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6430 :: Exporting string functions from DLLs

• ilitirit (orginal) [2020-06-13T16:01:42+02:00] view original

  I have this driver code:

  import dynlib
  
  type myLibFunction = proc(): cstring {.nimcall.}
  
  let lib = loadLib("library.dll")
  let functionCallAddr = lib.symAddr("myStringFunction")
  let libFunction = cast[myLibFunction](functionCallAddr)
  
  echo libFunction()
  Run

  And this library code:

  # nim c -d:release --header --noMain --app:lib library.nim
  
  proc internalStringFunction() : string =
      "I'm a Nim String"
  
  proc internalCStringFunction() : cstring =
      "I'm a CString"
  
  proc myStringFunction() : cstring {.exportc, cdecl, dynlib.} =
      internalCStringFunction() # works
      #"I'm a Nim String?" # works
      #internalStringFunction() # doesn't work
      #cstring(internalStringFunction()) # doesn't work
  Run

  How do I expose the value of internalStringFunction to callers?

  The error I get for the methods that don't work:

   (most recent call last)
  .\main.nim(9) main
  SIGSEGV: Illegal storage access. (Attempt to read from nil?)
  Error: execution of an external program failed: '.\main.exe
  Run

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• ilitirit (orginal) [2020-06-13T17:54:09+02:00] view original

  It looks like the issue is related to the way the DLL is compiled.

  It works if I exclude --noMain, but I'm not sure why:

  c -d:release --header --app:lib library.nim
  Run

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• Javi (orginal) [2020-06-13T21:00:54+02:00] view original

  It's normal, if you compile the DLL with --noMain then the function DllMain() is not created and the Nim system is not initializated calling NimMain() in DllMain() and the app crashes.

  If you compile the DLL without --noMain it generates this initialization code and NimMain() is called when the OS calls DllMain():

  BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fwdreason, LPVOID lpvReserved)
  {
          if(fwdreason == DLL_PROCESS_ATTACH)
          {
              NimMain();
          }
          return 1;
  }
  Run

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• PMunch (orginal) [2020-06-15T10:42:42+02:00] view original

  As @Javi alluded to NimMain is what sets up the GC. So without calling NimMain the GC won't work and thus no Nim strings as they require the GC. It's worth noting though that --noMain only removes the call to NimMain, the procedure is still generated. So you can import it into your Nim code as any other C function and call it yourself.

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