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From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: read past EOF?
Message-ID: <749@atari.UUCP>
Date: Fri, 5-Jun-87 13:37:24 EDT
Article-I.D.: atari.749
Posted: Fri Jun  5 13:37:24 1987
Date-Received: Wed, 10-Jun-87 03:26:45 EDT
References: <1987@ihuxy.ATT.COM>
Organization: Atari Corp., Sunnyvale CA
Lines: 52

in article <1987@ihuxy.ATT.COM>, nowlin@ihuxy.ATT.COM (Jerry Nowlin) says:
> If
> there are only 300 bytes in the file and I request 1000 Fread() will return
> 301 and the 301st byte is a ^Z.  Since ^Z is the EOF character in gemdos It
> appears that Fread() is actually reading one character past/into the end of
> file and sticking that character at the end of the input buffer.

^Z doesn't mean EOF to GEMDOS: it means EOF to some braindamaged programs.
Programs like Mince, for instance.

If there is a ^Z at the end of your file, it is considered a character
just like any other character to GEMDOS.  It's up to the library to interpret
^Z as EOF.  There is a remark in the GEMDOS documentation to the effect
that "Some applications use ^Z to mark EOF in text files."  The intent
of this was that you should write your programs to be liberal: when
dealing with a text file, don't be surprised to see ^Z, but don't be
surprised not to.  Personally, I prefer no ^Z, because you already know
exactly how many bytes there are in a file.  But as far as GEMDOS is
concerned, files are untyped: there is no concept of a "text" file
versus a "binary file" -- all files are just a collection of bytes.

Some history:  ^Z came to mean EOF because in CP/M, files were allocated in
clusters of multiples of 128 bytes.  In the directory entry for a file,
there was no indication of *exactly* how many bytes were in the file.
This didn't matter for programs (binary files), but it did matter for
text files.  So ^Z was used as the end-of-text marker in text files.

MuShDOS preserved this braindamage, even though it didn't need it because
it DOES keep track of EXACTLY how many bytes were written to a file.
GEMDOS doesn't even document any special treatment of ^Z, but some
programs use it to mark EOF in text files (e.g. Mince).

To address Mr. Nowlin's remarks specifically: how do you know there are
300 bytes left in the file?  Maybe there were 300 text bytes left and
one EOF byte (^Z).  Here's one way to find out how many bytes are REALLY
left in the file:

long bytes_left_in_file(fd)
int fd;
{
	long pos = Fseek(0L,fd,1);		/* get current offset */
	long end = Fseek(0L,fd,2);		/* seek to end, get offset */

	Fseek(pos,fd,0);			/* seek back to pos */
	return end-pos;
}


/----------------------------------------------\
| Opinions expressed above do not necessarily  |  -- Allan Pratt, Atari Corp.
| reflect those of Atari Corp. or anyone else. |     ...lll-lcc!atari!apratt
\----------------------------------------------/