Implement globals shared across DLLs on Windows.

doc/changelog.dox

@@ -350,8 +350,7 @@ See also:
     @ref Corrade::Containers::Array instead of being deleted
 -   Global state used by @ref GL::Context and @ref Audio::Context is no longer
     duplicated when Magnum is built statically and linked to more than one
-    dynamic library or executable. Currently that holds only for Linux and
+    dynamic library or executable.
-    macOS, Windows support will come next.
 -   The [base-qt](https://github.com/mosra/magnum-bootstrap/tree/base-qt)
     bootstrap project is fixed to use OpenGL 4.1 on macOS instead of being
     stuck on 2.1 (see [mosra/magnum-bootstrap#22](https://github.com/mosra/magnum-bootstrap/issues/22))

src/Magnum/Audio/Context.cpp

@@ -41,6 +41,10 @@
 
 #include "Magnum/Audio/Extensions.h"
 
+#if defined(CORRADE_TARGET_WINDOWS) && defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+#include "Magnum/Implementation/WindowsWeakSymbol.h"
+#endif
+
 namespace Magnum { namespace Audio {
 
 namespace {
@@ -113,14 +117,16 @@ std::vector<std::string> Context::deviceSpecifierStrings() {
     return list;
 }
 
-#ifndef MAGNUM_BUILD_STATIC
+#if !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
-/* (Of course) can't be in an unnamed namespace in order to export it below */
+/* (Of course) can't be in an unnamed namespace in order to export it below
+   (except for Windows, where we do extern "C" so this doesn't matter) */
 namespace {
 #endif
 
 /* Unlike GL, this isn't thread-local. Would need to implement
    ALC_EXT_thread_local_context first */
-#if defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS)
+#if !defined(MAGNUM_BUILD_STATIC) || (defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS))
+#ifdef MAGNUM_BUILD_STATIC
 /* On static builds that get linked to multiple shared libraries and then used
    in a single app we want to ensure there's just one global symbol. On Linux
    it's apparently enough to just export, macOS needs the weak attribute.
@@ -134,11 +140,36 @@ CORRADE_VISIBILITY_EXPORT
     #endif
 #endif
 Context* currentContext = nullptr;
+#else
+/* On Windows the symbol is exported unmangled and then fetched via
+   GetProcAddress() to emulate weak linking. */
+extern "C" CORRADE_VISIBILITY_EXPORT Context* magnumAudioUniqueCurrentContext = nullptr;
+#endif
 
-#ifndef MAGNUM_BUILD_STATIC
+#if !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
 }
 #endif
 
+/* Windows don't have any concept of weak symbols, instead GetProcAddress() on
+   GetModuleHandle(nullptr) "emulates" the weak linking as it's guaranteed to
+   pick up the same symbol of the final exe independently of the DLL it was
+   called from. To avoid #ifdef hell in code below, the currentContext is
+   redefined to return a value from this uniqueness-ensuring function. */
+#if defined(CORRADE_TARGET_WINDOWS) && defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+namespace {
+
+Context*& windowsCurrentContext() {
+    /* A function-local static to ensure it's only initialized once without any
+       race conditions among threads */
+    static Context** const uniqueGlobals = reinterpret_cast<Context**>(Magnum::Implementation::windowsWeakSymbol("magnumAudioUniqueCurrentContext"));
+    return *uniqueGlobals;
+}
+
+}
+
+#define currentContext windowsCurrentContext()
+#endif
+
 bool Context::hasCurrent() { return currentContext; }
 
 Context& Context::current() {

src/Magnum/Audio/Test/GlobalStateAcrossLibrariesALTest.cpp

@@ -44,12 +44,7 @@ GlobalStateAcrossLibrariesALTest::GlobalStateAcrossLibrariesALTest() {
 void GlobalStateAcrossLibrariesALTest::test() {
     Context context;
     CORRADE_VERIFY(Context::hasCurrent());
-    {
+    CORRADE_COMPARE(currentContextInALibrary(), &context);
-        #ifdef CORRADE_TARGET_WINDOWS
-        CORRADE_EXPECT_FAIL("Deduplication of global data across shared libraries isn't implemented on Windows yet.");
-        #endif
-        CORRADE_COMPARE(currentContextInALibrary(), &context);
-    }
 }
 
 }}}}

src/Magnum/CMakeLists.txt

@@ -66,6 +66,12 @@ set(Magnum_HEADERS
 set(Magnum_PRIVATE_HEADERS
     Implementation/ImageProperties.h)
 
+# Functionality specific to static Windows builds
+if(CORRADE_TARGET_WINDOWS AND NOT CORRADE_TARGET_WINDOWS_RT AND MAGNUM_BUILD_STATIC)
+    list(APPEND Magnum_SRCS Implementation/WindowsWeakSymbol.cpp)
+    list(APPEND Magnum_PRIVATE_HEADERS Implementation/WindowsWeakSymbol.h)
+endif()
+
 # Files shared between main library and math unit test library
 set(MagnumMath_SRCS
     Math/Angle.cpp

src/Magnum/GL/Context.cpp

@@ -65,6 +65,10 @@
 #include "Magnum/GL/Implementation/TransformFeedbackState.h"
 #endif
 
