diff --git a/include/malloc.h b/include/malloc.h
index e69de29b..35f8b19c 100644
--- a/include/malloc.h
+++ b/include/malloc.h
@@ -0,0 +1,25 @@
+#ifndef _MALLOC_H
+#define _MALLOC_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define __NEED_size_t
+
+#include <bits/alltypes.h>
+
+void *malloc (size_t);
+void *calloc (size_t, size_t);
+void *realloc (void *, size_t);
+void free (void *);
+void *valloc (size_t);
+void *memalign(size_t, size_t);
+
+size_t malloc_usable_size(void *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/malloc/malloc_usable_size.c b/src/malloc/malloc_usable_size.c
new file mode 100644
index 00000000..8cccd9d8
--- /dev/null
+++ b/src/malloc/malloc_usable_size.c
@@ -0,0 +1,17 @@
+#include <malloc.h>
+
+void *(*const __realloc_dep)(void *, size_t) = realloc;
+
+struct chunk {
+	size_t psize, csize;
+	struct chunk *next, *prev;
+};
+
+#define OVERHEAD (2*sizeof(size_t))
+#define CHUNK_SIZE(c) ((c)->csize & -2)
+#define MEM_TO_CHUNK(p) (struct chunk *)((char *)(p) - OVERHEAD)
+
+size_t malloc_usable_size(void *p)
+{
+	return CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD;
+}