How to use a dll outside of the system path

Question

I made an application that uses the openssl dlls (libeay32.dll and ssleay32.dll). It is indy using them, I don't call the dlls directly.

The simplest workaround i found to avoid an installer (i just delpoy an exe and i am ok with this approach) is to:

1. put the dlls as resources of the exe
2. on program start I extract them in the exe folder
3. the exe uses them

This is perfect but I would like to improve the approach by extracting the dlls in a temp folder and not on the exe folder (that in many cases is the desktop).

The problem is that I don't know how to force the application to use the dlls in the temp folder, because now the behaviour is: if dlls are not in current directory try to search in the directories defined in the system path.

Who knows a solution to force indy to use the dlls in my temp path? (like "temporarly registering the dlls")

Answer 1

You can use SetDllDirectory to manipulate the DLL search order.

Answer 2

If you are using an up-to-date version of Indy 10, the IdSSLOpenSSLHeaders unit has a public IdOpenSSLSetLibPath() function to tell Indy which custom folder to look in for the OpenSSL DLLs:

procedure IdOpenSSLSetLibPath(const APath: String);

Answer 3

Load the DLLs yourself with LoadLibrary as soon as you have written them to the temp folder.
This will make Indy's LoadLibrary use your DLLs when they need them:

If lpFileName does not include a path and there is more than one loaded module with the same base name and extension, the function returns a handle to the module that was loaded first.