From 3e4826cf36779f9650f1a73f470c92ae92612afc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nils=20O=2E=20Sel=C3=A5sdal?= <nos@utel.tech>
Date: Mon, 6 Apr 2026 14:37:00 +0200
Subject: [PATCH] Add simple static arena allocator

---
 include/ucore/sarena.h | 116 +++++++++++++++++++++++++++++++++++++++++
 src/sarena.c           |  30 +++++++++++
 2 files changed, 146 insertions(+)
 create mode 100644 include/ucore/sarena.h
 create mode 100644 src/sarena.c

diff --git a/include/ucore/sarena.h b/include/ucore/sarena.h
new file mode 100644
index 0000000..222039b
--- /dev/null
+++ b/include/ucore/sarena.h
@@ -0,0 +1,116 @@
+#ifndef UC_SARENA_H_
+#define UC_SARENA_H_
+
+#include <stddef.h>
+#include <stdint.h>
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** @file
+ * Static arena allocator.
+ *
+ *  An UCSArena allocates memory from pre-existing array
+ *
+ * The backing array should be suitable aligned to the types
+ * that will be allocated from it. Use .e.g
+ * @code
+ * _Alignas(16) char mem[1024];
+ * @endcode
+ * as the backing array.
+ *
+ * Once allocated, a piece of memory obtained from the
+ * allocator cannot be free'd individually, only the entire SArena can
+ * be free'd/reset
+*/
+typedef struct UCSArena UCSArena;
+struct UCSArena {
+    char *start; // start of user buffer
+    char *curr;  // next alloc point
+    char *end;   // one past end of user buffer
+};
+
+/** Initialize a UCSArena with predefined backing array.
+ * @data must be an array type
+ * Use as
+ * @code
+ * _Alignas(16) char v[128];
+ * SArana a = UC_BV_STATIC_INIT(v);
+ * @endcode
+ */
+#define UC_SAR_STATIC_INIT(data)\
+    UC_SAR_INIT((data), sizeof(data))
+
+/** Initialize a UCSArena with predefined memory
+ * Use as
+ * @code
+ * char v[128];
+ * SArana a = UC_BV_STATIC_INIT(v, sizeof v);
+ * @endcode
+ * or
+ * @code
+ * char *m = malloc(128);
+ * SArana a = UC_BV_INIT(m, 128);
+ * @endcode
+ */
+#define UC_SAR_INIT(data, sz)\
+    {\
+    (data),\
+    (data),\
+    (data) + sz\
+    }
+
+/** Initialize a UCSArena with an array as the backing memory to allocate from.
+ * The macro can be used at local scope to allocate memory automatic (stack) storage,
+ * or at global scope where the backing array will have static storage duration
+ *
+ * Use as:
+ * @code
+ * UC_SAR_ALLOC_DEF(my_allocator, 1024)
+ * struct foo *foo = uc_sar_alloc(&my_allocator);
+ * if (foo == NULL) {
+ *     // out of memory
+ * }
+ * @endcode
+ *
+ * @note, the backing store has alignment of 16. Use manual init if a different alignment
+ * is needed.
+ * @param var_name Variable name for the allocator
+ * @param size_  byte size of the backing array
+*/
+#define UC_SAR_ALLOC_DEF(var_name, size_) \
+    struct {\
+        _Alignas(16) char memory[size_]; \
+    } var_name ## _memory;\
+    UCSArena var_name = UC_SAR_STATIC_INIT(var_name ## _memory.memory)
+
+/** Allocate space for @type_ , returned memory is aligned to requirements
+ * of @type.
+ *
+ * Use as:
+ * @code
+ * UC_SAR_ALLOC_DEF(my_allocator, 1024)
+ * struct foo *foo = UC_SAR_ALLOC_ALIGNED(&my_allocator, struct foo);
+ * // Array types can be used too, room for null terminated string of lengh 10:
+ * char *str = UC_SAR_ALLOC_ALIGNED(&my_allocator, char[11]);
+ * @endcode
+ */
+#define UC_SAR_ALLOC_ALIGNED(allocator, type_) \
+    uc_sar_alloc_aligned((allocator), sizeof (type_), _Alignof(type_))
+
+// Allocate @sz bytes from the UCSArena. Returns NULL if not enough space
+void *uc_sar_alloc(UCSArena *a, size_t sz);
+
+// Allocate @sz bytes from the UCSArena, returned pointer is aligned to @align
+// Returns NULL if not enough space
+void *uc_sar_alloc_aligned(UCSArena *a, size_t sz, unsigned int align);
+
+static inline void uc_sar_reset(UCSArena *a)
+{
+    a->curr = a->start;
+}
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/sarena.c b/src/sarena.c
new file mode 100644
index 0000000..45e87c9
--- /dev/null
+++ b/src/sarena.c
@@ -0,0 +1,30 @@
+#include <assert.h>
+#include "ucore/sarena.h"
+
+void *uc_sar_alloc(UCSArena *a, size_t sz)
+{
+    if (a->curr + sz > a->end) {
+        return NULL;
+    }
+    void *start = a->curr;
+    a->curr += sz;
+    return start;
+}
+
+void *uc_sar_alloc_aligned(UCSArena *a, size_t sz, unsigned int align)
+{
+    assert(align != 0);
+    assert((align & (align - 1)) == 0); //power of 2
+
+    uintptr_t curr = (uintptr_t)a->curr;
+    uintptr_t aligned = (curr + (align - (uintptr_t)1)) & ~(align - (uintptr_t)1);
+    char *start = (char *)aligned;
+
+     if (start + sz > a->end) {
+        return NULL;
+     }
+
+     a->curr = start + sz;
+
+     return start;
+}