Re: Creating a dll in cygwin for use with jni without -mno-cygwin

[robbincatz <robbincatz at gmail dot com>]

I really appreciate your detailed reply Brian.  Having read your response I
realised that creating a dll in cygwin is way over my head especially as I
am limited on how much (more)time I can spend on this problem.  I have since
switched to using a linux platform and am having much more success.

Thanks a million

:)  


Brian Dessent wrote:
> 
> Brian Dessent wrote:
> 
>> [...] your own initialization procedure that
>> saves a copy of the bottom 4k, initializes Cygwin, and then restores the
>> saved parts.
> 
> Just to be clear, I don't mean that it should initialize Cygwin and then
> restore those parts of the stack.
> 
> To put it differently, when Cygwin initializes it expects to have free
> reign of the bottom X bytes of the stack for its own use.  You can
> accomplish this two ways: 
> 
> The designed way - have code that runs very early in the sequence of
> process initialization that simply allocates these bytes on the stack
> like a standard C auto variable, and since the stack is nearly empty
> they will be at the bottom.  This is what the Cygwin startup code
> (crt0.o) does when you run a Cygwin binary, and how it is designed to
> work.
> 
> The "fake it" way - Assume that you want to load the Cygwin DLL
> dynamically at runtime, and the stack has already been robustly
> allocated/populated by whatever application is already running.  You
> have no control over the bottom X bytes, so you're kind of screwed.  The
> best you can hope to do is make a copy of those X bytes, then initialize
> Cygwin (which now "owns" that area and will overwrite whatever's there
> with its own data) and hope that the control flow of the application is
> such that you will be able to unload the Cygwin DLL and restore those
> saved bytes before program flow reaches a point where the stack gets
> unwound down to that earliest frame.
> 
> If you don't take care of the deinitialization and restoration part then
> everything will seem right but your application will probably crash and
> die horribly when it terminates.
> 
> Obviously the key here is that you need to run this initialization code
> as early in the process init as possible.  If I recall right the cygload
> code tries to do this right at the beginning of main() and at that point
> there is only a small amount of data at the bottom of the stack to
> save/restore, which also means the best chances of that data not being
> referenced/used before it is restored.
> 
> [ X is sizeof(_cygtls) which has traditionally been less than 4K so 4K
> is a nice round number but the current cygload actually uses 32k for a
> safety margin, so the docs should be updated. ]
> 
> Brian
> 
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