| Subject: | Xbase-1.07: FPT file handling problem |
| Date: | Fri, 5 Jun 2015 14:38:58 -0400 |
| To: | bug-Xbase [...] rt.cpan.org |
| From: | Frank McKenney <frank_mckenney [...] mindspring.com> |
Module: Xbase-1.07 (1996)
Perl: This is perl 5, version 12, subversion 3 (v5.12.3) built
for i486-linux-thread-multi
OS: Linux amun 3.2.63-grsec #1 SMP Fri Oct 24 13:43:09 PDT 2014
i686 AMD Athlon(tm) II X2 250 Processor AuthenticAMD GNU/Linux
There is a bug in the section of Xbase.pm v1.07 that handles the
FPT files used by FoxBase, etc. to store MEMO fields.
Using Xbase v1.07, if the MEMO file is set up with a BLOCKSIZE
greater than 255 (the default is 64), or if non-zero data is present
in certain officially unused portions of the FPT file header,
attempts to retrieve the contents of MEMO fields will fail or
return invalid results.
This appears to be the result of a combination of "guessing" where
the offset and BLOCKSIZE values were being stored in the header and
assuming Intel (little endian) values rather the correct Motorola
(big endian) values.
A diff file that, if applied to a v1.07 Xbase.pm file (1996), will
fix this problem is attached. ( Since I don't know if e-mail bug
submission allows attachments, I've also appended the contents of
the -- short -- file below. )
Hope this helps someone else.
Frank McKenney
--
To be a first-rate scientist it is not necessary (and certainly not
sufficient) to be extremely clever, anyhow in a pyrotechnic sense. One
of the great social revolutions brought about by scientific research
has been the democratization of learning. Anyone who combines strong
common sense with an ordinary degree of imaginativeness can become a
creative scientist, and a happy one besides, in so far as happiness
depends upon being able to develop to the limit of one's abilities.
-- Peter Medawar, "Lucky Jim" 1968
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney aatt mindspring ddoott com
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0a1,37
Show quoted text
> #-------------------
> #
> # 2015-05-19 FAMcK
> #
> # Corrected several MEMO file (FPT) errors discovered while working
> # with the Harbour Project compiler (Clipper replacement):
> #
> # 1) FPT "BLOCKSIZE" is a 16-bit BIG-endian value contained in
> # bytes 6-7 of the header of the associated DBF file.
> # Old code only worked due to the default BLOCKSIZE of 64
> # fitting into one byte and the presence of trailing zeroes
> # in bytes 8-10 of most (but not all) FPT files.
> #
> # However, this would have failed with any FPT file with a
> # BLOCKSIZE > 255.
> #
> # 2) A MEMO block pointer of " 0" apparently indicates
> # an empty value (""). It definitely -- does not -- mean seek
> # to offset 0 of the FPT file. <grin>
> #
> # Note: The code used to obtain 'fpt_nextf', the next-free-block
> # pointer, also uses unpack("V") on a BIG-endian field, but
> # this does not cause any problems because that value is
> # never used. As the Xbase-1.07 Abstract says, this code
> # is intended only for reading.
> #
> # Frank McKenney, McKenney Associates
> #
> # References:
> #
> # Microsoft FoxPro Memo File Structure (.FPT)
> # https://msdn.microsoft.com/en-us/library/8599s21w.aspx
> #
> # Xbase File Format Description
> # http://www.clicketyclick.dk/databases/xbase/format/index.html
> #
> #-------------------
149c186
< $self->{'fpt_blksize'}=unpack("l",pack("L",unpack("V",substr($fpth,7,4))));
---
Show quoted text> $self->{'fpt_blksize'}=unpack("l",pack("L",unpack("n",substr($fpth,6,2)))); # 7,4 -> 6,2 FAMcK
467a505
Show quoted text> return "" if $memblk == 0; # Empty MEMO field, don't seek. FAMcK
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