diff --git a/rtcore.h b/rtcore.h
index 09440a4..321c585 100644
--- a/rtcore.h
+++ b/rtcore.h
@@ -153,6 +153,20 @@ typedef struct arena_cp
 	byte *cp;
 } arena_cp;
 
+static force_inline arena
+MakeArena(void *base, isize cap)
+{
+	return (cap > 0 && base != NULL) ?
+		(arena){
+			.begin = (byte *)base,
+			.end = (byte *)base + cap,
+		} :
+		(arena){
+			.begin = NULL,
+			.end = NULL
+		};
+}
+
 static force_inline arena_cp
 SaveArena(arena a)
 {