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Date: Thu, 20 Jun 2002 10:38:42 -0400
From: "Scott A. Smith" <ssmith AT magnet DOT fsu DOT edu>
Subject: DLLs
To: cygwin AT cygwin DOT com
Message-id: <PCEJJPDHEECNEKDDLPNLMEBNCAAA.ssmith@magnet.fsu.edu>
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Hello,

I've been having lots of fun trying to figure out how to build DLLs.
I should first mention that I have resisted putting declspec qualifiers
of any type into my volumes of code, so I have to use a .DEF file
to declare what is exported. From recent discussions on this list,
I guess that currently one can just invoke the compiler with a few
flags such as -shared and -Wl,--export-all-symbols to automatically
create a .DEF file (or at least the proper exports table) and build
a relocatable DLL that has the all the functions exported. I cannot find
this
documented anywhere, but I was able to copy a build of some other
software and it seems to work fine. Excellent. My makefile target looks
like

$(DLL_NAME) $(DLL_EXP_LIB): $(DLL_OBJS)
	g++ -shared -Wl,--out-implib=$(DLL_EXP_LIB) \
       -o $(DLL_NAME) $(DLL_OBJS) -Wl,--export-all-symbols $(DLL_LDLIBS)

Now, amidst the other scattered documentation on building DLLs there
are alternative methods mentioned. Some invoke dllwrap, dlltool, gcc
and/or ld, and they must use some repeat build commands to get a .DEF file,
.EXP file, .BASE file, and ultimately produce a relocatable DLL. It is
all quite confusing, and AFAIK, no documentation spells out exactly what
is happening in each of these steps. (Yes, I have looked at dllhelpers)
For example, the Cygwin Users Guide on Building and Using DLLs says to
do steps such as

gcc -s -Wl,--base-file,mydll.base -o mydll.dll mydll.o -Wl,-e,_mydll_init AT 12
dlltool --base-file mydll.base --def mydll.def --output-exp
mydll.exp --dllname mydll.dll
gcc -s -Wl,--base-file,mydll.base,mydll.exp -o mydll.dll
mydll.o -Wl,-e,_mydll_init AT 12
dlltool --base-file mydll.base --def mydll.def --output-exp
mydll.exp --dllname mydll.dll
gcc -Wl,mydll.exp -o mydll.dll mydll.o -Wl,-e,_mydll_init AT 12
dlltool --def mydll.def --dllname mydll.dll --output-lib mydll.a

I agree it is convoluted, and I vaguely understand why that had to be done.
As I recall, I had to make my .DEF file prior to this too, needed for
dlltool
(2nd command). Now for my questions.

 1.) Is creation of a DLL using just gcc the preferred method now? It is
     certainly quite easy to use, but is this documented anywhere? It seems
     to automatically put in its own DLL entry point function as well?
 2.) If trying to make a DLL with multiple commands, should one never use ld
     for linking (as I read on this list somewhere), but stick with gcc,
such
     as in the example above?
 3.) Is dllwrap supposed to just engulf the first 5 lines of the above
example?
 4.) Using this multiple command method, is there a way to get dlltool or
dllwrap
     to automatically build a .DEF file so that one need not explictly  use
nm on
     the object files prior to these commands?
 5.) Is dlltool and dllwrap to become obsolete? Are there man pages for
dllwrap?

My questions partially stem from a attempt to use the Dev-C++ IDE to manage
the build.
It automatically uses dllwrap, e.g. a target that looks like

$(DLL_NAME): $(DLL_OBJS)
	$(DLLWRAP) --export-all --output-def $(DEFFILE) --driver-name c++ \
       --implib $(DLL_EXP_LIB) $(DLL_OBJS) -o $(DLL_NAME) $(LIBS)

The resulting DLL causes linker errors complaining about undefined
references
to libBasics_a_iname and _nm(int0_t *).  Here "Basics" comes from the name
of
the DLL made/used. The man pages for ld is the only place I've found which
mentions the suffix _iname and how any such names are not exported.

  6.) Anyone know why it is producing the undefined reference errors?

I've tried so many differnt flags to try to stop this, but nothing works.
I'd just stick with one gcc line to make the DLL were it not for dllwrap
being embedded in the IDE. It would be nice to know what the differences
are.

At least I been having fun, or so I keep telling myself.

Thanks,
Scott


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