Cleanups

src/small_allocator.c

@@ -2,7 +2,18 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stddef.h>
-
+#include <assert.h>
+/**
+ * General purpose allocator.
+ * - Allocated pointers are aligned to 16 bytes.
+ * - An allocation "wastes" 32 bytes for housekeeping
+ * - Aallocation of > 0 bytes returns 16 bytes as a minimum
+ * - Allocation of 0 bytes returns a valid pointer with 0 usable space
+ *
+ * The allocator maintains a doubly linked list of blocks,
+ * when blocks are free'd, adjacent blocks are merged to keep
+ * fragmentation to a minimum
+ */
 #define ALIGNMENT 16
 #define ALIGN(n) (((n) + (ALIGNMENT - 1)) & ~(ALIGNMENT - 1))
 
@@ -12,8 +23,8 @@ typedef struct BlockHeader {
     size_t size_and_flags;       // size | free_bit
     struct BlockHeader* next;
     struct BlockHeader* prev;
-    // Padding to ensure we always return 16 byte aligned ponters
-    // remove padding & set ALIGNMENT to 8 to get 8 byte alignmet
+    // Padding to ensure we always return 16 byte aligned pointers.
+    // Remove padding & set ALIGNMENT to 8 to get 8 byte alignmet
     // (for 64 bit systems, 32 bit systems might need other adjustments)
     uint8_t pad[32 - (sizeof(size_t) + 2*sizeof(void*))];
 } BlockHeader;
@@ -32,30 +43,37 @@ typedef struct {
 
 /* --- Header helpers --- */
 
-static inline size_t block_size(BlockHeader* b) {
+static inline size_t block_size(BlockHeader* b)
+{
     return b->size_and_flags & ~BLOCK_FREE;
 }
 
-static inline int block_is_free(BlockHeader* b) {
+static inline int block_is_free(BlockHeader* b)
+{
     return (b->size_and_flags & BLOCK_FREE) != 0;
 }
 
-static inline void block_set_free(BlockHeader* b) {
+static inline void block_set_free(BlockHeader* b)
+{
     b->size_and_flags |= BLOCK_FREE;
 }
 
-static inline void block_set_used(BlockHeader* b) {
+static inline void block_set_used(BlockHeader* b)
+{
     b->size_and_flags &= ~BLOCK_FREE;
 }
 
-static inline void block_set_size(BlockHeader* b, size_t size) {
+static inline void block_set_size(BlockHeader* b, size_t size)
+{
     size_t flags = b->size_and_flags & BLOCK_FREE;
     b->size_and_flags = size | flags;
 }
 
-/* --- Allocator initialization --- */
+void small_allocator_init(SmallAllocator* a, void* backing_store, size_t size)
+{
+    // backing store must be aligned, otherwise  all alignment guarantees are lies
+    assert(((uintptr_t)backing_store & (ALIGNMENT - 1)) == 0);
 
-void small_allocator_init(SmallAllocator* a, void* backing_store, size_t size) {
     a->base = (uint8_t*)backing_store;
     a->size = size;
 
@@ -70,9 +88,8 @@ void small_allocator_init(SmallAllocator* a, void* backing_store, size_t size) {
     a->last_alloc = first;
 }
 
-/* Splitting */
-
-static void split_block(BlockHeader* block, size_t size) {
+static void split_block(BlockHeader* block, size_t size)
+{
     size = ALIGN(size);
     size_t bsize = block_size(block);
 
@@ -98,9 +115,8 @@ static void split_block(BlockHeader* block, size_t size) {
     block->next = new_block;
 }
 
-/* Coalescing */
-
-static void merge_with_next(BlockHeader* block) {
+static void merge_with_next(BlockHeader* block)
+{
     BlockHeader* next = block->next;
     if (!next || !block_is_free(next))
         return;
@@ -116,12 +132,8 @@ static void merge_with_next(BlockHeader* block) {
         next->next->prev = block;
 }
 
-/* Allocation (next-fit) */
-
-void* small_alloc(SmallAllocator* a, size_t size) {
-    if (size == 0)
-        return NULL;
-
+void* small_alloc(SmallAllocator* a, size_t size)
+{
     size = ALIGN(size);
 
     BlockHeader* start = a->last_alloc ? a->last_alloc : a->heap_start;
@@ -142,15 +154,20 @@ void* small_alloc(SmallAllocator* a, size_t size) {
 
 /* Free */
 
-void small_free(SmallAllocator* a, void* ptr) {
-    if (!ptr)
+void small_free(SmallAllocator* a, void* ptr)
+{
+    if (!ptr) {
         return;
+    }
 
-    BlockHeader* block =
-        (BlockHeader*)((uint8_t*)ptr - sizeof(BlockHeader));
+
+    BlockHeader* block = (BlockHeader*)((uint8_t*)ptr - sizeof(BlockHeader));
+
+    assert((uint8_t*)ptr >= a->base && (uint8_t*)ptr < a->base + a->size);
+    // double free detection
+    assert(!block_is_free(block));
 
     block_set_free(block);
-
     merge_with_next(block);
 
     if (block->prev && block_is_free(block->prev)) {