Identifying sparse files programatically in C

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  1. #1
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    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

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

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    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

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