LECTURE 29:DYNAMIC MEMORY ALLOCATION

PHY 107 – Programming For Science

Today’s Goal

Learn how arrays normally used in real programs Why a function returning an array causes

bugs How to declare array if size varies with

each run Using (& fixing problems) with malloc & free

All doing great, but pointers still hard

Arrays vs. Pointers

Arrays = Yams Pointers = Sweet Potatoes

Makes space at declaration

Variable value is address

Use [] to access entries Can also access using *

Can be assigned a pointer

Needs target to be useful

Variable value is address

Use * to access target Can also access using []

Can be assigned arrayOften use pointers & arrays interchangeably

Use Like Array Use Like Pointer

Arrays + Pointers =

char array[100];char * ptr = array;*array = ‘E’;*(ptr + 1) = ‘q’;*(ptr + 2) = ‘u’;*(array + 3) = ‘a’;*(ptr + 4) = ‘l’;*(array + 5) = ‘\0’;

char array[100];char * ptr = array;array[0] = ‘E’;ptr[1] = ‘q’;ptr[2] = ‘u’;array[3] = ‘a’;ptr[4] = ‘l’;array[5] = ‘\0’;

Pointers Can Be NULL

If pointer has no target, need a special value Not an actual address – NULL shows lack of

address No relation to null character – names are

coincidence If pointer is NULL, accessing value

causes crash Check for NULL before using unknown

pointer Check using equality: if (pointer != NULL)

To use NULL, several rules must be followed Cannot use null, Null, or nuLL; name must

be in CAPS #include <stdio.h> needed at program

start

Starting a New Company

Developing software for your new company Website selling engineering gear of all

possible types Plans to grow site to offer consulting &

other services Being as big as Amazon.com is your

eventual goal But, for the moment, your company is very

small

Developing the Software

Want to write code that scales with company Read inventory from user to make updates

easy By writing software in C/C++, can port to

new machines No hard limits (if possible) since guesses will

be wrong

First Attempt At Writing Code Use 2 arrays: one for names and one

for costs Work in parallel, so item data at same

index in arrays Number of items entered 1st to size

declared arrays Know that in the future will not want to

type Databases can be faster, but also more

expensive Put initial I/O in function so easy to update

later

Code is simple & easy; what could go wrong?

First Attempt At Writing Code Use 2 arrays: one for names and one

for costs Work in parallel, so item data at same

index in arrays Number of items entered 1st to size

declared arrays Know that in the future will not want to

type Databases can be faster, but also more

expensive Put initial I/O in function so easy to update

later

Code is simple & easy; what could go wrong?

First Attempt At Writing Code

Code is simple & easy; what could go wrong?

First Try At Code

double[] readCosts(int *nItems) {// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < *nItems; i++) { scanf(“%lf\n”, &costs[i]);}// And return the array…return costs;

}

Compile In MS Visual C++...

double[] readCosts(int *nItems) {// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < *nItems; i++) { scanf(“%lf\n”, &costs[i]);}// And return the array…return costs;

}

Compile In MS Visual C++...

double[] readCosts(int *nItems) {// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < nItems; i++) { fIn >> costs[i];}// And return the array…return costs;

}

Oops!Some compilers will allow this, but C++ standard requires that array lengths must be literal or constant

Compile on Eclipse + Mac OS…double[] readCosts(int *nItems) {

// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < *nItems; i++) { scanf(“%lf\n”, &costs[i]);}// And return the array…return costs;

}

Move to Eclipse + Mac OS

double[] readCosts(int *nItems) {// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < *nItems; i++) { scanf(“%lf\n”, &costs[i]);}// And return the array…return costs;

}

Compiles on Eclipse + Mac OS…double[] readCosts(int *nItems) {

// Read in the number of items & create the arrayscanf(“%d\n”, nItems);double costs[*nItems];// Now read the cost of each item from the filefor (int i = 0; i < *nItems; i++) { scanf(“%lf\n”, &costs[i]);}// And return the array…return costs;

}

…& Crashes on Eclipse + Mac OS Program behaves oddly without clear

reasons During program value of variables just

change Prices go up and down for no clear reason At some bizarre time, program eventually

crashes

…& Crashes on Eclipse + Mac OS Program behaves oddly without clear

reasons During program value of variables just

change Prices go up and down for no clear reason At some bizarre time, program eventually

crashes

How To Write Buggy Programs

Unsafe to return array declared in function Know how we draw boxes when we call

function Cross out box at end of that call to the

function Guess what? Those boxes really exist in

memory Within box are declared arrays’ memory

locations! Array uses the same address as other

things Odd behavior results as overwrite each

other’s values Important data held in shared memory

How To Write Buggy Programs

Unsafe to return array declared in function Know how we draw boxes when we call

function Cross out box at end of that call to the

function Guess what? Those boxes really exist in

memory Within box are declared arrays’ memory

locations! Array uses the same address as other

things Odd behavior results as overwrite each

other’s values Important data held in shared memory

What To Do?

Do not want to have to declare array in main Lots of rewrites needed if we do file I/O in

main Do not have money for huge array would

need later Cannot declare in function without risk of

crash

How to make long-lived or variable-length arrays?

Dynamic Storage Allocation!

Way of allocating arrays in own memory space No limits on space – array can be any

(integer) size Type must match variable, but no new

restrictions Memory reserved until explicitly returned to

system Entries NOT initialized to 0 when array

allocated Once it is allocated, use like normal

array Cannot tell difference given two valid

arrays Passing as parameter or returning from

function okay

Dynamic Storage Allocation

int * ptr = malloc(100 * sizeof(int));char * cstring;double * ddd;int * arr;const int FOUR = 5;int val;scanf(“%d”, &val);cstring = malloc(10 * sizeof(char));ddd = malloc((FOUR-1+2) * sizeof(double));arr = malloc(val* sizeof(int));arr[2] = 23;long * pirate = malloc(arr[2]*sizeof(long));

All Done Now

After being allocated, may not need array Normal arrays “die” when frame is crossed

out Not a problem here: dynamic arrays must be

killed Otherwise live forever, slowing system as it

runs If memory is completely full, program can

crash free(varName) dynamically allocated

array Argument must be address of 0th entry in

array SHOULD NOT USE ARRAY after it has been

freed Within program can call free() at any time

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");

Main Memory (RAM)

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");

Main Memory (RAM)

x

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;

Main Memory (RAM)

x

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;

Main Memory (RAM)

x

ptr

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;free(x);

Main Memory (RAM)

x

ptr

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;free(x);

Main Memory (RAM)

x ptr

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;free(x); // But I’m not dead yet!

Main Memory (RAM)

ptr

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;free(x); // But I’m not dead yet!char *y = malloc(6 * sizeof(char));strcpy(y, "sad");printf(“%s\n”, ptr); // oops…

Main Memory (RAM)

y ptr

Dangling Pointerschar *x = malloc(6 * sizeof(char));strcpy(x, "happy");char *ptr = x;free(x); // But I’m not dead yet!char *y = malloc(6 * sizeof(char));strcpy(y, "sad");printf(“%s\n”, ptr); // oops…

Hard-to-find bugs created throughout code

Not Always A Laughing Matter

Your Turn

Get into your groups and try this assignment

For Next Lecture

Review of pointers in class on Wednesday What questions do you have about how

they work? Are you ready to use & trace them in

programs? Create and delete pointers during program

running?

For Tuesday, D2L has Week #10 assignment

Midterm #2 in class on Friday