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Date: Fri, 21 Nov 2008 13:58:30 +0100
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Subject: Cygwin 1.7: O_DIRECT read from raw devices returns EOF
From: "Christian Franke" <Christian DOT Franke AT t-online DOT de>
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On cygwin 1.7.0-32, read() from raw devices returns 0 always if device
was opened with O_DIRECT.

Testcase:

$ dd if=/dev/sdb of=/dev/null count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.027 s, 19.0 kB/s

$ dd if=/dev/sdb of=/dev/null count=1 iflag=direct
0+0 records in
0+0 records out
0 bytes (0 B) copied, 0 s, Infinity B/s

Same for /dev/scd0, /dev/fd0.

Bug was probably introduced in fhandler_dev_floppy.cc 1.51:

void
fhandler_dev_floppy::raw_read (void *ptr, size_t& ulen)
{
...
  DWORD bytes_to_read = 0;
...
  if (devbuf)
    { /* !O_DIRECT ... */ }
  else
    {
      _off64_t current_position = get_current_position ();
      if (current_position + bytes_to_read >= drive_size)
	bytes_to_read = drive_size - current_position;
      if (bytes_to_read && !read_file (p, len, &bytes_read, &ret))
...

In the O_DIRECT case, bytes_to_read is always 0.



Christian




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