Updated free list interface

I realized my existing interface wouldn't work
if the coalescing or adding of a new node replaced
the head of the list, as there was no way to
communicate that back to the caller. As a result,
I've updated those interfaces to accept a pointer
to the free list head pointer. This will let them
change the free list head in alloc.c if necessary.

3 files changed, 39 insertions, 11 deletions

diff --git a/include/alloc_header.h b/include/alloc_header.h
new file mode 100644
index 0000000..bf4716c
--- /dev/null
+++ b/include/alloc_header.h
@@ -0,0 +1,27 @@
+/*
+ * include/alloc_header.h
+ *
+ * liballoc memory block header type.
+ * CISC 301 -- Operating Systems, Project 3
+ *
+ * Copyright (C) 2025  Douglas B. Rumbaugh <dbrumbaugh@harrisburgu.edu>
+ * 
+ * Distributed under the Modified BSD License
+ *
+ */
+#ifndef H_LIBALLOC_HEADER
+#define H_LIBALLOC_HEADER
+
+#include <stdlib.h>
+#include <assert.h>
+
+#include "constants.h"
+
+typedef struct header {
+  size_t size;
+  size_t magic_number;
+} header;
+
+static_assert(sizeof(header) % ALIGNMENT == 0, "Header improperly aligned");
+
+#endif
diff --git a/include/free_list.h b/include/free_list.h
index 887167b..1993aa7 100644
--- a/include/free_list.h
+++ b/include/free_list.h
@@ -15,6 +15,7 @@
 #include <stdlib.h>
 
 #include "constants.h"
+#include "alloc_header.h"
 
 typedef struct free_nd {
   size_t size;
@@ -34,9 +35,12 @@ free_nd *fl_find_first_fit(free_nd *free_list, size_t size);
  * specified node cannot be split (e.g., it's an exact match for size
  * within alignment restrictions), simply remove it from the list.
  *
+ * If the head of the free list is updated by this operation, the free
+ * list pointer passed as an argument will be updated to reflect this
+ *
  * Returns a pointer to the region of memory removed from the list.
  */
-void *fl_split_node(free_nd *free_list, free_nd *nd, size_t size);
+void *fl_split_node(free_nd **free_list, free_nd *nd, size_t size);
 
 /*
  * Scans the free list for adjacent nodes and merges them together
@@ -46,7 +50,10 @@ void fl_coalesce_nodes(free_nd *free_list);
 /*
  * Scans the free list for the correct location for the specified
  * pointer, and link it into the list at that point.
+ *
+ * If the head of the free list is updated by this operation, the free
+ * list pointer passed as an argument will be updated to reflect this
  */
-void fl_add_node(free_nd *free_list, void *ptr);
+void fl_add_node(free_nd **free_list, void *ptr);
 
 #endif
diff --git a/src/alloc.c b/src/alloc.c
index c13b1e8..830e49f 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -12,13 +12,7 @@
 #include "alloc.h"
 #include "constants.h"
 #include "free_list.h"
-
-typedef struct header {
-  size_t size;
-  size_t magic_number;
-} header;
-
-static_assert(sizeof(header) % ALIGNMENT == 0, "Header improperly aligned");
+#include "alloc_header.h"
 
 static void *heap_start = 0;
 static void *heap_end = 0;
@@ -59,7 +53,7 @@ void *allocate(size_t size) {
   /* first check for a suitable memory block on the free list */
   free_nd *nd = fl_find_first_fit(free_list, size);
   if (nd) {
-    void *return_region = fl_split_node(free_list, nd, size);
+    void *return_region = fl_split_node(&free_list, nd, size);
   }
 
   /*
@@ -82,6 +76,6 @@ void *allocate(size_t size) {
 }
 
 void release(void *ptr) {
-  fl_add_node(free_list, ptr);
+  fl_add_node(&free_list, ptr);
   fl_coalesce_nodes(free_list);
 }