v1.1.3 - Align Arena_Region data flexible array member

arena.h

@@ -1,4 +1,4 @@
-// arena.h - v1.0.3 - MIT License - https://github.com/seajee/arena.h
+// arena.h - v1.1.3 - MIT License
 // single header library for region-based memory management.
 //
 // [License and changelog]
@@ -21,29 +21,33 @@
 //     Note: every compile-time option listed here should be configured before
 //           the #include "arena.h"
 //
-//     #define ARENA_NO_ASSERT
-//
-//         This macro disables assertions that happen when a reallocation
-//         fails. The disabled assertions will be replaced with a simple
-//         condition that returns NULL when it fails. Note that ARENA_ASSERT
-//         is only used in arena_alloc().
-//
 //     #define ARENA_REGION_CAPACITY new_region_capacity_in_bytes (4096)
 //
 //         This macro defines the default capacity for arena regions.
 //
+//     #define ARENA_ALIGNMENT alignment (8)
+//
+//         This macro defines the memory alignment used for allocations.
+//
+//     #define ARENA_NO_ASSERT
+//
+//         This macro disables assertions that happen when allocations fail.
+//         The disabled assertions will be replaced with a simple condition
+//         that returns NULL when it fails. This macro affects every function
+//         that returns a pointer.
+//
 //     #define ARENA_ASSERT my_assert
 //
-//         This macro defines an alternative function for assertions. In this
-//         library, ARENA_ASSERT is only used when arena_alloc() fails. Will
-//         be overwritten if ARENA_NO_ASSERT is defined.
+//         This macro defines an alternative function for assertions. This has
+//         no effect when ARENA_NO_ASSERT is defined.
 //
-//     #define ARENA_REALLOC my_realloc
+//     #define ARENA_MALLOC my_malloc
 //     #define ARENA_FREE my_free
 //
 //         These macros define alternative functions for dynamic allocation
-//         and deallocation on the heap. They are only used for managing
-//         Arena_Region structures.
+//         and deallocation. They are only used for managing Arena_Region
+//         structures. If redefined ensure that your functions consider memory
+//         alignment.
 //
 // [Function documentation]
 //
@@ -53,15 +57,9 @@
 //
 //     Arena arena_create(size_t region_capacity)
 //
-// This function initializes an arena with a specified region capacity. By
-// default the region capacity is ARENA_REGION_CAPACITY, which can be
-// configured by redefining it before including the header file. This function
-// is not strictly necessary for initializing an arena (see example).
-//
-//     void *arena_alloc(Arena *a, size_t size)
-//
-// This function allocates a buffer of <size> bytes into the specified arena
-// and returns it's pointer.
+// This function is optional (see [Example] section): initializes an arena
+// with a specified region capacity. It will override the default
+// ARENA_REGION_CAPACITY.
 //
 //     void arena_free(Arena *a)
 //
@@ -77,6 +75,25 @@
 // Warning: this functions may cause fragmentation, consider setting an
 // appropriate region capacity.
 //
+//     void *arena_alloc(Arena *a, size_t size)
+//
+// This function allocates a buffer of <size> bytes into the specified arena
+// and returns it's pointer.
+//
+//     void *arena_copy(Arena *a, const void *src, size_t size);
+//
+// This function allocates a buffer of <size> bytes into the arena and copies
+// the contents of <src>.
+//
+//     char *arena_strdup(Arena *a, const char *s);
+//
+// This functions acts like the strdup() function but it instead allocates on
+// the specified arena. It duplicates a string in the arena.
+//
+// [Notes]
+//
+// This library is not Thread-safe.
+//
 // [Example]
 //
 #if 0
@@ -99,13 +116,19 @@ int main(void)
 #ifndef ARENA_H_
 #define ARENA_H_
 
+#include <stdalign.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <string.h>
 
 #ifndef ARENA_REGION_CAPACITY
 #    define ARENA_REGION_CAPACITY (8*1024)
 #endif // ARENA_REGION_CAPACITY
 
+#ifndef ARENA_ALIGNMENT
+#    define ARENA_ALIGNMENT alignof(max_align_t)
+#endif // ARENA_ALIGNMENT
+
 #ifndef ARENA_NO_ASSERT
 #    ifndef ARENA_ASSERT
 #        include <assert.h>
@@ -115,10 +138,10 @@ int main(void)
 #    define ARENA_ASSERT(...) ((void)0)
 #endif // ARENA_NO_ASSERT
 
-#ifndef ARENA_REALLOC
+#ifndef ARENA_MALLOC
 #    include <stdlib.h>
-#    define ARENA_REALLOC realloc
-#endif // ARENA_REALLOC
+#    define ARENA_MALLOC malloc
+#endif // ARENA_MALLOC
 
