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Message-ID: <3966D936.4FCEA748@ece.gatech.edu>
Date: Sat, 08 Jul 2000 03:33:10 -0400
From: Charles Wilson <cwilson AT ece DOT gatech DOT edu>
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To: cygwin AT sourceware DOT cygnus DOT com
Subject: YADLLQ: Yet Another DLL Question
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I'm trying to build libpng-1.0.7 as a dll, and I'm running into link
problems when building a application (okay, pngtest.exe).

pngtest.pic.o(.text+0x54f):pngtest.c: undefined reference to
`_imp__png_create_read_struct'

and the like. (Note: ONE underscore preceeding the 'imp')

Now, when building the dll, symbols (functions, actually) are declared
thusly:
   __declspec(dllexport) type __cdecl symbol
where 'type' is 'int *' or somesuch, and 'symbol' is the function
itself.

When compiling the object that is to be linked to the dll, the imported
functions get declared like this:
   __declspec(dllimport) type __cdecl symbol

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

Okay, so pngtest.pic.o (the object file importing stuff from the dll)
has symbols like:
  U __imp__png_create_read_struct
with 2 underscores preceeding the 'imp'

However, the import library libpng.dll.a, contains symbols like:
  I ___imp_png_create_read_struct
  T _png_create_read_struct
Note that the first one has 3 underscores preceeding the 'imp'

The dll itself contains only
  T _png_create_read_struct  

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

Since neither the import lib nor the dll contain
'__imp_png_create_read_struct' with exactly 2 underscores, the link
fails regardless of which one I try to link to. 

Why does the import lib have 3 underscores? What do the underscores
mean? How do I make 'ld --shared' generate the implib with only two
underscores?

--Chuck

P.S. I also observe that, in general, linking directly to a dll will
fail, if the library headers are done 'correctly' with
__declspec(dllimport). It's only when the headers are sloppy, and treat
functions imported from the dll exactly as if they were coming from a
static lib (that is, no __declspec at all) --- it's only in that case
that linking directly to the dll will work.

My intuition says this is probably why you can't import DATA when
linking directly from a dll. DATA *must* be declared
__declspec(dllimport)....

--
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