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7183 :: Nim compiled DLL, using Powerbuild programming language call, parameter return result garbled code.

• lihf8515 (orginal) [2020-12-01T11:18:11+01:00] view original

  There is such a simple function as follows, in Windows system compiled into a DLL file, using Powerbuild call, always can not get the correct results, the return is all messy code, if using C call this DLL, it can be successful. It should be noted that this DLL is compiled with Nim 1.4 x86 version, Pb does not support 64 bit. I want to understand, how to write DLL, can let Pb normal call.

  proc test(output: var cstring) {.dynlib, stdcall, exportc.} =
    output = "test".cstring
  Run

  This is a C program call, run results show 'test' string, run without error.

  #include
  #include
  int main()
  {
      typedef bool (*TEST)(char**); // Function pointer type
      HINSTANCE Hint = LoadLibrary("test.dll"); // Load the generated DLL
      TEST test = (TEST)GetProcAddress(Hint,"test@4"); // Get the method of DLL export
      char *out;
      test(&out);
      printf("%s\n", out);
      return 0;
  }
  Run

  This is the Powerbuild code, the external function declaration.

  subroutine test(ref string out) library "test.dll" alias for "test@4;Ansi"
  Run

  Call code:

  string ls_out
  ls_out = space(100)
  test(ls_out)
  messagebox('t',ls_out)
  Run

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• Araq (orginal) [2020-12-01T11:20:38+01:00] view original

  Probably Powerbuild's string type is not just a char*.

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• lihf8515 (orginal) [2020-12-01T11:28:00+01:00] view original

  #include<stdio.h> #include <windows.h>

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• PMunch (orginal) [2020-12-01T11:29:30+01:00] view original

  Any particular reason you want to take a var cstring argument instead of simply returning a string? Your powerbuilder code looks like it creates a string ls_out, then allocates space for it, then passes the pointer to that to Nim, which promptly rewrites the pointer to the new string, and then you output the old string (which is just 100 bytes of probably uninitialized memory). That being said I've never used powerbuilder so I might be way off. But the fact that the C version works fine I'm pretty sure it's your powerbuilder code which is the problem here.

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• Ward (orginal) [2020-12-01T12:49:43+01:00] view original

  In your nim code, test() only return the pointer of string. This is danger behavior because the caller don't know when the string will be destroyed. The code works becasue "test" is static string and it won't be destroyed.

  I have never used Powerbuild before, but in your code, it seems to create some buffer and then pass it to the test0 function. I think what you should do is to copy the string, not just to return the pointer of the string.

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• lihf8515 (orginal) [2020-12-01T13:11:25+01:00] view original

  Powerbuild doesn't seem to support DLL returning strings, so use parameters to return result strings.

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• lihf8515 (orginal) [2020-12-01T13:27:25+01:00] view original

  If I change the argument 'var cstring' to 'cstring', I won't be able to change its value. I need to return the string. In fact, the change to 'cstring' is called by PowerBuild, and you can see that it was passed to the DLL function successfully. Powerbuild 'ref string' is not compatible with 'var cstring', but is compatible with 'cstring'.

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• lihf8515 (orginal) [2020-12-02T02:15:23+01:00] view original

  If I change the argument 'var cstring' to 'var char' to pass a character, PowerBuild it works fine, but I want to pass a string instead of a single character.

  proc test(output: var char) {.dynlib, stdcall, exportc.} =
    var tmp = '8'
    output = tmp
  Run

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• lihf8515 (orginal) [2020-12-02T03:28:11+01:00] view original

  Thank you very much! 'var cstring' is not a string pointer, it is a pointer to a string pointer, so you cannot use 'ref string' in Powerbuild. Instead, use 'ref string []', which means' var string 'returns an array of strings instead of strings.

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