main.c

@@ -1,15 +1,156 @@
 #include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
 
-void* malloc(unsigned long size)
+#define ALIGN4(s) (((((s)-1)>>2)<<2)+4)
+#define BLOCK_SIZE sizeof(struct block)
+#define MINIMUM_BLOCK_SIZE  16
+
+/// The memory block's header.
+struct block
 {
-	return NULL;
+	size_t size;
+	struct block* prev;
+	struct block* next;
+	int free;
+};
+
+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* b = sbrk(0);
+
+    // Ensure the allocated size is at least the minimum block size
+    if (s < MINIMUM_BLOCK_SIZE) {
+        s = MINIMUM_BLOCK_SIZE;
+    }
+
+    if (sbrk(BLOCK_SIZE + s) == (void*)-1)
+        return NULL;
+
+    b->size = s;
+    b->prev = last;
+    b->next = NULL;
+    b->free =  0;
+
+    if (last)
+        last->next = b;
+
+    last = b;
+
+    return b;
 }
 
+/// 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* current = first;
+	while (current && (!current->free || current->size < s))
+		current = current->next;
+
+	return current;
+}
+
+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;
+
+	in->size -= totalSize;
+	in->next = b;
+
+	return b;
+}
+
+/// 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)
+{
+    struct block* b;
+
+    size_t alignedSize = ALIGN4(size);
+
+    // Enforce the minimum block size
+    if (alignedSize < MINIMUM_BLOCK_SIZE) {
+        alignedSize = MINIMUM_BLOCK_SIZE;
+    }
+
+    if (first)
+    {
+        b = find_first(alignedSize);
+        if (!b)
+            b = extend_heap(alignedSize);
+        else if (b->size > BLOCK_SIZE + alignedSize)
+            b = fragment_block(b, alignedSize);
+    }
+    else
+    {
+        b = extend_heap(alignedSize);
+        if (!b)
+            return NULL;
+
+        first = b;
+    }
+
+    return b +  1;
+}
+
+
+/// 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)
 {
+	if (!ptr)
+		return;
+
+	struct block* b = (struct block*)ptr - 1;
+	if (b->free)
+		return;
+
+	b->free = 1;
+
+	while (b->prev && b->prev->free)
+		b = b->prev;
+
+	while (b->next && b->next->free)
+	{
+		b->size += BLOCK_SIZE + b->next->size;
+		b->next = b->next->next;
+	}
+
+	if (!b->next)
+		brk(b);
 }
 
-int main(void)
+int main()
 {
+	int* a = (int*)malloc(sizeof(int));
+	int* b = (int*)malloc(sizeof(int));
+
+	*a = 5;
+	*b = 12;
+
+	printf("Test 1: %i\n", *a);
+	printf("Test 2: %i\n", *b);
+
+	free(a);
+	free(b);
+
+	int* c = (int*)malloc(sizeof(int));
+
 	return 0;
 }