This should fix compiler linkage.

selective-mocking/_inject.h

@@ -1,10 +1,12 @@
+#include <stddef.h> // for size_t
+
 // Replace all calls to malloc() with wrap_malloc() in the source code.
 // This change only affects _your_ code and not any 3rd party or
 // system libraries. The real malloc() will still be invoked by
 // 3rd party code.
-
-#include <stddef.h> // for size_t
-
 #define malloc wrap_malloc
 
+#if defined(WIN32) // Pay attention to compiler linkage!
+__declspec(dllexport)
+#endif
 void *wrap_malloc(size_t size);