Thread: 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.

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();
}

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

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?

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.

>>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