How to put a relative path for a DLL statically loaded?

Question

I have a DLL made in Delphi 7/Windows XP that I want to statically load in a host application on Windows (made in Delphi, too). I am using this line of code:

procedure Prepare_HTML_Email(var MailMessage : TIdMessage;
  const FileAddress, aDetail, aAlarmType : String); stdcall; external DLL_ADDRESS;

where DLL_ADDRESS must be the location the DLL is. But at this point I have a problem. The host application is a service, so it is running in C:\WINDOWS\System32, but I want to put the DLL in another directory, not in C:\WINDOWS\System32. The "external" keyword doesn't let to follow it with a function, it only admits a constant expression. So, How can I get the path of the DLL?

Answer 1

First, you're not "statically loading" anything. The D in DLL stands for dynamic; all DLLs are linked dynamically, no matter what. Static linking is how DCU and OBJ files get included in your program. You can't link statically to a DLL.

You're talking about load-time dynamic linking, where the OS loads the DLL for you implicitly due to the functions listed in your program's import table, as opposed to run-time dynamic linking, where you call LoadLibrary using whatever you want. When you use the external directive to define your function, you create an entry in the import table, and as far as I know, relative paths there are meaningless. The OS looks for DLLs at load time (and run time) using a certain documented search order. In general, it's the application's own directory, the current directory, the system directory, the Windows directory, and then everything else on the PATH environment variable.

In your case, the current directory and the system directory are the same place, and you don't have any control over them anyway. Don't put your DLLs in the Windows directory; that already has enough stuff that doesn't belong there.

Your best bet is to put your DLLs in the same directory as you've put your service EXE. If you don't want then, then you could put just enough to bootstrap your program in one DLL in that directory and then load everything else later with LoadLibrary using whatever private DLL directory you want.

You could put your DLLs someplace else and then add that directory to the PATH environment variable. That variable is a shared resource, though, so think twice before you change it.

Answer 2

1. Take a look at delayed dynamic link libraries, available since Delphi 2010. AFAIK you can't load the dll from a very specific path, but you can modify the environment path variable on the very first line of your host program to include the path where the dll is located before you use it's exported functions.
2. Changing your code to load the dll explicitly is not difficult and you can specify the full path to the LoadLibrary API Call. You'll found a example of implicit and explicit dll loading in the same article.