Is it possible for a function that is inside a DLL to unload the DLL? I need to do this so I can make sure the DLL is not in use, then write to the DLL's file.
As I understand it, it CAN be done and is MEANT to be done sometimes (for example in case of dll injection by CreateRemoteThread and other methods). So,
FreeLibraryAndExitThread(hModule, 0)
will do precisely that.
On the other hand, calling
FreeLibrary(hModule)
will not do here - from MSDN: "If they were to call FreeLibrary and ExitThread separately, a race condition would exist. The library could be unloaded before ExitThread is called." As a remark, ExitThread does some bookkeeping besides just returning from the thread function.
All this assumes that Your Dll obtained the hModule itself by calling LoadLibrary from inside the loaded Dll, or rather, by calling from inside the loaded Dll the following function:
GetModuleHandleEx
(
GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
(LPCTSTR)DllMain,
&hModule
)
This increments the reference count of the Dll so You know that if You free the library later using that handle and if the library is really unloaded then You had the last reference to it.
If You instead skip incrementing the Dll's reference count and obtain the hModule just from the argument to DllMain during DLL_PROCESS_ATTACH then You should not call FreeLibraryAndExitThread since the code that loaded the Dll is still using it and this module handle really isn't Yours to manage.
Use this when the dll has done it job:
CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)FreeLibrary, &__ImageBase, 0, NULL);
// terminate if dll run in a separate thread ExitThread(0);
// or just return out the dll
And the __ImageBase is your dll's PE header structure:
EXTERN_C IMAGE_DOS_HEADER __ImageBase;
If your asking if you can safely unload/unmap a DLL loaded in a process from code in the DLL itself, the answer is no - there isn't really a safe way to do this.
Think about it this way: Unloading a DLL is done by decrementing it's reference count using FreeLibrary(). The problem of course is that once the reference count of the DLL hits zero, the module is unmapped. Which means that the code in the DLL that called FreeLibrary() is gone.
Even if you could do this, you'd still need to ensure that there are no other threads executing any exported functions from the DLL.
I don't think it will work. Calling FreeLibrary with a handle from the outside (LoadLibrary would have been called from an area outside the DLL) as the code runs in a memory location that will not be valid anymore.
Even if this is possible, it smells like a bad design. Maybe you want to make some updater or alike. Explain a bit more what is the result you expect. Unloading a DLL from within itself is not the way to go.
I did solve my immediate problem at hand, but now I need to understand why it is solved. ;-)
So here I have couple more questions.
Suppose I have a class that is being exported from the DLL. Now this DLL should be loaded into the memory every time I call:
MyExportedClass *pb = new MyExportedClass;
and it should stay in memory and becomes unloaded only when I call:
delete pb;
Is this correct?
If I understood this correctly and the answer to the previous question is yes, then what should happen in the following scenario:
I have an interface which exported from the dll (dll1) and I have its implementation which is exported from another dll (dll2). So every time I execute:
MyInterface *pInterface = new MyImplementation;
both those dlls should be loaded in memory and they should stay in memory until I call:
delete pInterface;
Is this correct?
Now, if the answer to this question is yes - do I have a control/saying, which library will be unloaded first and which one will be second? Or the unload will always happen right after calling destructor of the appropriate class?
Now, is there a tool which checks if the library becomes unloaded and at which point? I can probably just use the fake DllMain() and check its process_detach case, but my impression always was: use DllMain when the library exports function and don't use DllMain when the library exports classes. I used this approach since MSVC 5/6 (following one of the books about C++).
Was I wrong and I can still use DLLMain in both cases?
Thank you.
DLL loading can be automatic (if listed in the import table) or manual (using LoadLibrary). Some managed frameworks, like .NET, will silently call LoadLibrary for you when they see a DLL import declaration in their metadata, but C++ is not one of these. The closest thing C++ has is delay-loading, where the function that calls LoadLibrary is (by default, you can substitute your own) provided by the compiler.
On the other hand, DLL unloading is always manual. Deleting an object never implicitly unloads a DLL. You have to call FreeLibrary (or FreeLibraryAndExitThread). And you had better not call FreeLibrary while objects defined in that library are still in use.
