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In Visual C++ .NET and Visual C++ 2003, DLLs compiled with the /clr compiler option could non-deterministically deadlock when loaded; this issue was called the mixed DLL loading or loader lock issue. In Visual C++ 2005, all non-determinism has been removed from the mixed DLL loading process. However, there are a few remaining scenarios for which loader lock can occur. For more information about this issue, see:

In Visual C++ 2005 there is still a restriction that code within DllMain must not access the CLR. This means that DllMain should make no calls to managed functions, directly or indirectly; no managed code should be declared or implemented in DllMain; and no garbage collection or automatic library loading should take place within DllMain.


Visual C++ 2003 provided _vcclrit.h to facilitate DLL initialization while minimizing opportunity for deadlock. For Visual C++ 2005, using _vcclrit.h is no longer necessary, and causes deprecation warnings to be produced during compilation. The recommended strategy is to remove dependencies on this file using the steps in How To: Remove Dependency on _vcclrit.h. Less ideal solutions include suppressing the warnings by defining _CRT_VCCLRIT_NO_DEPRECATE prior to including _vcclrit.h, or merely ignoring the deprecation warnings.

Causes of Loader Lock

With the introduction of the .NET platform there are two distinct mechanisms for loading an execution module (EXE or DLL): one for Windows, which is used for unmanaged modules, and one for the .NET Common Language Runtime (CLR) which loads .NET assemblies. The mixed DLL loading problem centers around the Microsoft Windows OS loader.

When an assembly containing only .NET constructs is loaded into a process, the CLR loader can perform all of the necessary loading and initialization tasks itself. However, for mixed assemblies, because they can contain native code and data, the Windows loader must be used as well.

The Windows loader guarantees that no code can access code or data in that DLL before it has been initialized, and that no code can redundantly load the DLL while it is partially initialized. To do this, the Windows loader uses a process-global critical section (often called the "loader lock") that prevents unsafe access during module initialization. As a result, the loading process is vulnerable to many classic deadlock scenarios. For mixed assemblies, the following two scenarios increase the risk of deadlock:

  • First, if users attempt to execute functions compiled to Microsoft intermediate language (MSIL) when the loader lock is held (from DllMain or in static initializers, for example), this can cause deadlock. Consider the case in which the MSIL function references a type in an assembly that has not been loaded. The CLR will attempt to automatically load that assembly, which may require the Windows loader to block on the loader lock. Since the loader lock is already held by code earlier in the call sequence, a deadlock results. However, executing MSIL under loader lock does not guarantee that a deadlock will occur, making this scenario difficult to diagnose and fix. In some circumstances, such as where the DLL of the referenced type contains no native constructs and all of its dependencies contain no native constructs, the Windows loader is not required to load the .NET assembly of the referenced type. Additionally, the required assembly or its mixed native/.NET dependencies may have already been loaded by other code. Consequently, the deadlocking can be difficult to predict, and can vary depending on the configuration of the target machine.

  • Second, when loading DLLs in versions 1.0 and 1.1 of the .NET Framework, the CLR assumed that the loader lock was not held and performed several actions that are invalid under loader lock. Assuming the loader lock is not held is a valid assumption for purely .NET DLLs, but, because mixed DLLs execute native initialization routines, they require the native Windows loader and therefore the loader lock. Consequently, even if the developer was not attempting to execute any MSIL functions during DLL initialization, there was still a small possibility of nondeterministic deadlock with versions 1.0 and 1.1 of the .NET Framework.

In Visual C++ 2005, all non-determinism has been removed from the mixed DLL loading process. This was accomplished with these changes:

  • The CLR no longer makes false assumptions when loading mixed DLLs.

  • Unmanaged and managed initialization is performed in two separate and distinct stages. Unmanaged initialization takes place first (via DllMain), and managed initialization takes place afterwards, through a .NET-supported construct called a .cctor. The latter is completely transparent to the user unless /Zl or /NODEFAULTLIB are used. See/NODEFAULTLIB (Ignore Libraries) and /Zl (Omit Default Library Name) for more information.

Loader lock can still occur, but now it occurs deterministically, and is detected. If DllMain contains MSIL instructions, the compiler will generate warning Compiler Warning (level 1) C4747. Furthermore, both the CRT and CLR will detect and report attempts to execute MSIL under loader lock, but the CRT detection is only performed in Debug builds. CRT detection results in runtime diagnostic C Run-Time Error R6031.

The remainder of this document describes the remaining scenarios for which MSIL can execute under the loader lock, resolutions for the issue under each of those scenarios, and debugging techniques.


There are several different situations under which user code can execute MSIL under loader lock. The developer must ensure that the user code implementation does not attempt to execute MSIL instructions under each of these circumstances. The following subsections describe all possibilities with a discussion of how to resolve issues in the most common cases.