 #ifndef ARENA_FREE
 #    include <stdlib.h>
@@ -135,7 +158,7 @@ struct Arena_Region {
     Arena_Region *next;
     size_t count;
     size_t capacity;
-    uint8_t data[];
+    alignas(ARENA_ALIGNMENT) uint8_t data[];
 };
 
 typedef struct Arena {
@@ -144,11 +167,16 @@ typedef struct Arena {
     size_t region_capacity;
 } Arena;
 
+// Arena management
 Arena arena_create(size_t region_capacity);
-void *arena_alloc(Arena *a, size_t size);
 void arena_free(Arena *a);
 void arena_reset(Arena *a);
 
+// Allocation other utilities
+void *arena_alloc(Arena *a, size_t size);
+void *arena_copy(Arena *a, const void *src, size_t size);
+char *arena_strdup(Arena *a, const char *s);
+
 #ifdef __cplusplus
 }
 #endif // __cplusplus
@@ -163,72 +191,12 @@ extern "C" { // Prevent name mangling of functions
 
 Arena arena_create(size_t region_capacity)
 {
-    Arena a = {0};
+    Arena a;
+    memset(&a, 0, sizeof(a));
     a.region_capacity = region_capacity;
     return a;
 }
 
-void *arena_alloc(Arena *a, size_t size)
-{
-    if (a == NULL || size == 0) {
-        ARENA_ASSERT(!"Invalid, parameters");
-        return NULL;
-    }
-
-    size_t region_capacity = (a->region_capacity == 0
-            ? ARENA_REGION_CAPACITY : a->region_capacity);
-
-    // Empty arena
-    if (a->head == NULL) {
-        size_t alloc_size = (size > region_capacity ? size : region_capacity);
-        a->head = (Arena_Region*)ARENA_REALLOC(
-                NULL, sizeof(*a->head) + alloc_size);
-
-        if (a->head == NULL) {
-            ARENA_ASSERT(!"Reallocation failed");
-            return NULL;
-        }
-
-        a->head->next = NULL;
-        a->head->count = size;
-        a->head->capacity = alloc_size;
-        a->tail = a->head;
-        return a->head->data;
-    }
-
-    // Not enough capacity
-    if (size > a->tail->capacity - a->tail->count) {
-        // Find first suitable region
-        while (a->tail->next != NULL
-                && size > a->tail->capacity - a->tail->count) {
-            a->tail = a->tail->next;
-        }
-
-        // If not found create a new region
-        if (size > a->tail->capacity - a->tail->count) {
-            size_t alloc_size =
-                (size > region_capacity ? size : region_capacity);
-
-            a->tail->next = (Arena_Region*)ARENA_REALLOC(
-                    NULL, sizeof(*a->tail) + alloc_size);
-
-            if (a->tail->next == NULL) {
-                ARENA_ASSERT(!"Reallocation failed");
-                return NULL;
-            }
-
-            a->tail = a->tail->next;
-            a->tail->next = NULL;
-            a->tail->count = size;
-            a->tail->capacity = alloc_size;
-            return a->tail->data;
-        }
-    }
-
-    a->tail->count += size;
-    return a->tail->data + a->tail->count - size;
-}
-
 void arena_free(Arena *a)
 {
     if (a == NULL) {
@@ -260,6 +228,104 @@ void arena_reset(Arena *a)
     a->tail = a->head;
 }
 