Now, the COM system in Windows is a bit more complicated, because it manages DLL lifetime for you. But DLL lifetime is still not controlled by object deletion, rather by calling a DllCanUnloadNow function in the DLL. Usually you need to maintain a count of active objects in order to write that function correctly. But it still requires your manual implementation, and you can simply always return false to never unload (this is a bit of a pain during development because you have to close the entire application to try a new plugin version, etc... but actually freeing every usage of a DLL and successfully unloading it is rare anyway)
And certainly there is nothing that will automatically unload a DLL listed in the import table, those get loaded during process startup and never unloaded, no matter how many object instances are created or destroyed.
When you have a base class in your main executable and subclasses (say, plugins) defined in DLLs, what happens when you want to get a plugin?
I am looking for an article/answer that'd clarify what happens when you
load a DLL
call a DLL's function that returns a plugin* (it has virtual functions)
use that plugin in your main executable's code
delete, unload
I am thinking about the vtable and other C++ issues. For instance, if you unload the DLL stil having some plugins running... The "code" will be gone?
Since you are talking about plugins, you must be doing something like LoadLibrary. Assuming Windows:
Load a library. You would normally call LoadLibrary followed by GetProcAddress. The DLL is loaded into the process address space and you have the pointer to the function exposed.
Call a DLL's function that returns a plugin* (it has virtual functions). You would cast a the return value from GetProcAddress to the function pointer type, and call it. Everything should work as normal.
Use that plugin in your main executable's code. There is nothing special.
Delete the plugin. I assume you use a function in the DLL to do so, and it should be fine. Directly calling delete can be dangerous, as the DLL may have a separate memory manager (depending on what runtime you use).
Unload the DLL. You call FreeLibrary, and the code is gone. The pointers you previously get from GetProcAddress become invalid.
I am not aware specific vtable issues. If you unload the DLL while the code is still running, I would assume the program will crash at this point, as the code address space becomes invalid.
If I load a DLL with LoadLibrary, is that DLL guaranteed to stay loaded? If not, how can I prevent re-loading.
Actual situation: I have a dispatcher which, depending on some messages, needs to load one of several dll's and execute some function from them. I can't link against them at compile time, so I use LoadLibrary. Because there can potentially be a lot of calls, I'd prefer not to have to call LoadLibrary every time, since it turns out it's a bottleneck. So I was thinking to only call it once per DLL, calling GetProcAddress to get the function also only once per dll, and cache it somewhere. But is it safe? Am I guaranteed that calling that function will be ok on any subsequent call? If not, how can I have this guarantee?
LoadLibrary increases the reference count of the executable, and FreeLibrary decreases it.
When the reference count reach 0, the executable is unloaded. So you normally don't have to worry about it. As long as no one calls FreeLibrary in your process, the Dll will stay there.
If you read the MSDN: http://msdn.microsoft.com/en-us/library/windows/desktop/ms684175%28v=vs.85%29.aspx it states what the behaviour will be if the dll is not already loaded and what happens if the dll is already loaded so you should not worry about this overhead.
If the specified module is a DLL that is not already loaded for the
calling process, the system calls the DLL's DllMain function with the
DLL_PROCESS_ATTACH value. If DllMain returns TRUE, LoadLibrary returns
a handle to the module.
What's the best way for loading a dll from a dll ?
My problem is I can't load a dll on process_attach, and I cannot load the dll from the main program, because I don't control the main program source. And therefore I cannot call a non-dllmain function, too.
After all the debate that went on in the comments, I think that it's better to summarize my positions in a "real" answer.
First of all, it's still not clear why you need to load a dll in DllMain with LoadLibrary. This is definitely a bad idea, since your DllMain is running inside another call to LoadLibrary, which holds the loader lock, as explained by the documentation of DllMain:
During initial process startup or after a call to LoadLibrary, the system scans the list of loaded DLLs for the process. For each DLL that has not already been called with the DLL_PROCESS_ATTACH value, the system calls the DLL's entry-point function. This call is made in the context of the thread that caused the process address space to change, such as the primary thread of the process or the thread that called LoadLibrary. Access to the entry point is serialized by the system on a process-wide basis. Threads in DllMain hold the loader lock so no additional DLLs can be dynamically loaded or initialized.
