FELICE@AGR03.TO.CNR.IT ("Felice G. Marchesi * +39-39-6555.563 *") (07/28/88)
Dear Info-Vaxers,
I'm forwarding to you this question for one of my friends.
Please reply directly to him to the address:
VOLPONES@AGR04.TO.CNR.IT
----------------- original message follows ---------------------
From : Volpones Carlo
I was working on adapting a C program, currently running under UNIX, to VMS.
I had some problems with the use of "ftell" and "fseek" .
After a call to "fseek" and a number 'n' of calls to "fgetc" are done,
the next "ftell" don't return the actual position ( n bytes after the last
"fseek"), but return the position of the last "fseek".
I think that could be some problems into the implementation of "fseek" and
"getc" so that the pointer to the position into the file is not correctly
handled by this functions.
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Felice Marchesi Felice@agr03.to.cnr.itLEICHTER@VENUS.YCC.YALE.EDU ("Jerry Leichter ", LEICHTER-JERRY@CS.YALE.EDU) (08/01/88)
I was working on adapting a C program, currently running under UNIX, to VMS. I had some problems with the use of "ftell" and "fseek" . After a call to "fseek" and a number 'n' of calls to "fgetc" are done, the next "ftell" don't return the actual position ( n bytes after the last "fseek"), but return the position of the last "fseek". I think that could be some problems into the implementation of "fseek" and "getc" so that the pointer to the position into the file is not correctly handled by this functions. On VMS, files may be record- or stream-oriented. A record-oriented file, which is what you get by default, permits seeking only to the beginning of a record. This is fully discussed in the VAX C documentation. -- Jerry
carl@CITHEX.CALTECH.EDU (Carl J Lydick) (08/01/88)
First, I'm sending this to INFO-VAX because my system never heard of the node AGR04.TO.CNR.IT. > I was working on adapting a C program, currently running under UNIX, to > VMS. I had some problems with the use of "ftell" and "fseek" . After a > call to "fseek" and a number 'n' of calls to "fgetc" are done, the next > "ftell" don't return the actual position ( n bytes after the last "fseek"), > but return the position of the last "fseek". I think that could be some > problems into the implementation of "fseek" and "getc" so that the pointer > to the position into the file is not correctly handled by this functions. In section 2.5.3 of the VAX-C Run-Time Library Reference Manual, ftell is documented as follows: The ftell function measures the offset from the beginning of the file. With record files, ftell returns the starting position of the current record, not the current byte offset. I think that the category "record files" doesn't include stream files. It should also be noted that, except for the first record of a file, ftell considers the current record to be the last record from which data was read; this means that if you read one record in its entirety, do an ftell then an fseek, you'll be back at the beginning of the record you just read. You can get around this by doing an ungetc(getc()) just before the call to ftell.
jdc@naucse.UUCP (John Campbell) (08/04/88)
From article <8808031204.AA26428@ucbvax.berkeley.edu>, by LEICHTER@VENUS.YCC.YALE.EDU ("Jerry Leichter ", LEICHTER-JERRY@CS.YALE.EDU):
>
> I was working on adapting a C program, currently running under UNIX,
> to VMS. I had some problems with the use of "ftell" and "fseek" .
> After a call to "fseek" and a number 'n' of calls to "fgetc" are done,
> the next "ftell" don't return the actual position ( n bytes after the
> last "fseek"), but return the position of the last "fseek". I think
> that could be some problems into the implementation of "fseek" and
> "getc" so that the pointer to the position into the file is not
> correctly handled by this functions.
>
> On VMS, files may be record- or stream-oriented. A record-oriented file,
> which is what you get by default, permits seeking only to the beginning of
> a record.
>
> This is fully discussed in the VAX C documentation.
>
> -- Jerry
Jerry has helped me a lot in the past (probably without remembering doing
so) and I don't want this comment to be construed as any criticism at all,
but...
There is a problem with ftell in VMS 'C' which thinking of things as record
oriented doesn't intuitively explain. This "feature" has been discussed
on the net before--namely an fgets followed by an ftell (which all sounds
record-like) won't properly position you to the start of the record you
expect unless the file is a stream-lf file. Current solutions include
doing an ungetc(fgetc(...)) prior to the ftell. The first time I ran into
this I did not feel it was "fully discussed in the VAX C documentation".
As always, if you don't like this behavior, please submit an SPR to DEC
(I finally got an acknowledgement that they received one of the two I sent
in on this).
--
John Campbell ...!arizona!naucse!jdc
unix? Sure send me a dozen, all different colors.