Add arena_free()

include/arena.h

@@ -3,19 +3,22 @@
 #define ARENA_H
 
 #include <stddef.h> // ptrdiff_t
+#include <stdint.h> // uintptr_t
 
-typedef unsigned char byte_t;
+#define _WORD_SIZE (sizeof(intptr_t))
 
+// linked list terminology:
+// https://web.archive.org/web/20230604145332/https://i.stack.imgur.com/2FbXf.png
 typedef struct arena {
-    size_t next, prev, cap;
-    void* data;
-} __attribute__((aligned(sizeof(void*)))) arena_t;
+    struct page *head, *last;
+} __attribute__((aligned(_WORD_SIZE))) arena_t;
 
-arena_t arena_new();
+arena_t arena_new(void);
 void arena_reset(arena_t* a);
 void* arena_alloc(arena_t* a, size_t len);
 void* arena_calloc(arena_t* a, size_t nmemb, size_t size);
 void* arena_realloc_tail(arena_t* a, size_t len);
 void arena_delete(arena_t* a);
+void arena_free(arena_t* a, void* p);
 
 #endif