The entry-point function should perform only simple initialization or termination tasks. It must not call the LoadLibrary or LoadLibraryEx function (or a function that calls these functions), because this may create dependency loops in the DLL load order. This can result in a DLL being used before the system has executed its initialization code. Similarly, the entry-point function must not call the FreeLibrary function (or a function that calls FreeLibrary) during process termination, because this can result in a DLL being used after the system has executed its termination code.
(emphasis added)
So, this on why it is forbidden; for a clear, more in-depth explanation, see this and this, for some other examples about what can happen if you don't stick to these rules in DllMain see also some posts in Raymond Chen's blog.
Now, on Rakis answer.
As I already repeated several times, what you think that is DllMain, isn't the real DllMain of the dll; instead, it's just a function that is called by the real entrypoint of the dll. This one, in turn, is automatically took by the CRT to perform its additional initialization/cleanup tasks, among which there is the construction of global objects and of the static fields of the classes (actually all these from the compiler's perspective are almost the same thing). After (or before, for the cleanup) it completes such tasks, it calls your DllMain.
It goes somehow like this (obviously I didn't write all the error checking logic, it's just to show how it works):
/* This is actually the function that the linker marks as entrypoint for the dll */
BOOL WINAPI CRTDllMain(
__in HINSTANCE hinstDLL,
__in DWORD fdwReason,
__in LPVOID lpvReserved
)
{
BOOL ret=FALSE;
switch(fdwReason)
{
case DLL_PROCESS_ATTACH:
/* Init the global CRT structures */
init_CRT();
/* Construct global objects and static fields */
construct_globals();
/* Call user-supplied DllMain and get from it the return code */
ret = DllMain(hinstDLL, fdwReason, lpvReserved);
break;
case DLL_PROCESS_DETACH:
/* Call user-supplied DllMain and get from it the return code */
ret = DllMain(hinstDLL, fdwReason, lpvReserved);
/* Destruct global objects and static fields */
destruct_globals();
/* Destruct the global CRT structures */
cleanup_CRT();
break;
case DLL_THREAD_ATTACH:
/* Init the CRT thread-local structures */
init_TLS_CRT();
/* The same as before, but for thread-local objects */
construct_TLS_globals();
/* Call user-supplied DllMain and get from it the return code */
ret = DllMain(hinstDLL, fdwReason, lpvReserved);
break;
case DLL_THREAD_DETACH:
/* Call user-supplied DllMain and get from it the return code */
ret = DllMain(hinstDLL, fdwReason, lpvReserved);
/* Destruct thread-local objects and static fields */
destruct_TLS_globals();
/* Destruct the thread-local CRT structures */
cleanup_TLS_CRT();
break;
default:
/* ?!? */
/* Call user-supplied DllMain and get from it the return code */
ret = DllMain(hinstDLL, fdwReason, lpvReserved);
}
return ret;
}
There isn't anything special about this: it also happens with normal executables, with your main being called by the real entrypoint, which is reserved by the CRT for the exact same purposes.
Now, from this it will be clear why the Rakis' solution isn't going to work: the constructors for global objects are called by the real DllMain (i.e. the actual entrypoint of the dll, which is the one about the MSDN page on DllMain talks about), so calling LoadLibrary from there has exactly the same effect as calling it from your fake-DllMain.
Thus, following that advice you'll obtain the same negative effects of calling directly LoadLibrary in the DllMain, and you'll also hide the problem in a seemingly-unrelated position, which will make the next maintainer work hard to find where this bug is located.
As for delayload: it may be an idea, but you must be really careful not to call any function of the referenced dll in your DllMain: in fact, if you did that you would trigger a hidden call to LoadLibrary, which would have the same negative effects of calling it directly.
Anyhow, in my opinion, if you need to refer to some functions in a dll the best option is to link statically against its import library, so the loader will automatically load it without giving you any problem, and it will resolve automatically any strange dependency chain that may arise.
Even in this case you mustn't call any function of this dll in DllMain, since it's not guaranteed that it's already been loaded; actually, in DllMain you can rely only on kernel32 being loaded, and maybe on dlls you're absolutely sure that your caller has already loaded before the LoadLibrary that is loading your dll was issued (but still you shouldn't rely on this, because your dll may also be loaded by applications that don't match these assumptions, and just want to, e.g., load a resource of your dll without calling your code).
