dynamically allocated memory string cgs 3460, lecture 33 apr 3, 2006 hen-i yang

Dynamically Allocated Memory String

CGS 3460, Lecture 33Apr 3, 2006Hen-I Yang

Previously… Dynamic Memory Allocation Dynamic Allocated Arrays

Agenda

Dynamic Allocated Arrays Dynamic Allocated Strings Deallocation Strings

Malloc an Array

int *a;a = malloc(n * sizeof(int)); Can we just say a = malloc(n* 4)?

Not a good idea After malloc, you have all the space neede

d for the array allocated After that, you can use a[i] or *(a+i) to acc

ess the elements

Calloc an Array

int *a;A = calloc(n, sizeof(int)); Why do we use calloc? All elements are i

nitialized to 0 at the time of allocation

Realloc an Array

int *a;a = malloc(n * sizeof(int));a = realloc(a, m * sizeof(int)); Why do we use realloc? So we can chang

e its size after we malloc or calloc If first argument is null, realloc = malloc If second argument is zero, the memory is

freed

Strings

In C, strings are equivalent to a collection (array) of characters

How do we setup a string? char quarterback[] = “Palmer”; 7 bytes are allocate for variable name. What if Mannings replaced Palmer as QB? char director[9]; Let’s say, a guy named Roethlisberger comes along. char * quarterback;

quarterback = (char *) malloc (15);

How do we use malloc with strings?

char * temp_buffer = (char *) malloc(20); char * name; char * title; scanf(“%s”, temp_buffer); name = (char *) malloc(strlen(temp_buffer)); strcpy(name, temp_buffer); strcpy(temp_buffer, “□ □ □ □…□”);

□ x 20 scanf(“%s”, temp_buffer); title = (char *) malloc(strlen(temp_buffer)); strcpy(title, temp_buffer);

Array of strings

If there are multiple strings, we can use a 2 dimensional array to store it.

But it waste too much space, instead we store an array of pointers to strings

char * strings[NUMBER_OF_STRINGS]; strings[0], strings[1], …

Recycle

Memory leak Freep = malloc(…);free(p);

Calling free to a memory location, not previously allocated dynamically (e.g. an element of array, a variable) can have unpredictable effect

Dangling pointer: refer to the pointer pointing to an address that is already freed

Some programmers put p = null; whenever they free the memory p points to.

Strings

String: an array of characters String constant (String literals) String variable

Summary

Dynamic Allocated Arrays Dynamic Allocated Strings Deallocation Strings

Before you go

Nothing for today. Enjoy your plan for tonight.