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Author: LPJE

old_malloc.c

@@ -0,0 +1,174 @@
+#include<stdio.h>
+
+#define MEMSIZE 1000
+
+struct test{
+
+  int a;
+
+  
+  int tf()
+  {
+
+    return a;
+
+  }
+  
+
+};
+
+
+
+
+
+
+
+char mem[MEMSIZE]; // 1000 bytes that currently only store character data types. 
+
+struct m_data{
+
+  struct m_data* next;
+  struct m_data* prev;
+  int isFree;
+  int size;
+};
+
+
+struct m_data* root = (struct m_data*)mem;  // it's more useful to make root global
+
+
+// it could well be that you reach the end of the linked list without ever finding a suitable fit
+
+
+// finds first occurrence of suitable slot
+// what is the termination condition if there is no suitable space available? If no suitable space is available then when will ultimately have reached the end of the queue. 
+
+struct m_data* find(int m_size)
+{
+ struct m_data*  p = root;
+
+ while(p->next !=NULL && (p->size <  (m_size + sizeof(struct m_data)) || !p->isFree))
+    {
+      p = p->next;
+
+    }
+ 
+    return p;
+
+}
+
+
+// still confused about typecasting pointers into void pointer; what is allowable to return in case of function with void pointer.
+
+void* mymalloc(int m_size)
+//void* mymalloc()
+{
+
+  //initialisation
+  static int init = 0;
+  // static  struct m_data* root;
+  struct m_data* q;
+ 
+
+  if(!init)
+    {
+      init = 1;
+      root = (struct m_data*)mem;
+      root -> isFree = 1;
+      root -> next = NULL;
+      root -> prev = NULL;
+      root -> size = MEMSIZE - sizeof(struct m_data);
+    }
+
+  q = find(m_size); // first suitable block
+ 
+  // set up new m_data block to point at new free space
+
+   struct m_data* newB = (struct m_data*)((char*)q + sizeof(struct m_data) + m_size); // need to type cast to char in order to use pointer arithmetic
+   newB -> next = q->next;;//?????
+   newB -> prev = q;
+   newB -> isFree = 1;
+   newB -> size = (q->size - m_size - sizeof(struct m_data));
+   printf("newB = %p\n", newB);
+
+
+
+  //  printf("%p\n", find(m_size));
+
+  q->isFree = 0;
+  q->size = m_size;
+  q->next = newB;
+
+
+  return (void*)(q+1);
+  
+}
+
+
+
+
+void myfree(void* ptr)
+{
+
+
+  struct m_data* t = (struct m_data*)ptr;
+  t->isFree = 1;
+
+
+
+  struct m_data* curr = root;
+
+  while(curr->next != NULL)
+    {
+
+
+      if(!(curr -> isFree)|| !(curr -> next -> isFree))
+	{
+	  curr = curr -> next;
+	}
+      else
+	{
+	  //merge
+
+	  curr-> size += curr->size + sizeof(struct m_data) + curr->next->size;
+	  curr -> next -> next -> prev = curr;
+	  curr -> next = curr->next-> next;
+
+	}
+
+
+
+    }
+
+
+
+
+}
+
+
+
+
+
+
+
+int main()
+{
+
+
+
+
+  printf("mymalloc() = %p\n ", mymalloc(128));
+  printf("mem[24]  %p\n", &mem[24]);
+  printf("mem[152] = %p\n", &mem[152]);
+  //  printf("mymalloc(32) again = %p\n", mymalloc(32));
+  int* p = mymalloc(32);
+
+
+  myfree(p);;
+
+
+
+
+  return 0;
+
+}