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Message-ID: <3A7E3CF4.39BEB71F@ece.gatech.edu>
Date: Mon, 05 Feb 2001 00:41:08 -0500
From: Charles Wilson <cwilson AT ece DOT gatech DOT edu>
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To: Jerome BENOIT <JGMBenoit AT Wanadoo DOT fr>, cygwin AT sources DOT redhat DOT com
Subject: Re: [ANNOUNCEMENT] Updated: libpng-1.0.9-3
References: <200102042131 DOT f14LVNr13796 AT rtl DOT cygnus DOT com> <3A7DFCAC DOT 6F846E99 AT Wanadoo DOT fr> <3A7E1D98 DOT 59466F66 AT ece DOT gatech DOT edu>
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Charles Wilson wrote:
> 
> Jerome BENOIT wrote:
> >
> > Bonjour:
> >
> > When I launch one of my EXE build with the previous version I get
> > an "Error Starting Program" window with the message:
> >
> > The X.EXE file is linkeked to missing export CYGPNG2.DLL:png_init_io
> >
> > I hope it helps,
> > Jerome BENOIT

Well, I'm afraid I can't help you.  It turns out that this is a "fix". 
Here's what happened:

In 1.0.8, "png_init_io" was not listed in the .def file that determines
what functions/variables are or are not exported from the dll.  The last
defined export was at ordinal #168 = png_access_version_number.  Note
that cygwin uses the msvc .def file -- rather than maintain a completely
separate but almost identical one.  Cygwin just munges the format a
little bit during the build process.

However, if a function in the source code is declared
__declspec(dllexport), then gcc will automatically add it to the export
table -- even if it is not in the .def file.  Well, png_init_io was one
of those functions -- the developers marked it __declspec(dllexport) but
forgot (in 1.0.8) to add it to the .def file.

So, in 1.0.8, you had an "unofficial" exported function; as it happens,
it was automagically assigned ordinal #170.

However, in 1.0.9 they fixed the oversight, and explicitly assigned to
it the ordinal number #171.  

Now, if your application linked to dll exports via the name table, there
would be no problem; but apparently your app was "link by ordinal" (I'm
not sure how to force ld to do one or the other).  Thus, your app went
looking for png_init_io at #170, but found nothing there (here's where
it gets confusing):

At one point during the release candidate cycle for 1.0.9, the following
exports were added to the .def file after png_access_version_number:
  png_sig_bytes    @169
  png_libpng_ver    @170
  png_init_io    @171
  png_convert_to_rfc1123     @172
  png_set_invalid  @173
  png_permit_mng_features  @174
  png_mmx_support  @175

However, for various reasons it was decided that some of these should be
removed until the 1.2.0 series -- so the final tail of the .def file
looks like:
;  png_sig_bytes    @169
;  png_libpng_ver    @170
   png_init_io    @171
   png_convert_to_rfc1123     @172
   png_set_invalid  @173
;To be added at version 1.2.0
;  png_permit_mng_features  @174
;To be added at version 1.2.0
;  png_mmx_support  @175

Notice that there is NO export at #170.  *THAT'S* why your program is
breaking.  Unfortunately, this is a png-developer level thing, and not a
cygwin-specific thing that *I* am able to change.  The only recourse,
I'm afraid, is one the following:

(1) stick with 1.0.8-2
(2) recompile your app with 1.0.9-3.

One reason they did not notice a problem is this: on pure windows
builds, PNG_NO_STDIO is defined -- which causes png_init_io to be
#ifdef'ed out completely.  Only on cygwin do you get (a) a dll, plus (b)
!PNG_NO_STDIO. 

sorry for the difficulty, but the above choices are the best I can give
you.

--Chuck

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