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Path: utzoo!watmath!clyde!rutgers!husc6!ut-sally!ut-ngp!dlnash
From: dlnash@ut-ngp.UUCP
Newsgroups: comp.lang.c
Subject: Re: Help needed ->  fseek() and ftell() in VMS VAX C
Message-ID: <4685@ut-ngp.UUCP>
Date: Thu, 12-Feb-87 12:25:26 EST
Article-I.D.: ut-ngp.4685
Posted: Thu Feb 12 12:25:26 1987
Date-Received: Fri, 13-Feb-87 19:54:33 EST
References: <384@umnd-cs-gw.umnd-cs.UUCP>
Organization: UTexas Computation Center, Austin, Texas
Lines: 55
Keywords: It doesn't do what the manual says ..

In article <384@umnd-cs-gw.umnd-cs.UUCP>, jwabik@umnd-cs.UUCP (Jeff Wabik) writes:
> I'm trying to fseek my way around a large (1.5MB) file using VAX VMS C,
> and am having absolutely NO LUCK.  
> 
> Precise Problem Statement:
> 
> 	1)  File is opened properly. 
> 	2)  I've converted LONG ftell variables from UNIX to INT 
> 	    ftell variables for VMS.  I've followed other instructions
> 	    in the manual as well.
> 	3)  fseek(fp,n,0) does NOT bring me to byte n in the file....
> 

VAX C manual p. 16-21:  "With record files, an fseek to a position that
was not returned by ftell causes unpredictable behavior."  I also tripped
over this problem.  If the file you are trying to fseek around in is not
a Stream_LF file, then fseek will only seek to record boundaries, not
character positions, or something broken like that.  If you brought the
data file over from another system, then it is probably not in Stream_LF
format.  The only way around this problem is to write a program which
does something like:

        while ((c = fgetc(in)) != EOF)
            fputc(c, out);

to make a copy of the file in Stream_LF format (which the stdio library
creates by default).  Then try fseeking around in the copy.  It's a
kludge, but it works.  BTW, don't rely on feof, it is broken too.  If
you read the last byte from a file, so that the next read will return
EOF, feof will not indicate the eof condition.  If you try to read and
get EOF, then try feof, it will indicate the eof condition.  That in my
mind is broken.  It should indicate eof when there is nothing left to
read, not after a read returns EOF. 

Oh, one more thing, make sure the variable 'c' in the above example is
an int, not a char.  The character 0xff is a perfectly valid character
on a machine with an 8-bit character set, especially if your are reading
and writing files with binary data in them.  Fgetc will return 0x000000ff
if the character it reads is 0xff.  The constant EOF is 0xffffffff.  The
above loop will work correctly only if 'c' is an int, not a char.

				Don Nash

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