  • DllMain

  • Static Initializers

  • User-Supplied Functions Affecting Startup

  • Custom Locales


The DllMain function is a user defined entry point for a DLL. Unless the user specifies otherwise, DllMain is invoked every time a process or thread attaches to or detaches from the containing DLL. Since this invocation can occur while the loader lock is held, no user-supplied DllMain function should be compiled to MSIL. Furthermore, no function in the call tree rooted at DllMain can be compiled to MSIL. To resolve issues here, the code block that defines DllMain should be modified with #pragma unmanaged, or the object file containing DllMain should be compiled without /clr. The same should be done for every function that DllMain calls.

In cases where these functions must call a function that requires an MSIL implementation for other calling contexts, a duplication strategy can be used where both a .NET and a native version of the same function are created.

Alternatively, if DllMain is not required or if it does not need to be executed under loader lock, the user-provided DllMain implementation can be removed, which will eliminate the problem.

If DllMain attempts to execute MSIL directly, Compiler Warning (level 1) C4747 will result. However, the compiler cannot detect cases where DllMain calls a function in another module that in turn attempts to execute MSIL.

Initializing Static Objects

Initializing static objects can result in deadlock if a dynamic initializer is required. For simple cases, such as when a static variable is simply assigned to a value known at compile time, no dynamic initialization is required, so there is no risk of deadlock. However, static variables initialized by function calls, constructor invocations, or expressions that cannot be evaluated at compile time all require code to execute during module initialization.

The code below shows examples of static initializers that require dynamic initialization: a function call, object construction, and a pointer initialization. (These examples aren't static, but are assumed to be defined in the global scope, which has the same effect.)

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// dynamic initializer function generated
int a = init();
CObject o(arg1, arg2);  
CObject* op = new CObject(arg1, arg2);

This risk of deadlock depends on whether the containing module is compiled with /clr and whether MSIL will be executed. Specifically, if the static variable is compiled without /clr (or resides in a #pragma unmanaged block), and the dynamic initializer required to initialize it results in the execution of MSIL instructions, deadlock may occur. This is because, for modules compiled without /clr, the initialization of static variables is performed by DllMain. In contrast, static variables compiled with /clr are initialized by the .cctor, after the unmanaged initialization stage has completed and the loader lock has been released.

There are a number of solutions to deadlock caused by the dynamic initialization of static variables (arranged roughly in order of time required to fix the problem):

  • The source file containing the static variable can be compiled with /clr.

  • All functions called by the static variable can be compiled to native code using either the #pragma unmanaged directive, or by compiling the containing source file without /clr.

  • Manually clone the code that the static variable depends upon, providing both a .NET and a native version with different names. Developers can then call the native version from native static initializers and call the .NET version elsewhere.

User-Supplied Functions Affecting Startup

There are several user-supplied functions on which libraries depend for initialization during startup. For example, when globally overloading operators in C++ such as the new and delete operators, the user-provided versions are used everywhere, including in STL initialization and destruction. As a result, STL and user-provided static initializers will invoke any user-provided versions of these operators.

If the user-provided versions are compiled to MSIL, then these initializers will be attempting to execute MSIL instructions while the loader lock is held. A user-supplied malloc has the same consequences. To resolve this problem, any of these overloads or user-supplied definitions must be implemented as native code using the #pragma unmanaged directive.

Custom Locales

If the user provides a custom global locale, this locale will be used for initializing all future I/O streams, including those that are statically initialized. If this global locale object is compiled to MSIL, then locale-object member functions compiled to MSIL may be invoked while the loader lock is held.

There are three options for solving this problem:

The source files containing all global I/O stream definitions can be compiled using the /clr option. This will prevent their static initializers from being executed under loader lock.

The custom locale function definitions can be compiled to native code, either by using the #pragma unmanaged directive or by compiling the containing source files without the /clr option.

Refrain from setting the custom locale as the global locale until after the loader lock is released. Then explicitly configure I/O streams created during initialization with the custom locale.

Impediments to Diagnosis

In some case it is difficult to detect the source of deadlocks. The following subsections discuss these scenarios and ways to work around these issues.

Implementation in Headers

Function implementations insider header files can complicate diagnosis. Nevertheless, inline functions and template code both require that functions be specified in a header file. The C++ language specifies the One Definition Rule, which forces all implementations of functions with the same name to be semantically equivalent. Consequently, the C++ linker need not make any special considerations when merging object files that have duplicate implementations of a given function. The linker simply chooses the largest of these semantically equivalent definitions, to accommodate forward declarations and scenarios when different optimization options are used for different source files. This creates a problem for mixed native/.NET DLLs.

