malloc: defined set_footer, get_footer, init_gc_chunk find_free_chunk and merge_chunk

include/gc_malloc.h

@@ -2,11 +2,13 @@
 #define GC_MALLOC_H
 
 #include <stdint.h>
+#include <stdio.h>
 #include <unistd.h>
 
 // Generally, inline functions are preferable to macros resembling functions.
 // https://www.kernel.org/doc/html/v4.10/process/coding-style.html#data-structures
 
+#define ALIGN 8 /* Number of bytes to which chunk size is aligned */
 #define OVERHEAD 1 /* Number of 8-byte segments for chunk size & flags */
 #define MAGIC 2 /* Number of 8-byte segments from top of chunk to fields */
 #define MIN 3 /* Minimum number of 8-byte segments for data in a chunk */
@@ -14,34 +16,47 @@
 /*
  * Metadata and user data for allocated memory
  *
- * footer: user data for previous chunk
+ * footer: size of previous block or user data of previous block if allocated
  * size: size of chunk in bytes
- * merge: flag to identify when the chunk can be coalesced
+ * merge: flag to identify when the chunk can be coalesced,
- * free: flag to identify when the chunk is free
+ *        possible values 0 (false, don't merge) or non-zero (true, do merge)
+ * free: flag to identify when the chunk is free,
+ *       possible values 0 (false, allocated) or non-zero (true, chunk is free)
  * next: pointer to next chunk in the free list
  * prev: pointer to previous chunk in the free list
  */
 struct gc_chunk {
 	uint64_t footer;
 	uint32_t size;
-	uint16_t merge;
+	uint16_t merge; /* 1 = do merge, initialize to 0 */
-	uint16_t free;
+	uint16_t free; /* 0 = allocated, initialize to 1 */
 	struct gc_chunk *next;
 	struct gc_chunk *prev;
 };
 
 // Gets a pointer to the next chunk
-struct gc_chunk *get_next_chunk(struct gc_chunk *curr);
+struct gc_chunk *find_next_chunk(struct gc_chunk *curr);
 
 // Gets a pointer to the previous chunk, ensuring that current can be coalesced
-struct gc_chunk *get_prev_chunk(struct gc_chunk *curr);
+struct gc_chunk *find_prev_chunk(struct gc_chunk *curr);
 
-// Updates chunk footer information
+// Updates chunk footer information (remember that the current chunk's footer
+// data is actually part of the next chunk)
 void set_footer(struct gc_chunk *curr);
 
-// Gets footer information for the current chunk
+// Gets footer information for the current chunk - useful for debugging
 int get_footer(struct gc_chunk *curr);
 
+// Creates a new chunk with sentinel
+struct gc_chunk *init_gc_chunk(void);
+
+// Gets a pointer to a suitable chunk from the free list, otherwise creates a
+// new chunk and returns it's pointer
+struct gc_chunk *find_free_chunk(size_t size);
+
+// Coalesces chunks when merge flag is set
+void merge_chunk(struct gc_chunk *curr);
+
 // Performs allocations of SIZE bytes
 void *gc_malloc(size_t size);
 

src/gc_malloc.c

@@ -1,9 +1,12 @@
 #include <assert.h>
 #include <stdint.h>
 #include <sys/mman.h>
+#include <unistd.h>
 
 #include "gc_malloc.h"
 
+static struct gc_chunk *freelist = NULL;
+
 // Hides chunk header
 static inline void *hide(struct gc_chunk *curr)
 {
@@ -17,26 +20,131 @@ static inline struct gc_chunk *magic(void *mem)
 }
 
 // Aligns the amount of memory requested by the user with chunk size
-static inline size_t align(size_t size)
+/*
+ * Example: User requests 1 byte
+ * LHS
+ *   requested size              = 0b00000001
+ *   alignment - 1               = 0b00000111
+ *     => size + (alignment - 1) = 0b00001000
+ * RHS
+ *   ~(alignment - 1)            = 0b11111000
+ * LHS & RHS                     = 0b00001000
+ */
+// TODO: should probably be 16? would need to change chunk struct
+static inline uint32_t align(uint32_t size)
 {
-	const size_t alignment = 8;
+	size = (size + (ALIGN - 1)) & ~(ALIGN - 1);
-	size = (size + (alignment - 1)) & ~(alignment - 1);
 
+	// REVIEW: unnecessary with bit-wise math above?
 	if (size <= MIN)
 		return MIN;
 	else
 		return size;
 }
 
-struct gc_chunk *get_next_chunk(struct gc_chunk *curr)
+struct gc_chunk *find_next_chunk(struct gc_chunk *curr)
 {
 	unsigned size = curr->size;
 	return (struct gc_chunk *)((uint64_t *)curr + MAGIC + (size - 1));
 }
 
-struct gc_chunk *get_prev_chunk(struct gc_chunk *curr)
+struct gc_chunk *find_prev_chunk(struct gc_chunk *curr)
 {
 	assert(curr->merge);
 	unsigned size = curr->footer;
 	return (struct gc_chunk *)((uint64_t *)curr - (size - 1) - MAGIC);
 }
+
+void set_footer(struct gc_chunk *curr)
+{
+	struct gc_chunk *next = find_next_chunk(curr);
+	next->footer = curr->size;
+}
+
+int get_footer(struct gc_chunk *curr)
+{
+	struct gc_chunk *next = find_next_chunk(curr);
+	return next->footer;
+}
+
+struct gc_chunk *init_gc_chunk(void)
+{
+	const size_t page_size = getpagesize();
+	const size_t len = page_size * sizeof(uint64_t);
+	struct gc_chunk *chunk =
+		(struct gc_chunk *)mmap(NULL, len, PROT_READ | PROT_WRITE,
+					MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+	if (chunk == MAP_FAILED)
+		return NULL;
+
+	chunk->size = page_size - MAGIC - OVERHEAD;
+	chunk->merge = 0;
+	chunk->free = 1;
+	chunk->next = NULL;
+	chunk->prev = NULL;
+
+	struct gc_chunk *sentinel = find_next_chunk(chunk);
+	sentinel->footer = chunk->size;
+	sentinel->size = 0;
+	sentinel-> merge = 1;
+	sentinel->free = 1;
+	sentinel->next = NULL;
+	sentinel->prev = chunk;
+
+	return chunk;
+}
+
+struct gc_chunk *find_free_chunk(size_t size)
+{
+	struct gc_chunk *found;
+
+	while (1) {
+		found = freelist;
+		while (found != NULL) {
+			if (found->size >= size) {
+				// found, TODO: split when appropriate
+				break;
+			}
+			found = found->next;
+		}
+
+		if (found == NULL) {
+			found = init_gc_chunk();
+			if (found == NULL) {
+				return found;
+			}
+			freelist = found;
+			break;
+
+		} else {
+			break;
+		}
+	}
+
+	return found;
+}
+
+void merge_chunk(struct gc_chunk *curr)
+{
+	if (curr->merge) {
+		struct gc_chunk *prev = find_prev_chunk(curr);
+		struct gc_chunk *next = find_next_chunk(curr);
+		if (next->free) {
+			// TODO Case 1: merge prev and next
+
+		} else {
+			// TODO Case 2: merge with prev
+		}
+		set_footer(prev);
+	} else {
+		struct gc_chunk *next = find_next_chunk(curr);
+		if (next->free) {
+			// TODO Case 3: merge with after
+		} else {
+			// TODO Case 4: can't merge anything
+		}
+		curr->free = 1;
+		set_footer(curr);
+	}
+}