Re: zlib's gzseek return gabbage and fails intermittently under wine.

[Hin-Tak Leung]

--- On Sun, 14/4/13, Vincent Povirk  wrote:
> Well, here's a simple thing you can
> check: Does your zlib dll link to
> _lseek or _lseeki64? The first one uses a 32-bit offset.
> Wine's
> implementation (http://source.winehq.org/source/dlls/msvcrt/file.c#L1090)
> expands that to 64-bit and later truncates the file offset
> to 32-bit.
> For a file larger than 2 GB, that could account for the
> large negative
> value you're seeing.
>
> And since this would only matter in cases where zlib uses
> lseek (the
> first time through I guess it wouldn't, as it has to read
> the whole
> everything up to the offset you give at least once) and is
> at least 2
> GB into the file, that might also explain why it doesn't
> fail
> initially.
>
> But without really digging into the zlib code, all I can do
> it speculate.
>
> I should probably also check coapp's build of zlib
> sometime.

It is not a dll - as you suggested and I already wrote, due to past experience of other's packaging of slightly outdated, it is being built against a private *source* copy of the latest zlib.

Also the bogus offset is not large negative but large (larger than 2^32) positive.

Here is an example of the debug output under wine:

---------------
set_filepos failed at 34307 returning 134127533721091
Re-opening to re-try
Retry successful
set_filepos failed at 96919 returning 146686018157207
Re-opening to re-try
Retry successful
set_filepos failed at 128254 returning 12103217968382
Re-opening to re-try
Retry successful
...
---------------

This is generated by this code snipplet which is called inside a loop, all wrapped in the c++ class:

---------------
    	off_t offset = gzseek(gzvcf_in, filepos, SEEK_SET);
    	if (offset != filepos) { //implicitly converted to off_t by template streamoff()
    	    LOG.printLOG("set_filepos failed at " + LOG.streampos2str(filepos)
    	      + " returning " + LOG.off_t2str(offset) + "\n");
    	    LOG.printLOG("Re-opening to re-try\n");
    	    close(); open();
    	    off_t offset1 = gzseek(gzvcf_in, filepos, SEEK_SET);
    	    if (offset1 == filepos)
    	      LOG.printLOG("Retry successful\n");
    	    else
    	      LOG.error("Retry failed\n"); // this also aborts
    	}

-------------------

This code runs silently on linux i.e. the "if (offset != filepos)" condition is not triggered.