Because the same header may be included both by a CPP files with /clr enabled and disabled, it is possible to have both MSIL and native versions of functions that provide implementations in headers. MSIL and native implementations have different semantics with respect to initialization under the loader lock, which effectively violates the one definition rule. Consequently, when the linker chooses the largest implementation, it may choose the MSIL version of a function, even if it was explicitly compiled to native code elsewhere using the #pragma unmanaged directive. To ensure that an MSIL version of a template or inline function is never called under loader lock, every definition of every such function called under loader lock must be modified with the #pragma unmanaged directive, or compiled in a source file with /clr disabled. Often, the easiest way to achieve this is to push and pop the #pragma unmanaged directive around the #include directive for the offending header file. (See managed, unmanaged for an example.) However, this strategy will not work for headers that contain other code that must directly call .NET APIs.

Diagnosing in Debug Mode

All diagnoses of loader lock problems should be done with Debug builds. Release builds may not produce diagnostics, and the optimizations performed in Release mode may mask some of the MSIL under loader lock scenarios.

How to Debug Loader Lock Issues

The diagnostic that the CLR generates when an MSIL function is invoked causes the CLR to suspend execution. In turn, this causes the Visual C++ 2005 mixed-mode debugger to be suspended as well when running the debuggee in-process. However, when attaching to the process, it is not possible to obtain a managed callstack for the debuggee using the mixed debugger.

To identify the specific MSIL function that was called under loader lock, developers should complete the following steps:

  1. Ensure that symbols for mscoree.dll and mscorwks.dll are available.

    This can be done in two ways. First, the PDBs for mscoree.dll and mscorwks.dll can be added to the symbol search path. To do this, open the symbol search path options dialog. (From the Tools menu, click Options. In the left pane of the Options dialog box, Open the Debugging node and click Symbols.) Add the path to the mscoree.dll and mscorwks.dll PDB files to the search list. These PDBs are installed with the .NET SDK to the %VSINSTALL%\SDK\v2.0\symbols. Click OK.

    Second, the PDBs for mscoree.dll and mscorwks.dll can be downloaded from the Microsoft Symbol Server. To configure Symbol Server, open the symbol search path options dialog. (From the Tools menu, click Options. In the left pane of the Options dialog box, Open the Debugging node and click Symbols.) Add the following search path to the search list: Add a symbol cache directory to the symbol server cache text box. Click OK.

  2. Set debugger mode to native-only mode.

    To do this, open the Properties grid for the startup project in the solution. Under the Configuration Properties subtree, select the Debugging Node. Set the Debugger Type Field to Native-Only.

  3. Start the Debugger (F5).

  4. When the /clr diagnostic is generated, click Retry and then click Break.

  5. Open the call stack window. (From the Debug menu, click Windows, then Call Stack.) If the offending DllMain or static initializer is identified with a green arrow. If the offending function is not identified, the following steps must be taken to find it.

  6. Open the immediate window (From the Debug menu, click Windows, then Immediate.)

  7. Type .load sos.dll into the immediate window to load the SOS debugging service.

  8. Type !dumpstack into the immediate window to obtain a complete listing of the internal /clr stack.

  9. Look for the first instance (closest to the bottom of the stack) of either _CorDllMain (if DllMain causes issue) or VTableBootstrapThunkInitHelperStub (if static initializer causes issue). The stack entry just below this call is the invocation of the MSIL implemented function that attempted to execute under loader lock.

  10. Go to the source file and line number identified in Step 9 and correct the problem using the scenarios and solutions described in the Scenarios section.


The following sample shows how to avoid loader lock by moving code from DllMain into the constructor of a global object.

In this sample, there is a global managed object whose constructor contains the managed object that was originally in DllMain. The second part of this sample references the assembly, creating an instance of the managed object to invoke the module constructor which does the initialization.

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// initializing_mixed_assemblies.cpp
// compile with: /clr /LD 
#pragma once
#include <stdio.h>
#include <windows.h>
struct __declspec(dllexport) A {
   A() {
      System::Console::WriteLine("Module ctor initializing based on global instance of class.\n");

   void Test() {
      printf("Test called so linker does not throw away unused object.\n");
#pragma unmanaged
// Global instance of object
A obj;
extern "C"
BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved) {
   // Remove all managed code from here and put it in constructor of A.
   return true;


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// initializing_mixed_assemblies_2.cpp
// compile with: /clr initializing_mixed_assemblies.lib
#include <windows.h>
using namespace System;
#include <stdio.h>
#using "initializing_mixed_assemblies.dll"
struct __declspec(dllimport) A {
   void Test();

int main() {
   A obj;


Module ctor initializing based on global instance of class.

Test called so linker does not throw away unused object.

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