main.c

@@ -1,36 +1,39 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <unistd.h>
-#include <sys/syscall.h>
+#include <stdlib.h>
 
-#define ALIGN4(s) (((((s)-1)>>2)<<2)+4)
+#define ALIGN16(s) (((s) + 15) & ~0x0F)
 #define BLOCK_SIZE sizeof(struct block)
-#define MINIMUM_BLOCK_SIZE  4
+#define MINIMUM_BLOCK_SIZE (sizeof(struct block) + 16)
 
 /// The memory block's header.
 struct block
 {
 	size_t size;
-	struct block* prev;
-	struct block* next;
+	struct block *prev;
+	struct block *next;
 	int free;
 };
 
-struct block* first = NULL;
-struct block* last = NULL;
+struct block *first = NULL;
+struct block *last = NULL;
 
 /// Extends heap memory upwards, towards zero.
 /// @param [in] s The size of the memory needed aligned by 4 bytes.
 /// @returns The new memory block.
-struct block* extend_heap(size_t s)
+struct block *extend_heap(size_t s)
 {
 	// Ensure the allocated size is at least the minimum block size
 	if (s < MINIMUM_BLOCK_SIZE)
 		s = MINIMUM_BLOCK_SIZE;
 
-	struct block* b = (struct block*)syscall(SYS_brk, NULL);
+	struct block *b = (struct block *)sbrk(0); // Get the current break
 
-	if ((void*)syscall(SYS_brk, (char*)(b + 1) + s) == (void*)-1)
+	// Add the size of the block header to the requested size
+	s += BLOCK_SIZE;
+
+	if (sbrk(s) == (void *)-1) // Extend the break by s bytes
 		return NULL;
 
 	b->size = s;
@@ -49,9 +52,9 @@ struct block* extend_heap(size_t s)
 /// Finds the first block that will fit the given size.
 /// @param [in] s The 4 byte aligned size to look for.
 /// @returns The matching available memory block.
-struct block* find_first(size_t s)
+struct block *find_first(size_t s)
 {
-	struct block* current = first;
+	struct block *current = first;
 	while (current && (!current->free || current->size < s))
 		current = current->next;
 
@@ -62,112 +65,204 @@ struct block* find_first(size_t s)
 /// @param [in] in The memory block to fragment.
 /// @param [in] s The size of the new memory block.
 /// @returns The new memory block.
-struct block* fragment_block(struct block* in, size_t s)
+struct block *fragment_block(struct block *in, size_t s)
 {
-	size_t totalSize = BLOCK_SIZE + s;
-	struct block* b = (struct block*)((char*)(in + 1) + (in->size - totalSize));
-	b->size = s;
-	b->prev = in;
-	b->next = in->next;
-	b->free = 0;
+	// Calculate the size of the new block, including the block header
+	size_t newBlockSize = s + BLOCK_SIZE;
 
-	in->size -= totalSize;
-	in->next = b;
+	// Check if the current block can be split
+	if (in->size <= newBlockSize)
+	{
+		// Cannot split, return the original block
+		return in;
+	}
 
-	return b;
+	// Calculate the size of the remainder block
+	size_t remainderSize = in->size - newBlockSize;
+
+	// Create the new block in the remainder space
+	struct block *newBlock = (struct block *)((char *)(in + 1) + s);
+	newBlock->size = remainderSize;
+	newBlock->prev = in;
+	newBlock->next = in->next;
+	newBlock->free = 1; // Set the new block as free
+
+	// Update the current block to reflect the reduced size
+	in->size = newBlockSize;
+	in->next = newBlock;
+
+	// Insert the new block into the linked list of blocks
+	if (newBlock->next)
+	{
+		newBlock->next->prev = newBlock;
+	}
+
+	return newBlock;
+}
+
+struct block *find_best_fit(size_t s)
+{
+	struct block *current = first;
+	struct block *best_fit = NULL;
+	while (current)
+	{
+		if (current->free && current->size >= s)
+	{
+			if (best_fit && current->size < best_fit->size) // Check for NULL before comparison
+		{
+			{
+				best_fit = current;
+			}
+		}
+		current = current->next;
+	}
+	return best_fit;
+	}
 }
 
 /// Will find or allocate a memory block.
 /// @param [in] size The size of the memory block to request.
 /// @returns The requested memory on the heap.
 /// @todo Fragmenting functionality.
-void* malloc(size_t size)
+void *malloc(size_t size)
 {
-	struct block* b;
+	if (size == 0)
+	{
+		return NULL;
+	}
 
