v1.2.5 - Fix alignment above alignof(max_align_t) and stale marker rewinds

.gitignore

@@ -1 +1,2 @@
 test
+test_align64

Makefile

@@ -1,6 +1,10 @@
+all: test test_align64
 
 test: test.c arena.h
 	clang -Wall -Wextra -pedantic -std=c11 -o test test.c
 
+test_align64: test.c arena.h
+	clang -Wall -Wextra -pedantic -std=c11 -DARENA_ALIGNMENT=64 -o test_align64 test.c
+
 clean:
-	rm -rf test
+	rm -rf test test_align64

arena.h

@@ -1,4 +1,4 @@
-// arena.h - v1.2.4 - MIT License
+// arena.h - v1.2.5 - MIT License
 // single header library for region-based memory management.
 //
 // [License and changelog]
@@ -69,7 +69,8 @@
 //     #define ARENA_ALIGNMENT alignment (alignof(max_align_t))
 //
 //         This macro defines the memory alignment used for allocations.
-//         Must be a power of two.
+//         Must be a power of two. Values greater than alignof(max_align_t)
+//         are supported.
 //
 //     #define ARENA_REGION_CAPACITY new_region_capacity_in_bytes (8*1024)
 //
@@ -97,8 +98,9 @@
 //
 //         These macros define alternative functions for dynamic allocation
 //         and deallocation. They are only used for managing Arena__Region
-//         structures. If redefined ensure that your functions consider memory
-//         alignment.
+//         structures. If redefined ensure that your functions return memory
+//         aligned at least like malloc() does. ARENA_ALIGNMENT is handled by
+//         the library itself.
 //
 // [Function documentation]
 //
@@ -177,6 +179,11 @@
 // Note: this function does not free any newly allocated regions internally.
 // See arena_trim()
 //
+// Warning: a marker is invalidated when its region is freed by arena_trim() or
+// arena_free(). Rewinding to a marker whose region is no longer part of the
+// arena is treated as an error and leaves the arena untouched. This takes
+// linear time in the number of regions.
+//
 // [Notes]
 //
 // This library does not account for thread safety.
@@ -225,9 +232,10 @@
 
 typedef struct Arena_Region {
     struct Arena_Region *next;
+    uint8_t *data; // Start of user buffers, aligned to ARENA_ALIGNMENT
     size_t count;
     size_t capacity;
-    alignas(ARENA_ALIGNMENT) uint8_t data[];
+    uint8_t storage[];
 } Arena__Region;
 
 typedef struct {
@@ -272,6 +280,7 @@ void arena_rewind(Arena *arena, ArenaMarker marker);
 
 // Private functions
 static size_t arena__align(size_t n);
+static Arena__Region *arena__region_create(size_t capacity);
 
 Arena arena_create(size_t region_capacity)
 {
@@ -351,21 +360,10 @@ void *arena_alloc(Arena *arena, size_t size)
                                  ? aligned_size
                                  : default_capacity);
 
-        if (alloc_capacity > SIZE_MAX - sizeof(Arena__Region)) {
-            ARENA_ASSERT(false && "Region allocation size overflow");
-            return NULL;
-        }
+        arena->head = arena->tail = arena__region_create(alloc_capacity);
+        if (arena->head == NULL) { return NULL; }
 
-        arena->head = arena->tail = (Arena__Region *)ARENA_MALLOC(
-                sizeof(*arena->head) + alloc_capacity);
-        if (arena->head == NULL) {
-            ARENA_ASSERT(false && "Memory allocation failed");
-            return NULL;
-        }
-
-        arena->head->next = NULL;
         arena->head->count = aligned_size;
-        arena->head->capacity = alloc_capacity;
         return arena->head->data;
     }
 
@@ -396,22 +394,11 @@ void *arena_alloc(Arena *arena, size_t size)
                              ? aligned_size
                              : next_capacity);
 
-    if (alloc_capacity > SIZE_MAX - sizeof(Arena__Region)) {
-        ARENA_ASSERT(false && "Region allocation size overflow");
-        return NULL;
-    }
-
-    last_region->next = (Arena__Region *)ARENA_MALLOC(
-            sizeof(*last_region->next) + alloc_capacity);
-    if (last_region->next == NULL) {
-        ARENA_ASSERT(false && "Memory allocation failed");
-        return NULL;
-    }
+    last_region->next = arena__region_create(alloc_capacity);
+    if (last_region->next == NULL) { return NULL; }
 
     arena->tail = last_region->next;
-    arena->tail->next = NULL;
     arena->tail->count = aligned_size;
-    arena->tail->capacity = alloc_capacity;
     return arena->tail->data;
 }
 
