Specify a "back-up" load path with Load-Time Dynamic Linking? - windows

Question

When using 'Load-Time Dynamic Linking'(using an import library .lib in compilation that loads a dll when the module is loaded) in a Win32 application, is it possible to affect the search order?
The goal is to have a library loaded using the normal search order, with a back-up path if one is not found on the normal path.
Example: foo.lib is linked in the project. At load time:
If foo.dll is present in the System path or any of the other paths mentioned in Dynamic-Link Library Search Order then it will be loaded.
Else mySpecifiedPath/foo.dll will be loaded.
Clarification: The back-up dll should only be loaded if no version of the dll is found in the standard LoadLibrary search paths.

Answer 1

Leaving the old answer at the bottom and adding a new one at the top:
The combination of delay-loading and the delay-load Failure Hook as described at provides a nice way to handle this.
Register the failure hook that loads the back-up dll before using any symbol from the library.
#include <Windows.h>
#include "foo.h"
#include <delayimp.h>
//access to set the failure hook
ExternC PfnDliHook __pfnDliFailureHook2;
const char* dllName = "foo.dll";
const char* dllBackupPath = "pathToBackup\\foo.dll";
FARPROC WINAPI DelayLoadHook(unsigned dliNotify, PDelayLoadInfo pdli)
{
//if the failure was failure to load the designated dll
if (dliNotify == dliFailLoadLib &&
strcmp(dllName, pdli->szDll) == 0 &&
pdli->dwLastError == ERROR_MOD_NOT_FOUND)
{
printf("loading back-up dll\n");
//return the successfully loaded back-up lib,
//or 0, the LoadLibrary fails here
HMODULE lib = LoadLibraryA(dllBackupPath);
return (FARPROC)lib;
}
return 0;
}
int main()
{
//set the Failure Hook
__pfnDliFailureHook2 = &DelayLoadHook;
//when this function is called the library will be loaded
//from standard paths. If it is not found the Failure Hook
//set above will be called.
int test = ::intReturningFuncFromFooDll();
printf("%d", test);
getchar();
return 0;
}
===========old answer below==============
Looks like what I was looking for is delay-loading, as mentioned in the comment by IInspectable. I found information regarding this:
Linker Support for Delay-Loaded DLLs
Here is some code demonstrating the usage I mentioned in the original post:
//load the dll using the normal search
HMODULE lib = LoadLibrary( L"foo.dll" );
//if unsuccessful, try a specified path
if (lib == NULL)
{
LoadLibrary( L"mySpecifiedPath/foo.dll" );
}
if (lib == NULL)
{
//make sure that the library is not used,
//or exit the application, as it was not found
}
Thanks for the help!
edit to old answer: This dynamic loading would be used before any symbol from the library is used. The delay loaded fills out the symbol addresses using the module loaded here the first time a symbol from the library is accessed.

Answer 2

Looking at the Dynamic-Link Library Search Order documentation, it should be obvious, that the final location searched is the PATH environment variable. Applying that knowledge, the easiest solution that meets all your requirements is to append the backup location to the PATH variable.
This could be done in two ways:
An external launcher: The CreateProcess API call allows an application, to pass a custom environment to the new process through the lpEnvironment argument. The current process' environment block can be queried by calling GetEnvironmentStrings.
Enable delay-loading of the desired DLL(s) using the /DELAYLOAD linker option, and modify the process' environment at application startup. After retrieving the PATH environment variable (using GetEnvironmentVariable), it can be modified and updated in the environment block by calling SetEnvironmentVariable.