Full rewrite

include/arena.h

@@ -2,23 +2,60 @@
 #ifndef ARENA_H
 #define ARENA_H
 
-#include <stddef.h> // ptrdiff_t
-#include <stdint.h> // uintptr_t
+#include <stdbool.h>
+#include <stddef.h> // size_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 {
-    struct page *head, *last;
-} __attribute__((aligned(_WORD_SIZE))) arena_t;
+    char  *data;
+    size_t size;
+    size_t cap;
+    bool   grow;
+} arena_t;
 
-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);
+/**
+ * Allocate a new arena.
+ * The underlying memory is allocated with mmap.
+ */
+arena_t arena_new();
+
+/**
+ * Delete memory mapped for arena.
+ * Should only be used with arenas from arena_new().
+ */
+void arena_delete(arena_t *a);
+
+/**
+ * Attach an arena to an existing memory region.
+ * The arena will not expand the region if space is exceeded.
+ */
+static inline arena_t arena_attach(void *ptr, size_t size)
+{
+    return (arena_t) { .data = ptr, .size = 0, .cap = size, .grow = false };
+}
+
+/**
+ * Detach an arena from an existing memory region.
+ */
+static inline void *arena_detatch(arena_t arena)
+{
+    return arena.data;
+}
+
+/**
+ * Reset an arena.
+ */
+void arena_reset(arena_t *a);
+
+/**
+ * Allocate memory from an arena.
+ * Returns NULL and sets errno on failure.
+ */
+void *arena_alloc(arena_t *a, size_t len);
+
+/**
+ * Allocate and zero memory from an arena.
+ * Returns NULL and sets errno on failure.
+ */
+void *arena_calloc(arena_t *a, size_t nmemb, size_t size);
 
 #endif