40 lines
2.0 KiB
Plaintext
40 lines
2.0 KiB
Plaintext
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Being able to run code 'before main' isn't just a magic trick. In a situation
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where there may be more than one layer with dynamic linking and more than one
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.dll (plugin system for instance), the maintenance burden of setting up each
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layer in a central DllMain or main is significant enough to be a burden.
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It is possible to get this effect on Windows through the CL compiler, but it
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would be a stretch to say that it is "supported". The way I show here works
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by relying on the fact that a special section does exist that contains function
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pointers that run before 'main' or 'DllMain'. We can use CL's compiler
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extensions to add a function pointer to that section just by declaring it as a
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global variable and marking it up with:
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__declspec(allocate(".CRT$XCU"))
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If you look up this method on the internet, you will find claims that under
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certain types of whole-program optimization, this won't work. In particular
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this happens if you use the option /GL in CL.
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This happens because the global variable appears to be unused from the
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perspective of the compiler & linker. Since it is never directly referenced,
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there is no C-level semantical reason to think this global variable is doing
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anything.
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However, in this example I show how we can still make it work. We have to
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make sure the linker won't eliminate the global variable that we are trying to
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place into the ".CRT$XCU" section. I achieve this by marking it as an export
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symbol. Export symbols can't be eliminated even if they aren't used locally.
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From what I've seen in testing, this works as desired, even with the /GL option.
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IMPORTANT RESTRICTION: Because this creates an export symbol, each time we use
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this within a binary it must have a unique name. Generally I would recommend
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naming before-main symbol by scoping it to the layer where it exists.
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CLANG NOTE: Interestingly, clang can build this, but it can also use the
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__attribute__((constructor)) extension on Windows, which is a lot closer to
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"supporting" this feature. I suspect that When I am building with clang I will
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prefer to go with this option most of the time.
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