As pointed out by the article I linked before,
The thing is, as far as your binary is concerned, DllMain gets called at a truly unique moment. By that time OS loader has found, mapped and bound the file from disk, but - depending on the circumstances - in some sense your binary may not have been "fully born". Things can be tricky.
In a nutshell, when DllMain is called, OS loader is in a rather fragile state. First off, it has applied a lock on its structures to prevent internal corruption while inside that call, and secondly, some of your dependencies may not be in a fully loaded state. Before a binary gets loaded, OS Loader looks at its static dependencies. If those require additional dependencies, it looks at them as well. As a result of this analysis, it comes up with a sequence in which DllMains of those binaries need to be called. It's pretty smart about things and in most cases you can even get away with not following most of the rules described in MSDN - but not always.
The thing is, the loading order is unknown to you, but more importantly, it's built based on the static import information. If some dynamic loading occurs in your DllMain during DLL_PROCESS_ATTACH and you're making an outbound call, all bets are off. There is no guarantee that DllMain of that binary will be called and therefore if you then attempt to GetProcAddress into a function inside that binary, results are completely unpredictable as global variables may not have been initialized. Most likely you will get an AV.
(again, emphasis added)
By the way, on the Linux vs Windows question: I'm not a Linux system programming expert, but I don't think that things are so different there in this respect.
There are still some equivalents of DllMain (the _init and _fini functions), which are - what a coincidence! - automatically took by the CRT, which in turn, from _init, calls all the constructors for the global objects and the functions marked with __attribute__ constructor (which are somehow the equivalent of the "fake" DllMain provided to the programmer in Win32). A similar process goes on with destructors in _fini.
Since _init too is called while the dll loading is still taking place (dlopen didn't return yet), I think that you're subject to similar limitations in what you can do in there. Still, in my opinion on Linux the problem is felt less, because (1) you have to explicitly opt-in for a DllMain-like function, so you aren't immediately tempted to abuse of it and (2), Linux applications, as far as I saw, tend to use less dynamic loading of dlls.
In a nutshell
No "correct" method will allow you to reference to any dll other than kernel32.dll in DllMain.
Thus, don't do anything important from DllMain, neither directly (i.e. in "your" DllMain called by the CRT) neither indirectly (in global class/static fields constructors), especially don't load other dlls, again, neither directly (via LoadLibrary) neither indirectly (with calls to functions in delay-loaded dlls, which trigger a LoadLibrary call).
The right way to have another dll loaded as a dependency is to - doh! - mark it as a static dependency. Just link against its static import library and reference at least one of its functions: the linker will add it to the dependency table of the executable image, and the loader will load it automatically (initializing it before or after the call to your DllMain, you don't need to know about it because you mustn't call it from DllMain).
If this isn't viable for some reason, there's still the delayload options (with the limits I said before).
If you still, for some unknown reason, have the inexplicable need to call LoadLibrary in DllMain, well, go ahead, shoot in your foot, it's in your faculties. But don't tell me I didn't warn you.
I was forgetting: another fundamental source of information on the topic is the [Best Practices for Creating DLLs][6] document from Microsoft, which actually talks almost only about the loader, DllMain, the loader lock and their interactions; have a look at it for additional information on the topic.
Addendum
No, not really an answer to my question. All it says is: "It's not possible with dynamic linking, you must link statically", and "you musn't call from dllmain".
Which *is* an answer to your question: under the conditions you imposed, you can't do what you want. In a nutshell of a nutshell, from DllMain you can't call *anything other than kernel32 functions*. Period.
Although in detail, but I'm not really interested in why it doesn't work,
You should, instead, because understanding why the rules are made in that way makes you avoid big mistakes.
fact is, the loader is not resolving dependenies correctly and the loading process is improperly threaded from Microsoft's part.
No, my dear, the loader does its job correctly, because *after* LoadLibrary has returned, all the dependencies are loaded and everything is ready to be used. The loader tries to call the DllMain in dependency order (to avoid problems with broken dlls which rely on other dlls in DllMain), but there are cases in which this is simply impossible.
For example, there may be two dlls (say, A.dll and B.dll) that depend on each other: now, whose DllMain is to call first? If the loader initialized A.dll first, and this, in its DllMain, called a function in B.dll, anything could happen, since B.dll isn't initialized yet (its DllMain hasn't been called yet). The same applies if we reverse the situation.
