RemoveDllImportHook

Please note that an import table hooks affects all threads in a process. The hook may be removed at any time, but the callback may not be released unless you are sure that no thread is currently using it.

* The Cockpit System

* HookAPI.PRG

* Sample program showing how to hook API Functions

#INCLUDE "CockpitPlatformLibs.ch"

#INCLUDE "Windows.ch"

#INCLUDE "DllImportHook.ch"

STATIC originalAddress

FUNCTION main

LOCAL hookInfo,envValue

* Hook any call from "xpprt1.dll" to "GetEnvironmentVariableA" in dll "kernel32.dll"

* to the callback function "MyHookFunc"

hookInfo := InstallDllImportHook("xpprt1.dll","GetEnvironmentVariableA",;

"kernel32.dll",MyHookFunc())

* That didn't work?

IF hookInfo == NIL

MessageBox(0,"Hooking failed")

CANCEL

ENDIF

* Save the original address for passsing on the call to the original function

originalAddress := hookInfo[DLLHOOKINFO_INDEX_ORIGINALPROCADDRESS]

MessageBox(0,"Hooking successful.")

* HOOKINFO is our special value

envValue := GETENV("HOOKINFO")

* Display what we received

MessageBox(0,SVI("Returned value for HOOKINFO: &1",envValue))

* Any other value should return the real value of the environment variable

envValue := GETENV("SystemRoot")

* Display what we received

MessageBox(0,SVI("Returned value for SystemRoot: &1",envValue))

* Now remove the Hook

RemoveDllImportHook(hookInfo)

MessageBox(0,"Hook removed.")

* Now let's see what GETENV returns

envValue := GETENV("HOOKINFO")

* Display what we received (should be empty this time but it isn't, hence

* Xbase++ caches environment variables!)

MessageBox(0,SVI("Returned value for HOOKINFO after unhooking: &1",envValue))

RETURN NIL

* This function is called instead of the original GetEnvironmentVariableA function

CALLBACK FUNCTION MyHookFunc(p1,p2,p3)

* Display the passed parameters

MessageBox(0,SVI("GetEnvironmentVariableA(&1,&2,&3)",CopyMemoryToString(p1),p2,p3),;

"Hook function called")

* Handle our special case...

IF CopyMemoryToString(p1) == "HOOKINFO"

* Size Request?

IF p3 == 0

RETURN 11

ENDIF

* Return our special result value

WriteMemory(p2,"Holy Moly!"+CHR(0),11)

* Return the size

RETURN 10

ENDIF

* Pass the call on to the original function

RETURN CallAddress(originalAddress,p1,p2,p3)