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From: CDCKAB%EMUVM1.BITNET@cunyvm.cuny.edu ( Karl Brendel)
Newsgroups: comp.lang.pascal
Subject: (R)Blockread/write problem
Message-ID: <24352@adm.BRL.MIL>
Date: 31 Aug 90 12:56:55 GMT
Sender: news@adm.BRL.MIL
Lines: 46


In <26DE0F04.26212@ics.uci.edu> Alastair Milne <milne@ics.uci.edu>
  writes:

>In <1990Aug29.123053.17412@ux1.cso.uiuc.edu> dslg0849@uxa.cso.uiuc.edu (Daniel
> S. Lewart) writes:
>
>>>1. When I run this program, I don't get the whole file copied (I miss severa
>>>   bytes).
>>>   FILE.1  TEMP   missing bytes:
>>>   7056    7040    16
>>>   6899    6784    115
>>>   6296    6272    24
>>>   3040    2944    96
>
>>Notice that all the TEMP sizes are multiples of 128, the default record size.
>>Try reset(fil1,1) instead.
>
>    Doing an entire BlockRead for a single byte seems a waste of the routine,
>    and probably invites a lot of overhead.  If you want to do single-characte
>    I/O, skip BlockIO altogether and just use FILE OF BYTE, with plain READs.
>    May even be faster than BlockIO.

The suggestion is not to BlockRead one byte at a time, but to use a
record size of one byte. The BlockRead can still access as many bytes as
in the original. I suspect that all typed file I/O becomes equivalent to
BlockI/O in the end, so I doubt that Read(some file of byte) can be any
faster than BlockRead.

>   I suspect you are not getting the whole file copied because BlockRead's
>   last call causes a non-zero IOResult because of not being able to read a
>   full 128-byte block.  This would keep BlockWrite from doing any further

Because the original code provided a variable for the optional "result"
parameter, BlockRead would not cause an I/O error (or a non-zero
IOResult) when reading the last partial block. It _would_, indeed, read
that partial block into the buffer. Unfortunately, there would be no
explicit indication that a partial block had been read.

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