Thread: Identifying sparse files programatically in C

How would I identify sparse files through a C program ?I know that for a sparse file, its real size would be less than that returned by stat(). How would I get the real size ?

Thanks

You should read more about stat, because that's the system call that you need.

Look at the stat structure. One crude heurestic is that if the size of the file (st_size) is greater than the number of blocks (st_blocks) multipied by the size of a block, then you have a sparse file.

The GNU core utilities which support the --sparse option use this heuristic. From copy.c:

Code:

#if HAVE_STRUCT_STAT_ST_BLOCKS
          /* Use a heuristic to determine whether SRC_NAME contains any sparse
             blocks.  If the file has fewer blocks than would normally be
             needed for a file of its size, then at least one of the blocks in
             the file is a hole.  */
          if (x->sparse_mode == SPARSE_AUTO && S_ISREG (src_open_sb.st_mode)
              && ST_NBLOCKS (src_open_sb) < src_open_sb.st_size / ST_NBLOCKSIZE)
            make_holes = true;
#endif