>>>>> On Wed, 12 Aug 1992 06:35:34 GMT, jwinstea@jarthur.claremont.edu
quoted text
>>>>> (Jim Winstead Jr.) said:

Jim> Not necessarily true - GNU cp contains the optimization that it will
Jim> use lseek() to 'write' holes in a file instead of writing out all the
Jim> individual null bits.  This is what causes shoelace to break for some
Jim> people.  I'm not sure what effect this has on copying to a floppy with
Jim> the cp command - but I *know* dd is safe.

An examination of the source code shows that GNU cp only performs this
optimization if it thinks that the original file also had holes.  The
way GNU cp determines this is by comparing the number of blocks used
with the actual file size.  Furthermore, it only does this when it is
compiled without ST_BLOCKS_MISSING defined.  Note that the kernel is
created by build and does not have any holes.

So if your cp is creating holes ROUTINELY then:

* fstat is broken and returns incorrect values for st_blocks, or
* you manually defined an incorrect value of DEV_BSIZE (i.e. too small)

Here is the relevant code from GNU cp.c:

+-----
| #ifndef ST_BLOCKS_MISSING
|   if (S_ISREG (sb.st_mode))
|     {
|       /* Find out whether the file contains any sparse blocks. */
|
|       if (fstat (source_desc, &sb))
|         {
|           error (0, errno, "%s", src_path);
|           return_val = -1;
|           goto ret;
|         }
|
|      /* 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 (S_ISREG (sb.st_mode) &&
|           sb.st_size - (sb.st_blocks * DEV_BSIZE) >= DEV_BSIZE)
|         make_holes = 1;
|     }
| #endif
+-----
--
Rick Sladkey
jrs@world.std.com