From - Thu Oct 23 07:38:08 1997
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From: hoffman@xdelta.enet.dec.nospam (Stephen Hoff Hoffman)
Newsgroups: comp.os.vms
Subject: C stat() and 2GB files (was Re: Problem ZIPing 2gig file)
Date: 22 Oct 1997 20:03:16 GMT
Organization: Digital Equipment Corp
Lines: 45
Distribution: world
Message-ID: <62lm64$vsl@usenet.pa.dec.com>
References: <1997Oct22.115415@alpha.vitro.com>
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In article <1997Oct22.115415@alpha.vitro.com>, vaxs09@alpha.vitro.com (John Briggs) writes:
..
:The problem is that the file size is stored in a signed longword and
:tested for values less than -2.  (-2 and -1 are used as codes indicating
:special file types).
:
:I started to hack up my copy of ZIP to use 64 bit integers and got the
:thing to compile OK.  But that just pushes the problem back.  ZIP is
:using stat() to return the file size and stat returns a longword.
..

  The C RTL stat() st_size field can overflow at present, due to the
  way the calculations are performed.

  The following is a suggestion from one of the DEC C RTL engineers:

|  I would recommend the following approach:
|
|     1.  Open the file using the "ACC" callback to grab the address of the
|         RMS FAB used for the file.
|
|     2.  Once the open returns, use the saved FAB address to locate the
|         address of the XABFHC block walking through the XAB's.
|
|     3.  Use the following calculation:
|
|            unsigned __int64 mysize;
|            if (fhc->xab$l_ebk != 0)
|                 mysize = (fhc->xab$l_ebk - 1) * 512 + fhc->xab$w_ffb;
|            else mysize = fhc->xab$l_hbk * 512 + fhc->xab$w_ffb;
|
|     4.  Close the file

  The engineer then goes on to admit that this is neither the cleanest
  nor the prettiest of solutions. :-)

  There are folks on the C team looking at ways of allowing off_t and
  other similar structure fields to expand to 64 bits, but this won't
  happen immediately.

  -------------------------- pure personal opinion ---------------------------
   Hoff (Stephen) Hoffman   OpenVMS Engineering   h*ffman@xdelta.enet.dec.c*m
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