diff --git a/libports/src/lib/libc/libc_mem_alloc.cc b/libports/src/lib/libc/libc_mem_alloc.cc
index 19ba2796a9..ecb7a2ea24 100644
--- a/libports/src/lib/libc/libc_mem_alloc.cc
+++ b/libports/src/lib/libc/libc_mem_alloc.cc
@@ -88,7 +88,7 @@ namespace Libc {
 						_ram_session = ram, _rm_session = rm; }
 			};
 
-			Lock           _lock;
+			Lock   mutable _lock;
 			Dataspace_pool _ds_pool;      /* list of dataspaces */
 			Allocator_avl  _alloc;        /* local allocator    */
 			size_t         _chunk_size;
@@ -114,6 +114,7 @@ namespace Libc {
 
 			void *alloc(Genode::size_t size, Genode::size_t align_log2);
 			void free(void *ptr);
+			Genode::size_t size_at(void const *ptr) const;
 	};
 }
 
@@ -223,6 +224,16 @@ void Libc::Mem_alloc_impl::free(void *addr)
 }
 
 
+Genode::size_t Libc::Mem_alloc_impl::size_at(void const *addr) const
+{
+	/* serialize access of heap functions */
+	Lock::Guard lock_guard(_lock);
+
+	/* forward request to our local allocator */
+	return _alloc.size_at(addr);
+}
+
+
 Libc::Mem_alloc *Libc::mem_alloc()
 {
 	static Libc::Mem_alloc_impl inst;
diff --git a/libports/src/lib/libc/libc_mem_alloc.h b/libports/src/lib/libc/libc_mem_alloc.h
index f8fc3c2fa9..3939fb1704 100644
--- a/libports/src/lib/libc/libc_mem_alloc.h
+++ b/libports/src/lib/libc/libc_mem_alloc.h
@@ -16,6 +16,7 @@ namespace Libc {
 	{
 		virtual void *alloc(Genode::size_t size, Genode::size_t align_log2) = 0;
 		virtual void free(void *ptr) = 0;
+		virtual Genode::size_t size_at(void const *ptr) const = 0;
 	};
 
 	/**