+#if defined(CORRADE_TARGET_WINDOWS) && defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+#include "Magnum/Implementation/WindowsWeakSymbol.h"
+#endif
+
 namespace Magnum { namespace GL {
 
 namespace {
@@ -473,8 +477,9 @@ Containers::ArrayView<const Extension> Extension::extensions(Version version) {
     CORRADE_ASSERT_UNREACHABLE(); /* LCOV_EXCL_LINE */
 }
 
-#ifndef MAGNUM_BUILD_STATIC
+#if !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
-/* (Of course) can't be in an unnamed namespace in order to export it below */
+/* (Of course) can't be in an unnamed namespace in order to export it below
+   (except for Windows, where we do extern "C" so this doesn't matter) */
 namespace {
 #endif
 
@@ -485,8 +490,7 @@ CORRADE_THREAD_LOCAL
 /* On static builds that get linked to multiple shared libraries and then used
    in a single app we want to ensure there's just one global symbol. On Linux
    it's apparently enough to just export, macOS needs the weak attribute.
-   Windows not handled yet, as it needs a workaround using DllMain() and
+   Windows handled differently below. */
-   GetProcAddress(). */
 CORRADE_VISIBILITY_EXPORT
     #ifdef __GNUC__
     __attribute__((weak))
@@ -496,8 +500,35 @@ CORRADE_VISIBILITY_EXPORT
 #endif
 Context* currentContext = nullptr;
 
-#ifndef MAGNUM_BUILD_STATIC
+#if !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS)
+}
+#endif
+
+/* Windows can't have a symbol both thread-local and exported, moreover there
+   isn't any concept of weak symbols. Exporting thread-local symbols can be
+   worked around by exporting a function that then returns a reference to a
+   non-exported thread-local symbol; and finally GetProcAddress() on
+   GetModuleHandle(nullptr) "emulates" the weak linking as it's guaranteed to
+   pick up the same symbol of the final exe independently of the DLL it was
+   called from. To avoid #ifdef hell in code below, the currentContext is
+   redefined to return a value from this uniqueness-ensuring function. */
+#if defined(CORRADE_TARGET_WINDOWS) && defined(MAGNUM_BUILD_STATIC) && !defined(CORRADE_TARGET_WINDOWS_RT)
+extern "C" CORRADE_VISIBILITY_EXPORT Context*& magnumGLUniqueCurrentContext() {
+    return currentContext;
+}
+
+namespace {
+
+Context*& windowsCurrentContext() {
+    /* A function-local static to ensure it's only initialized once without any
+       race conditions among threads */
+    static Context*&(*const uniqueGlobals)() = reinterpret_cast<Context*&(*)()>(Magnum::Implementation::windowsWeakSymbol("magnumGLUniqueCurrentContext"));
+    return uniqueGlobals();
 }
+
+}
+
+#define currentContext windowsCurrentContext()
 #endif
 
 bool Context::hasCurrent() { return currentContext; }

src/Magnum/GL/Test/GlobalStateAcrossLibrariesGLTest.cpp

@@ -42,12 +42,7 @@ GlobalStateAcrossLibrariesGLTest::GlobalStateAcrossLibrariesGLTest() {
 
 void GlobalStateAcrossLibrariesGLTest::test() {
     CORRADE_VERIFY(GL::Context::hasCurrent());
-    {
+    CORRADE_COMPARE(currentContextInALibrary(), &GL::Context::current());
-        #ifdef CORRADE_TARGET_WINDOWS
-        CORRADE_EXPECT_FAIL("Deduplication of global data across shared libraries isn't implemented on Windows yet.");
-        #endif
-        CORRADE_COMPARE(currentContextInALibrary(), &GL::Context::current());
-    }
 }
 
 }}}}

src/Magnum/Implementation/WindowsWeakSymbol.cpp

@@ -0,0 +1,43 @@
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019
+              Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include "WindowsWeakSymbol.h"
+
+#include <Corrade/Utility/Assert.h>
+
+#define WIN32_LEAN_AND_MEAN 1
+#define VC_EXTRALEAN
+#include <windows.h>
+
+namespace Magnum { namespace Implementation {
+
+void* windowsWeakSymbol(const char* name) {
+    /* FARPROC?! I want either a function pointer or a variable pointer */
+    void* address = reinterpret_cast<void*>(GetProcAddress(GetModuleHandleA(nullptr), name));
+    CORRADE_INTERNAL_ASSERT(address);
+    return address;
+}
+
+}}

src/Magnum/Implementation/WindowsWeakSymbol.h

@@ -0,0 +1,44 @@
+#ifndef Magnum_Implementation_WindowsWeakSymbol_h
+#define Magnum_Implementation_WindowsWeakSymbol_h
+/*
+    This file is part of Magnum.
+
+    Copyright © 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019
+              Vladimír Vondruš <mosra@centrum.cz>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include <Magnum/configure.h>
+
+/* This is a copy of Corrade/Utility/Implementation/windowsWeakSymbol.{h,cpp},
+   just adapted for Magnum. Yeah, it's only in order to avoid including
+   windows.h in all the code. */
+
+#if !defined(CORRADE_TARGET_WINDOWS) || !defined(MAGNUM_BUILD_STATIC) || defined(CORRADE_TARGET_WINDOWS_RT)
+#error this file is only meant to be used in non-RT Windows static builds
+#endif
+
+namespace Magnum { namespace Implementation {
+
+void* windowsWeakSymbol(const char* name);
+
+}}
+
+#endif