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flat assembler > Main > Equating a data entry to an import macro entry

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ssjcoder



Joined: 20 Nov 2018
Posts: 6
<data section>
_b dd CS_Print

<import section>
library _Lib, 'msvcrt.dll'
import _Lib, CS_Print, 'printf'

for some reason _b does not equate to CS_Print when the program starts, I guess I'm not sure exactly how to correctly get its value?

Any help appreciated
Post 15 May 2019, 21:29
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7272
Location: Kraków, Poland
CS_Print is not an address of imported function, it is an address of a 32-bit data field that contains an address of a function once the imports have been resolved. You call the function indirectly through this variable:
Code:
call [CS_Print]    
This field is filled with a correct address only at run-time, after PE loader has processed the imports.

Therefore if you need [_b] to have the address of imported function, you have to copy the value like:
Code:
mov eax,[CS_Print]
mov [_b],eax    
Post 15 May 2019, 21:47
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ssjcoder



Joined: 20 Nov 2018
Posts: 6
thank you for your answer it's much appreciated.

It turns out _b stored the address of CS_Print rather than matching it (obviously)

So I needed to alter my code to this:
Code:
mov eax, dword [_b]
call dword [eax]    


In order to actually call the correct address, and as you have mentioned I need to copy it at the start of the program to get the correct value (without needing to go through extra instructions).

Thanks
Post 15 May 2019, 21:59
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