From: matz@... (Yukihiro Matsumoto)
Date: 2000-12-16T14:04:08+09:00
Subject: [ruby-talk:7396] Re: BUG? IO.seek conflicts with IO.sysread

Hi,

In message "[ruby-talk:7370] BUG? IO.seek conflicts with IO.sysread"
    on 00/12/16, Robert Read <rread@datarithm.net> writes:

|The error comes from io.c:rb_io_sysread around line 1157:
|
|    if (READ_DATA_PENDING(fptr->f)) {
|	rb_raise(rb_eIOError, "sysread for buffered IO");
|    }
|
|READ_DATA_PENDING seems to be confused by the seek.  I'm new to Ruby,
|so I'm not sure if this is my fault or a real bug.

Hmm, the simple case like

  ruby -e 'f=open("ChangeLog"); p f.sysread(10); f.seek(100,0); p f.sysread(10)'

works.  But fseek(3) sometimes fill the buffer in certain case.
It will be fixed in the future version, which will not use stdio.

Anyway here's the workaround.

class DCheck 
  def initialize(dev, blk_size, blk_count)
    @dev = dev
    @blk_size = blk_size
    @blk_count = blk_count
  end
  
  def verifyBlocks()
    disk = File.new(@dev)
    fd = disk.fileno
    disk.sync = true
    begin
      for i in 0..@blk_count
	f = IO.new(fd)
	f.seek(i*@blk_size)
	block = f.sysread(@blk_size)
        f.close
      end
    rescue
      $stderr.print "IO failed: " + $! + "\n"
      disk.close
      raise
    end
  end
end