rand functions in a DLL

This is a discussion on rand functions in a DLL within the C++ Programming forums, part of the General Programming Boards category; I am trying to use the srand and rand functions in a DLL, but for some reason I keep getting ...

  1. #1
    carry on JaWiB's Avatar
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    rand functions in a DLL

    I am trying to use the srand and rand functions in a DLL, but for some reason I keep getting access violations (and this function worked before it was part of a DLL).

    Here's a short example of my problem:
    Code:
    //this is the DLL
    #include <windows.h>
    #include <ctime>
    
    extern "C"
    {
    __declspec(dllexport) int Foo(char* str)
    {
    int len = strlen(str);
    //if I change the following three lines to this:
    //str[5] = '^';
    //everything works
    srand(time(0));
    int num = rand()%len;
    str[num] = '^';
    return 0;
    }
    }
    Code:
    #include <windows.h>
    #include <iostream>
    
    typedef int (*MYDLLPROC)(char*);
    
    MYDLLPROC Foo;
    
    int main()
    {
      char str[] = "Hello World!";
      HMODULE hDLL = LoadLibrary("dllClassTest.dll");
      Foo = (MYDLLPROC)GetProcAddress(hDLL,"Foo");
      Foo(str); //access violation
      std::cout<<str;
    }
    I'm using VS.NET 2003 if that makes a difference.
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  2. #2
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    Two suggestions:
    1. Do error checking. I suspect LoadLibrary() failed to find the dll
    2. Copy the dll into the main project's source directory or into a directory that is in your PATH environment variable.
    3. It works when compiled with Dev-C++ and the dll is in the test programs's source directory.

    Code:
    int main()
    {
      char str[] = "Hello World!";
      HMODULE hDLL = LoadLibrary("dllClassTest.dll");
      if( hDLL == 0)
      {
          std::cout << "LoadLibrary error" << std::endl;
          std::cin.ignore();
          return 1;
      }
      Foo = (MYDLLPROC)GetProcAddress(hDLL,"Foo");
      if( Foo == 0)
      {
          std::cout << "GetProcAddress error" << std::endl;
          std::cin.ignore();
          return 1;
      }
      Foo(str); //access violation
      std::cout<<str;
      std::cin.ignore();
    }
    Last edited by Ancient Dragon; 03-14-2006 at 09:06 PM.

  3. #3
    carry on JaWiB's Avatar
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    No, I'm sure LoadLibrary is successful for two reasons. First, other functions in the DLL work correctly. Second, if remove any lines from the test DLL that I posted, the function works correctly without any errors.

    If it works with Dev-C++ it sounds like an VS problem or possibly something wrong with my project settings (but I'm not sure what I should change...)

    Edit: And in my actual project I do error checking, just so you know
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  4. #4
    carry on JaWiB's Avatar
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    I just discovered a way I can get it to work, but it causes other problems. In my linker options I have:

    Force Symbol References: __DllMainCRTStartup@12
    Resource Only DLL: Yes (/NOENTRY)

    If I remove the first, and select no for the second, the program works if I'm not debugging it. However, I had those two lines because if I try to debug without them, I receive an unhandled exception. So, damned if I do, damned if I don't?
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

  5. #5
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    The problem might be VC++ 2003/2005 specific, I know Microsoft made a lot of major changes in that compiler. I'm in the process of porting from VC++ 6.0 to 2005 and the port is not very nice. A lot of MFC classes are now templates! and don't work well in DLLs. I don't know if your problem is similar.

  6. #6
    Yes, my avatar is stolen anonytmouse's Avatar
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    Could it be a mismatch between the standard library being used by the DLL and the EXE?

    http://msdn.microsoft.com/library/de..._libraries.asp
    http://msdn2.microsoft.com/en-us/library/abx4dbyh.aspx

  7. #7
    and the hat of wrongness Salem's Avatar
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    Calling srand(time(0)) every time you want a new rand() can't be good.
    Remember, time() is a constant (more or less) in short-lived programs.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  8. #8
    carry on JaWiB's Avatar
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    >>Could it be a mismatch between the standard library being used by the DLL and the EXE?

    Yup, I needed to use the /MD option. Now it works
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

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