Main: add single-instance checking.

src/main.cpp

@@ -18,7 +18,25 @@
 
 #include <fstream>
 
+#include <errhandlingapi.h>
+#include <synchapi.h>
+#include <winerror.h>
+
 int main(int argc, char** argv) {
+    void* mutex_handle = CreateMutexW(nullptr, 0, L"MassBuilderSaveTool");
+
+    if(!mutex_handle) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Error initialising the app",
+                                 "There was an error initialising the mutex.",nullptr);
+        return EXIT_FAILURE;
+    }
+
+    if(GetLastError() == ERROR_ALREADY_EXISTS) {
+        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Error initialising the app",
+                                 "There can be only one running instance of the application.",nullptr);
+        return EXIT_FAILURE;
+    }
+
 #ifndef SAVETOOL_DEBUG_BUILD
     std::ofstream output{"SaveToolLog.txt", std::ios::trunc|std::ios::out};
 
@@ -28,5 +46,9 @@ int main(int argc, char** argv) {
 #endif
 
     SaveTool app({argc, argv});
-    return app.exec();
+    Int result = app.exec();
+
+    ReleaseMutex(mutex_handle);
+
+    return result;
 }