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Message-ID:	<e550670e-417d-4fa2-9686-aa8464633be5@dronecode.org.uk>
Date:	Thu, 30 May 2024 14:05:28 +0100
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Subject:	Re: LD_PRELOAD for Win32?
To:	Martin Wege <martin DOT l DOT wege AT gmail DOT com>
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From:	Jon Turney via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Jon Turney <jon DOT turney AT dronecode DOT org DOT uk>
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On 25/05/2024 22:55, Martin Wege via Cygwin wrote:
> Hello,
> 
> Does Cygwin or Win32 have something like LD_PRELOAD, so I can
> override/substitute functions in a DLL or EXE, like it is common for
> UNIX/Linux ELF shared libraries?

This is not generally available on Win32, due to limitations of the PE 
file format.

(basically, the import table lists symbols which are imported, and where 
they are imported from, so the module (shared library or executable) 
which provides a given symbol is determined when the module is *linked*, 
not when it is loaded.


However, when googling "cygwin LD_PRELOAD" (which surely you must have 
done before asking this question?) I discover that there's some voodoo 
in the cygwin DLL (hookapi.cc) which claims to do something like that, 
but it seems (i) this requires the preloaded DLL to explicitly hook 
functions it's overriding, and (ii) maybe only works on functions 
imported by the running executable.

It would be nice to have some documentation for that, and/or a simple 
example which could serve as a testcase.

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