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Aug 4th, 2009
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Win32 functions in NASM

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Hello. I was wondering if someone here could tell me what is needed to call a win32 function from NASM. I thought I could declare MessageBox for example with EXTERN and then use LINK to link my object module with user32.lib (from visual studio) but I get "unresolved symbol MessageBox" error when linking. Any ideas what I'm missing or what I'm doing wrong here?
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kindnumbernine is offline Offline
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Aug 4th, 2009
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Re: Win32 functions in NASM

Here's a link to a forum with someone with a similar problem.
http://www.nabble.com/How-to-call-Me...d23420607.html

For this kind of problem, posting code would be more beneficial!
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wildgoose is offline Offline
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Aug 5th, 2009
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Re: Win32 functions in NASM

Thank you for the link. I had to rename the function to _MessageBoxA@16 in my source and it worked. Now I wonder why functions are named like this in the visual studio libraries. Is it a requirement of the linker? How would one go about writing their own routine in assembly to import a function directly from a DLL?
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kindnumbernine is offline Offline
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Aug 5th, 2009
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Re: Win32 functions in NASM

Didn't use to be that way, but due to multi-language there are various libraries.

MessageBox is merely a macro mapping to functions such as:
MessageBoxA Single-Byte Character System
MessageBoxW Wide-Character (Unicode)
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wildgoose is offline Offline
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Aug 9th, 2009
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Re: Win32 functions in NASM

An alternative is to use the Alink downloads:

http://alink.sourceforge.net/download.html

Assembly Syntax (Toggle Plain Text)
  1. ; To assemble: nasm -f obj hw.asm 
  2. ; To link: alink -oPE hw .obj win32.lib 
  3.  
  4. extern MessageBoxA 
  5. extern ExitProcess
  6.  
  7. segment .data USE32 
  8.  
  9. title db 'My First Win32 ASM Program', 0 
  10. message db 'Hello, World!', 0 
  11.  
  12. segment .text USE32 
  13.  
  14. ..start 
  15. push dword 0Eh
  16. push dword title 
  17. push dword message 
  18. push dword 0 
  19. call MessageBoxA 
  20. push dword 0
  21. call ExitProcess
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Evenbit is offline Offline
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Aug 16th, 2009
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Re: Win32 functions in NASM

Thanks for the responses! I'm still confused though as to why functions are named like that. You say its because of multi-language - could you elaborate on that? Also, Microsoft's linker requires the entry point label to begin with an underscore but not other global names I put in my code. Could someone explain why that is?
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kindnumbernine is offline Offline
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This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.
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