.

[Lakshmipathi]

Hi all, i need to find out how many blocks a file is being allocated (in actual) in a c program. I tried this code:

lstat(filepath, &info) ;
printf("block size = %d\n", (int)(info.st_blksize)) ;

Which gives me 4096, but the command sfdisk -l gives 1024 which seems to agree with ls -lsi command and therefore correct. So the question is how do i get the actual (correct)block size in a c program ?? Plz help. Thanx in advance.

I think you would need understand the difference between two sizes in stat -

blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */

What you have retrieved is st_blksize which is nothing but your File System block size. (by default its 4096 or 4KB)
For example, see below file bit.c its size is just 146 bytes

$ ls -ltr bit.c
-rw-rw-r--. 1 laks laks 146 2011-05-30 23:16 bit.c

But using stat command you can see

[laks@space ~]$ stat bit.c
File: `bit.c'
Size: 146 Blocks: 8 IO Block: 4096 regular file
Device: fd00h/64768d Inode: 249733 Links: 1
Access: (0664/-rw-rw-r--) Uid: ( 500/ laks) Gid: ( 500/ laks)
Access: 2011-05-30 23:16:06.157773774 +0530
Modify: 2011-05-30 23:16:04.844524535 +0530
Change: 2011-05-30 23:16:04.972521528 +0530

Its says Blocks : 8 and IO Block as 4096
Blocks = IO block / 512
which is
Blocks = 4096 / 512 = 8
If I'm not wrong - I think Blocks=8 ( st_blocks ) is rarely used .

So now

, i need to find out how many blocks a file is being allocated

its

if (st_size < 4096 )
then actually used blocks = 1
else
blocks = st_size / st_blksize

Btw,Whats your File system? If it belongs to ext family (ext2/ext3/ext4) and you want to dig deeper into them, try using ext2fs libraries.

If you are looking for file's data block then you should be looking at i_block[14] in inode structure

[dumb_terminal]

@Lakshmipathi Thank u a lot, i think i need to look at the inode structure, as i am implementing ls which needs to show the no of blocks occupied by a file when -s flag is given. Yhank u again. very helpful.