From: Robert Read <rread@...>
Date: 2000-12-16T10:53:14+09:00
Subject: [ruby-talk:7370] BUG? IO.seek conflicts with IO.sysread

I'm writing a simple tool to test a disk driver.  I want to write lots
of blocks and then read them back to verify them.  When I use seek and
sysread like this, it fails after 3 or four reads.  Sample code:

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)
    disk.sync = true
    begin
      for i in 0..@blk_count
	disk.seek(i*@blk_size)
	block = disk.sysread(@blk_size)
      end
    rescue
      $stderr.print "IO failed: " + $! + "\n"
      disk.close
      raise
    end
  end
end

I realize that in this simple case the seek is not needed, but the
final version will need to do more random io.

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.

thanks,
 robert