ENEE150 – 0102 ANDREW GOFFIN Dynamic Memory

Dynamic vs Static Allocation Dynamic  On the heap  Amount of memory chosen at runtime  Can change allocated memory during runtime Goffin – ENEE150 Static  On the stack  Amount of memory chosen at compiletime  Memory is allocated for entirety of runtime

Dynamic Memory Motivation More optimized data structures Optimal memory usage More powerful data structures overall  Linked lists, binary search trees, etc. Goffin – ENEE150

Dynamic Allocation A few possible functions to allocate memory Three main ones: void *malloc(int)  most important!!! void *calloc(int, int) void *realloc(void *, int) These functions are all in stdlib.h Goffin – ENEE150

Malloc Parameter determines how many contiguous bytes of memory to allocate  % of time you use sizeof( ) Good to explicitly cast output of malloc, but not necessary Example: int *x = (int *)malloc(sizeof(int));  Allocates one “integer size” of memory and stores the address of the first byte to the pointer x  Explicit cast with (int *)  If you want an array, multiply sizeof(int) by array size Goffin – ENEE150

Notes about Malloc When there is no memory, malloc will return a NULL pointer  ALWAYS check for this, otherwise program will go haywire Should use free() for all dynamically allocated memory Goffin – ENEE150

Calloc and Realloc Calloc assumes you want an array and takes two arguments: size of one element and number of elements int *a = (int *)calloc(sizeof(int), 5); // allocates memory for int array of size 5 Realloc takes dynamically allocated memory and changes how much of it there is  Good for increasing/decreasing length of arrays, for example int *b = (int *)realloc(a, 3); // returns pointer to a but with int array of only size 3 Goffin – ENEE150

Intro to Linked Lists Essentially a more dynamic array! Goffin – ENEE150

Some Linked List Vocab Node  Each element in the list: stores data and a “next pointer” Head pointer  Points to first element in the linked list  Is NULL for an empty list Next pointer  Pointer in each node that indicates the next node in the list  Is NULL for the last element in the list Tail pointer (if applicable)  Points to last element in the linked list  Is NULL for an empty list Goffin – ENEE150

Defining a linked list Use a struct to define a single node in the list typedef struct node_type{ // type of data depends on application! int data1; float data2; // NEXT POINTER, SUPER IMPORTANT struct node_type *next; } node, *pnode; Goffin – ENEE150

Starting a linked list int main(void){ pnode head = NULL; head = (pnode)malloc(sizeof(node)); head->next = NULL; } A single node is allocated Next pointer is NULL, indicating that the node is the last (and only) node in the list Goffin – ENEE150