@@ -531,7 +518,19 @@ void arena_rewind(Arena *arena, ArenaMarker marker)
         return;
     }
 
-    arena->tail = marker.region;
+    // Reject markers whose region is not part of this arena anymore, e.g.
+    // freed by arena_trim() or arena_free(), or taken from another arena
+    Arena__Region *region = arena->head;
+    while (region != NULL && region != marker.region) {
+        region = region->next;
+    }
+
+    if (region == NULL || marker.count > region->capacity) {
+        ARENA_ASSERT(false && "Invalid marker");
+        return;
+    }
+
+    arena->tail = region;
     arena->tail->count = marker.count;
 
     // Reset any new regions that were appended
@@ -545,11 +544,41 @@ static size_t arena__align(size_t n)
     return (n + (ARENA_ALIGNMENT - 1)) & ~(ARENA_ALIGNMENT - 1);
 }
 
+static Arena__Region *arena__region_create(size_t capacity)
+{
+    // Extra space to align data ourselves, since ARENA_MALLOC only guarantees
+    // alignof(max_align_t), which may be less than ARENA_ALIGNMENT
+    size_t padding = ARENA_ALIGNMENT - 1;
+    if (capacity > SIZE_MAX - sizeof(Arena__Region) - padding) {
+        ARENA_ASSERT(false && "Region allocation size overflow");
+        return NULL;
+    }
+
+    Arena__Region *region = (Arena__Region *)ARENA_MALLOC(
+            sizeof(*region) + padding + capacity);
+    if (region == NULL) {
+        ARENA_ASSERT(false && "Memory allocation failed");
+        return NULL;
+    }
+
+    uintptr_t misalignment = (uintptr_t)region->storage
+                             & ((uintptr_t)ARENA_ALIGNMENT - 1);
+    region->data = region->storage + (misalignment == 0
+                                      ? 0
+                                      : ARENA_ALIGNMENT - misalignment);
+    region->next = NULL;
+    region->count = 0;
+    region->capacity = capacity;
+    return region;
+}
+
 #endif // ARENA_IMPLEMENTATION
 
 /*
  * Revision history:
  *
+ *     1.2.5 (2026-10-28) Support ARENA_ALIGNMENT > alignof(max_align_t);
+ *                        reject invalid markers in arena_rewind()
  *     1.2.4 (2026-09-20) Major rewrite
  *     1.2.3 (2026-07-07) New function: arena_calloc()
  *     1.2.2 (2026-07-07) Fix various memory problems;

test.c

@@ -160,6 +160,45 @@ static void test_snapshot_rewind_trim_logic(void)
     arena_free(&arena);
 }
 
+static void test_invalid_markers(void)
+{
+    printf("Running invalid marker tests...\n");
+    Arena arena = arena_create(128);
+
+    void *first = arena_alloc(&arena, 100); // Region 1 (Cap: 128)
+    assert(is_aligned(first));
+    ArenaMarker mark_r1 = arena_snapshot(&arena);
+
+    arena_alloc(&arena, 200); // Region 2 (Cap: 256)
+    ArenaMarker mark_r2 = arena_snapshot(&arena);
+
+    // Free Region 2, invalidating mark_r2
+    arena_rewind(&arena, mark_r1);
+    arena_trim(&arena);
+    assert(count_regions(&arena) == 1);
+
+    // Stale marker must be rejected without touching freed memory
+    arena_rewind(&arena, mark_r2);
+    assert(arena.tail == mark_r1.region);
+    assert(arena.tail->count == mark_r1.count);
+
+    // Marker from another arena
+    Arena other = arena_create(128);
+    arena_alloc(&other, 10);
+    arena_rewind(&arena, arena_snapshot(&other));
+    assert(arena.tail == mark_r1.region);
+    assert(arena.tail->count == mark_r1.count);
+    arena_free(&other);
+
+    // Marker with a count beyond the region capacity
+    ArenaMarker forged = mark_r1;
+    forged.count = mark_r1.region->capacity + 1;
+    arena_rewind(&arena, forged);
+    assert(arena.tail->count == mark_r1.count);
+
+    arena_free(&arena);
+}
+
 static void test_massive_string_formatting(void)
 {
     printf("Running massive string formatting tests...\n");
@@ -204,6 +243,7 @@ int main(void)
     test_overflows();
     test_geometric_growth_and_stress();
     test_snapshot_rewind_trim_logic();
+    test_invalid_markers();
     test_massive_string_formatting();
     test_zero_capacity_fallback();