-	size = ALIGN4(size);
+	size = ALIGN16(size);				   // First align the requested size
+	size_t total_size = size + BLOCK_SIZE; // Then add the size of the block header
+
+	struct block *b;
 
 	if (first)
 	{
-		b = find_first(size);
+		b = find_best_fit(size);
 		if (!b)
-			b = extend_heap(size);
-		else if (b->size > BLOCK_SIZE + size)
+		{
+			b = extend_heap(total_size);
+			if (!b)
+			{
+				return NULL; // Check if heap extension failed
+			}
+		}
+		else if (b->size > total_size + MINIMUM_BLOCK_SIZE)
+		{
 			b = fragment_block(b, size);
 		}
+	}
 	else
 	{
-		b = extend_heap(size);
+		b = extend_heap(total_size);
 		if (!b)
+		{
 			return NULL;
-
+		}
 		first = b;
 	}
 
-	return b + 1;
+	b->free = 0;				   // Mark the block as used
+	return (char *)b + BLOCK_SIZE; // Return a pointer to the usable memory
 }
 
-void* realloc(void* ptr, size_t new_size)
+void *realloc(void *ptr, size_t new_size)
 {
 	if (!ptr)
+	{
 		return malloc(new_size);
+	}
 
-	if (!new_size)
+	if (new_size == 0)
+	{
+		free(ptr);
 		return NULL;
+	}
 
-	struct block* b = (struct block*)ptr - 1;
-	if (b->free)
-		return NULL;
+	struct block *b = (struct block *)((char *)ptr - BLOCK_SIZE);
+	if (b->size >= new_size + BLOCK_SIZE)
+	{
+		return ptr; // The block is already big enough
+	}
 
-	if (b->size == new_size)
-		return ptr;
-	else if (b->size > BLOCK_SIZE + new_size)
-		return fragment_block(b, new_size) + 1;
+	void *new_ptr = malloc(new_size);
+	if (!new_ptr)
+	{
+		return NULL; // Allocation failed
+	}
+	memcpy(new_ptr, ptr, b->size - BLOCK_SIZE); // Copy old data to new block, excluding the header size
+	free(ptr);									// Free the old block
 
-	return NULL;
+	return new_ptr;
 }
 
 /// Will flag the provided memory as free and will defragment other blocks adjacent to it.
 /// @param [in] ptr The memory to flag as free.
 /// @note If all data after the provided memory is free, it will reduce the heap size.
-void free(void* ptr)
+void my_custom_free(void *ptr)
 {
     if (!ptr)
+    {
         return;
+    }
 
-	struct block* b = (struct block*)ptr - 1;
+    struct block *b = (struct block *)((char *)ptr - BLOCK_SIZE);
     if (b->free)
-		return;
+    {
+        fprintf(stderr, "Double free detected at block %p.\n", ptr);
+        abort(); // Terminate the program immediately due to serious error
+    }
 
     b->free =  1;
 
+    // Coalesce free blocks
     while (b->prev && b->prev->free)
+    {
+        // Merge with previous block
+        b->prev->size += BLOCK_SIZE + b->size;
+        b->prev->next = b->next;
         b = b->prev;
+    }
 
-	while (b->next && b->next->free)
+    // If there is a next block and it's free, merge with it
+    if (b->next && b->next->free)
     {
         b->size += BLOCK_SIZE + b->next->size;
         b->next = b->next->next;
+        if (b->next)
+        {
+            b->next->prev = b;
+        }
     }
 
+    // After merging, update the 'last' pointer if necessary
     if (!b->next)
-		syscall(SYS_brk, b);
-}
+    {
+        last = b;
+    }
 
-/// Retrieves the total heap usage in bytes.
-/// @returns Usage in bytes.
-size_t get_heap_size()
-{
-	if (!first || !last)
-		return 0;
-
-	return ((char*)last + last->size) - (char*)first;
+    // Check if we can shrink the heap
+    if (b == last)
+    {
+        // Update 'last' to the previous block or NULL if there's no previous block
+        last = b->prev;
+        if (last)
+        {
+            last->next = NULL;
+        }
+        else
+        {
+            first = NULL;
+        }
+        // Reduce the program break to release the memory
+        sbrk(0 - (b->size + BLOCK_SIZE));
+    }
 }
 
 int main()
 {
-	int* a = (int*)malloc(1);
-	int* b = (int*)malloc(1);
+	int *a = (int *)malloc(sizeof(int));
+	int *b = (int *)malloc(sizeof(int));
 
 	*a = 5;
 	*b = 12;
@@ -179,7 +274,7 @@ int main()
 	free(a);
 	free(b);
 
-	int* c = (int*)malloc(sizeof(int));
+	int *c = (int *)malloc(sizeof(int));
 
 	return 0;
 }