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    <blockquote> <p>1) At what point does web.dll dynamically link to business.dll? You notice a lot in Windows HDD thrashing for seemingly small tasks when using Word etc and I reckon that this Word going off and dynamically linking in functionality from other DLL's?</p> </blockquote> <p>1) I think you are confusing linking with loading. The link is when all the checks and balances are tested to be sure that what is asked for is available. At load time, parts of the dll are loaded into memory or swapped out to the pagefile. This is the HD activity you are seeing. </p> <p>Dynamic linking is different from static linking in that in static linking, all the object code is put into the main .exe at link time. With dynamic linking, the object code is put into a separate file (the dll) and it is loaded at a different time from the .exe. </p> <p>Dynamic linking can be implicit (i.e. the app links with a import lib), or explicit (i.e. the app uses LoadLibrary(ex) to load the dll). </p> <p>In the implicit case, /DELAYLOAD can be used to postpone the loading of the dll until the app actually needs it. Otherwise, at least some parts of it are loaded (mapped into the process address space) as part of the process initilazation. The dll can also request that it never be unloaded while the process is active. </p> <p>COM uses LoadLibrary to load COM dlls. Note that even in the implicit case, the system is using something similar to LoadLibrary to load the dll either at process startup or on first use. </p> <blockquote> <p>2) What actually executes the code in the DLL? Does it get executed by the processor or is there another stage of translation or compilation before the processor will understand the code inside the DLL?</p> </blockquote> <p>2) Dlls contain object code just like .exes. The format of the dll file is almost identical to the format of an exe file. I have heard that there is only one bit that is different in the headers of the two files. </p> <p>In the case of a DLL built from C# .net, the .Net framework is running it. </p> <blockquote> <p>3) Does a DLL from say Linux work on a Windows system (if such a thing exists) or are they operating system specific?</p> </blockquote> <p>3) DLLs are platform specific. </p> <blockquote> <p>4) Are they specific to a particular framework? Can a DLL built using C# .Net be used by a DLL built with Borland C++ (example only)?</p> </blockquote> <p>4) Dlls can interoperate with other frameworks if special care is taken or some additional glue code is written. </p> <p>Dlls are very useful when a company sells multiple products that have overlapping capabilities. For instance, I maintain a raster i/o dll that is used by more than 30 different products at the company. If you have multiple products installed, one upgrade of the dll can upgrade all the products to new raster formats.</p> <blockquote> <p>5) Going back to the web.dll / business.dll example. To get a class type of customer I need to reference business.dll from web.dll. This must mean that business.dll contains a specification of some sort of what a customer class actually is. If I had compiled my business.dll file in say Delphi would C# understand it and be able to create a customer class - or is there some sort of header info or something that says "hey sorry you can only use me from another delphi dll".</p> </blockquote> <p>5) Depending on the platform, the capabilities of a dll are presented in various ways, thru .h files, .tlb files, or other ways on .net. </p> <blockquote> <p>6) On the subject of DLL hijacking, surely the replacement (bad) DLL must contain the exact method signatures, types as the one that is being hijacked. I suppose this wouldnt be hard to do if you could find out what methods etc were available in the original DLL.</p> </blockquote> <p>6) dumpbin /exports and dumbin /imports are interesting tools to use on .exe and .dlls</p>
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