+void *arena_alloc(Arena *a, size_t size)
+{
+    if (a == NULL || size == 0) {
+        ARENA_ASSERT(false && "Invalid parameters");
+        return NULL;
+    }
+
+    // Ensure memory alignment of requested size
+    size = (size + (ARENA_ALIGNMENT - 1)) & ~(ARENA_ALIGNMENT - 1);
+
+    // Check for overflow
+    if (size > SIZE_MAX - sizeof(Arena_Region)) {
+        ARENA_ASSERT(false && "Requested size too large");
+        return NULL;
+    }
+
+    size_t region_capacity = (a->region_capacity == 0
+                                ? ARENA_REGION_CAPACITY : a->region_capacity);
+
+    // Calculate and check region allocation size
+    size_t alloc_size = (size > region_capacity ? size : region_capacity);
+    if (alloc_size > SIZE_MAX - sizeof(Arena_Region)) {
+        ARENA_ASSERT(false && "Region size overflow");
+        return NULL;
+    }
+
+    // Empty arena: allocate first region
+    if (a->head == NULL) {
+        a->head = (Arena_Region*)ARENA_MALLOC(sizeof(*a->head) + alloc_size);
+        if (a->head == NULL) {
+            ARENA_ASSERT(false && "Allocation failed");
+            return NULL;
+        }
+
+        a->head->next = NULL;
+        a->head->count = size;
+        a->head->capacity = alloc_size;
+        a->tail = a->head;
+        return a->head->data;
+    }
+
+    // Find first suitable region
+    Arena_Region *r = a->tail;
+    while (r != NULL && size > r->capacity - r->count) {
+        r = r->next;
+    }
+
+    // If found allocate on it
+    if (r != NULL) {
+        if (r->count > SIZE_MAX - size) {
+            ARENA_ASSERT(false && "Region count overflow");
+            return NULL;
+        }
+        r->count += size;
+        return r->data + r->count - size;
+    }
+
+    // If not found append a new region
+    a->tail->next = (Arena_Region*)ARENA_MALLOC(sizeof(*a->tail) + alloc_size);
+    if (a->tail->next == NULL) {
+        ARENA_ASSERT(false && "Allocation failed");
+        return NULL;
+    }
+
+    a->tail = a->tail->next;
+    a->tail->next = NULL;
+    a->tail->count = size;
+    a->tail->capacity = alloc_size;
+    return a->tail->data;
+}
+
+void *arena_copy(Arena *a, const void *src, size_t size)
+{
+    if (a == NULL || src == NULL) {
+        ARENA_ASSERT(false && "Invalid parameters");
+        return NULL;
+    }
+
+    void *copy = arena_alloc(a, size);
+    if (copy == NULL) {
+        return NULL;
+    }
+
+    memcpy(copy, src, size);
+
+    return copy;
+}
+
+char *arena_strdup(Arena *a, const char *s)
+{
+    if (a == NULL || s == NULL) {
+        ARENA_ASSERT(false && "Invalid parameters");
+        return NULL;
+    }
+
+    return (char*)arena_copy(a, s, strlen(s) + 1);
+}
+
 #ifdef __cplusplus
 }
 #endif // __cplusplus
@@ -269,6 +335,10 @@ void arena_reset(Arena *a)
 /*
  * Revision history:
  *
+ *     1.1.3 (2026-01-14) Align Arena_Region data flexible array member
+ *     1.1.2 (2026-01-10) Minor changes; check result of alloc in arena_copy()
+ *     1.1.1 (2025-11-18) Implement memory alignment; improve docs
+ *     1.1.0 (2025-11-18) New helper functions: arena_copy(), arena_strdup()
  *     1.0.3 (2025-09-10) Renamed parameter bytes to size in arena_alloc()
  *     1.0.2 (2025-09-06) Bug fixes; new ARENA_NO_ASSERT macro
  *     1.0.1 (2025-08-02) Prevent name mangling of functions; don't reset

test.c

@@ -2,8 +2,8 @@
 #include <stdlib.h>
 
 #ifdef DEBUG
-#define realloc(p, s) (printf("%s:%d:%s: realloc(%p, %lu)\n",\
-            __FILE__, __LINE__, __func__, (p), (s)), realloc((p), (s)));
+#define malloc(s) (printf("%s:%d:%s: malloc(%lu)\n",\
+            __FILE__, __LINE__, __func__, (s)), malloc((s)));
 #define free(p)   (printf("%s:%d:%s: free(%p)\n",\
             __FILE__, __LINE__, __func__, (p)), free((p)));
 #endif // DEBUG
@@ -25,34 +25,28 @@ void arena_print(Arena arena)
 
 int main(void)
 {
+    printf("struct Arena_Region {\n");
+    printf("    Arena_Region *next; (%zu)\n", offsetof(struct Arena_Region, next));
+    printf("    size_t count;       (%zu)\n", offsetof(struct Arena_Region, count));
+    printf("    size_t capacity;    (%zu)\n", offsetof(struct Arena_Region, capacity));
+    printf("    uint8_t data[];     (%zu)\n", offsetof(struct Arena_Region, data));
+    printf("};\n");
+
+    printf("Alignment: %zu\n", ARENA_ALIGNMENT);
+    printf("================================================================\n");
+
     Arena a = {0};
 
-    printf("------ small alloc ------\n");
-
+    arena_alloc(&a, 4012);
     arena_alloc(&a, 100);
-    arena_alloc(&a, 100);
-    arena_alloc(&a, 100);
-    arena_alloc(&a, 100);
-    arena_print(a);
-
-    printf("\n=========================================================\n\n");
-    printf("------ big alloc ------\n");
-
-    arena_alloc(&a, 8000);
-    arena_print(a);
-
-    printf("\n=========================================================\n\n");
-    printf("------ reset ------\n");
+    arena_alloc(&a, 2301);
+    arena_alloc(&a, 4015);
+    arena_alloc(&a, 10000);
 
     arena_reset(&a);
-    arena_print(a);
 
-    printf("\n=========================================================\n\n");
-    printf("------ allocs after reset ------\n");
+    arena_alloc(&a, 1000);
 
-    arena_alloc(&a, 300);
-    arena_alloc(&a, 400);
-    arena_alloc(&a, 9000);
     arena_print(a);
 
     arena_free(&a);