There may be other cases in which similar problems may arise, so the simple rule is: don't call any external functions in DllMain, DllMain is just for initializing the internal state of your dll.
The problem is there is no other way then doing it on dll_attach, and all the nice talk about not doing anything there is superfluous, because there is no alternative, at least not in my case.
This discussion is going on like this: you say "I want to solve an equation like x^2+1=0 in the real domain". Everybody says you that it's not possible; you say that it's not an answer, and blame the math.
Someone tells you: hey, you can, here's a trick, the solution is just +/-sqrt(-1); everybody downvotes this answer (because it's wrong for your question, we're going outside the real domain), and you blame who downvotes. I explain you why that solution is not correct according to your question and why this problem can't be solved in the real domain. You say that you don't care about why it can't be done, that you can only do that in the real domain and again blame math.
Now, since, as explained and restated a million times, under your conditions your answer has no solution, can you explain us why on earth do you "have" to do such an idiotic thing as loading a dll in DllMain? Often "impossible" problems arise because we've chosen a strange route to solve another problem, which brings us to deadlock. If you explained the bigger picture, we could suggest a better solution to it which does not involve loading dlls in DllMain.
PS: If I statically link DLL2 (ole32.dll, Vista x64) against DLL1 (mydll), which version of the dll will the linker require on older operating systems?
The one that is present (obviously I'm assuming you're compiling for 32 bit); if an exported function needed by your application isn't present in the found dll, your dll is simply not loaded (LoadLibrary fails).
Addendum (2)
Positive on injection, with CreateRemoteThread if you wanna know. Only on Linux and Mac the dll/shared library is loaded by the loader.
Adding the dll as a static dependency (what has been suggested since the beginning) makes it to be loaded by the loader exactly as Linux/Mac do, but the problem is still there, since, as I explained, in DllMain you still cannot rely on anything other than kernel32.dll (even if the loader in general intelligent enough to init first the dependencies).
Still, the problem can be solved. Create the thread (that actually calls LoadLibrary to load your dll) with CreateRemoteThread; in DllMain use some IPC method (for example named shared memory, whose handle will be saved somewhere to be closed in the init function) to pass to the injector program the address of the "real" init function that your dll will provide. DllMain then will exit without doing anything else. The injector application, instead, will wait for the end of the remote thread with WaitForSingleObject using the handle provided by CreateRemoteThread. Then, after the remote thread will be ended (thus LoadLibrary will be completed, and all the dependencies will be initialized), the injector will read from the named shared memory created by DllMain the address of the init function in the remote process, and start it with CreateRemoteThread.
Problem: on Windows 2000 using named objects from DllMain is prohibited because
In Windows 2000, named objects are provided by the Terminal Services DLL. If this DLL is not initialized, calls to the DLL can cause the process to crash.
So, this address may have to be passed in another manner. A quite clean solution would be to create a shared data segment in the dll, load it both in the injector application and in the target one and have it put in such data segment the required address. The dll would obviously have to be loaded first in the injector and then in the target, because otherwise the "correct" address would be overwritten.
Another really interesting method that can be done is to write in the other process memory a little function (directly in assembly) that calls LoadLibrary and returns the address of our init function; since we wrote it there, we can also call it with CreateRemoteThread because we know where it is.
In my opinion, this is the best approach, and is also the simplest, since the code is already there, written in this nice article. Have a look at it, it is quite interesting and it probably will do the trick for your problem.
The most robust way is to link the first DLL against the import lib of the second. This way, the actual loading of the second DLL will be done by Windows itself. Sounds very trivial, but not everyone knows that DLLs can link against other DLLs. Windows can even deal with cyclic dependencies. If A.DLL loads B.DLL which needs A.DLL, the imports in B.DLL are resolved without loading A.DLL again.
I suggest you to use delay-loading mechanism. The DLL will be loaded at the fisrt time you call imported function. Moreover you can modify load function and error handling. See Linker Support for Delay-Loaded DLLs for more info.
One possible answer is through the use of LoadLibrary and GetProcAddress to access pointers to functions found/located inside the loaded dll - but your intentions/needs aren't clear enough to determine if this is a suitable answer.