diff --git a/src/os_win/mem.c b/src/os_win/mem.c
index b0f0366..d6c38d7 100644
--- a/src/os_win/mem.c
+++ b/src/os_win/mem.c
@@ -1,14 +1,17 @@
 
+#define DEFAULT_ALIGNMENT 16
+
 static HANDLE heap_handle;
 
-static bool is_power_of_two(size_t s) {
+static bool _is_power_of_two(size_t s) {
     return (s & (s-1)) == 0;
 }
 
-static intptr_t align_forward(intptr_t p, size_t a) {
+static intptr_t _align_forward(intptr_t p, size_t a) {
     return (p+a-1)&~(a-1);
 }
 
+
 static void _setup_heap() {
     heap_handle = GetProcessHeap();
     if (heap_handle == NULL) {
@@ -16,6 +19,57 @@ static void _setup_heap() {
     }
 }
 
+struct AllocationTag typedef AllocationTag;
+struct AllocationTag {
+    void *ptr;
+    size_t size;
+};
+
+// Retrieve the tag using allocated pointer
+static AllocationTag *_mem_block_tag(void *user_ptr) {
+    AllocationTag *tag = (void *)((char *)user_ptr - sizeof(AllocationTag));
+    return tag;
+}
+
+// Calculate the block size that is enough to hold user data, the tag, and
+// the necessary free space in case the pointer from HeapAlloc needs further
+// alignment
+static size_t _mem_min_block_size(size_t alignment, size_t user_size) {
+    size_t size = user_size + sizeof(AllocationTag);
+    if(alignment > 8) {
+        size += alignment;
+    }
+    return size;
+}
+
+// Calculate the start of the user data, such that it's aligned, attach the tag
+// to the allocated pointer
+static void *_mem_block_setup(void *block_start, size_t block_size, size_t alignment) {
+    intptr_t block_start_i = (intptr_t)block_start;
+    intptr_t aligned_block_start_i = _align_forward(block_start_i, alignment);
+    intptr_t free_space = aligned_block_start_i - block_start_i;
+    if(free_space < sizeof(AllocationTag)) {
+        aligned_block_start_i += alignment;
+    }
+    AllocationTag *tag = _mem_block_tag(block_start);
+    tag->ptr = block_start;
+    tag->size = block_size;
+    return (void *)aligned_block_start_i;
+}
+
+void *_mem_alloc(size_t alignment, size_t size) {
+    // HeapAlloc is guaranteed to return 8-byte aligned pointer,
+    // so if our alignment is 8 or less we don't have to overallocate.
+    // otherwise we'll reserve extra `alignment` bytes to later adjust our
+    // memory buffer according to required alignment.
+    size_t block_size = _mem_min_block_size(alignment, size);
+    void *block_start = HeapAlloc(heap_handle, 0, block_size);
+    if(block_start == NULL) {
+        return NULL;
+    }
+    return _mem_block_setup(block_start, block_size, alignment);
+}
+
 void *aligned_alloc(size_t alignment, size_t size) {
     if(size == 0) {
         return NULL;
@@ -23,22 +77,10 @@ void *aligned_alloc(size_t alignment, size_t size) {
     if(alignment == 0) {
         alignment = 8;
     }
-    if(!is_power_of_two(alignment)) {
+    if(!_is_power_of_two(alignment)) {
         return NULL;
     }
-    // HeapAlloc is guaranteed to return 8-byte aligned pointer,
-    // so if our alignment is 8 or less we don't have to overallocate.
-    // otherwise we'll reserve extra `alignment` bytes to later adjust our
-    // memory buffer according to required alignment.
-    size_t min_req_size = size;
-    if(alignment > 8) {
-        min_req_size += alignment;
-    }
-    void *block_start = HeapAlloc(heap_handle, 0, min_req_size);
-    intptr_t block_start_i = (intptr_t)block_start;
-    intptr_t aligned_block_start_i = align_forward(block_start_i, alignment);
-    void *aligned_block_start = (void *)aligned_block_start_i;
-    return aligned_block_start;
+    return _mem_alloc(alignment, size);
 }
 
 void *calloc(size_t nmemb, size_t size) {
@@ -48,32 +90,39 @@ void *calloc(size_t nmemb, size_t size) {
     if(nmemb > SIZE_MAX/size) {
         return NULL;
     }
-    void *block_start = HeapAlloc(heap_handle, HEAP_ZERO_MEMORY, size*nmemb);
-    return block_start;
+    return _mem_alloc(DEFAULT_ALIGNMENT, nmemb * size);
 }
 
 void free(void *ptr) {
     if(ptr == NULL) {
         return;
     }
-    HeapFree(heap_handle, 0, ptr);
+    AllocationTag *tag = (void *)((char *)ptr - sizeof(AllocationTag));
+    HeapFree(heap_handle, 0, tag->ptr);
 }
 
 void *malloc(size_t size) {
-    if(size == 0) return NULL;
-    return HeapAlloc(heap_handle, 0, size);
+    if(size == 0) {
+        return NULL;
+    }
+    return _mem_alloc(DEFAULT_ALIGNMENT, size);
 }
 
 void *realloc(void *ptr, size_t size) {
     if (ptr == NULL) {
-        if (size == 0) return NULL;
-
-        return HeapAlloc(heap_handle, 0, size);
+        if (size == 0) {
+            return NULL;
+        }
+        return _mem_alloc(DEFAULT_ALIGNMENT, size);
     } else if (size == 0) {
-        HeapFree(heap_handle, 0, ptr);
+        free(ptr);
         return NULL;
     } else {
-        return HeapReAlloc(heap_handle, 0, ptr, size);
+        AllocationTag *tag = _mem_block_tag(ptr);
+        void *old_block = tag->ptr;
+        size_t new_block_size = _mem_min_block_size(DEFAULT_ALIGNMENT, size);
+        void *new_block = HeapReAlloc(heap_handle, 0, old_block, new_block_size);
+        return _mem_block_setup(new_block, new_block_size, DEFAULT_ALIGNMENT);
     }
 }