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Explicitly Unloading a Delay-Loaded DLL

The /delay:unload linker option allows you to unload a DLL that was delay loaded. By default, when your code unloads the DLL (using /delay:unload and __FUnloadDelayLoadedDLL2), the delay-loaded imports remain in the import address table (IAT). However, if you use /delay:unload on the linker command line, the helper function will support the explicit unloading of the DLL, resetting the IAT to its original form; the now-invalid pointers will be overwritten. The IAT is a field in the ImgDelayDescr that contains the address of a copy of the original IAT (if it exists).

// link with /link /DELAYLOAD:MyDLL.dll /DELAY:UNLOAD
#include <windows.h>
#include <delayimp.h>
#include "MyDll.h"
#include <stdio.h>

#pragma comment(lib, "delayimp")
#pragma comment(lib, "MyDll")
int main()
{
    BOOL TestReturn;
    // MyDLL.DLL will load at this point
    fnMyDll();

    //MyDLL.dll will unload at this point
    TestReturn = __FUnloadDelayLoadedDLL2("MyDll.dll");
   
    if (TestReturn)
        printf_s("\nDLL was unloaded");
    else
        printf_s("\nDLL was not unloaded");
}

Important notes on unloading a delay-loaded DLL:

  • You can find the implementation of the __FUnloadDelayLoadedDLL2 function in the file \VC7\INCLUDE\DELAYHLP.CPP.

  • The name parameter of the __FUnloadDelayLoadedDLL2 function must exactly match (including case) what the import library contains (that string is also in the import table in the image). You can view the contents of the import library with DUMPBIN /DEPENDENTS. If a case-insensitive string match is desired, you can update __FUnloadDelayLoadedDLL2 to use one of the CRT string functions or a Windows API call.

See Unloading a Delay-Loaded DLL for more information.

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