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Memory Management

Junaed SattarSeptember 17, 2008

Lecture 3

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Today

memory management

C++ memory management issues

dynamic memory (de)allocation

pointers

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Memory structure (simplified)

Command line argumentsEnvironment variables

Stack(Function/method arguments, locals, return addresses)

Free store (Heap)

Dynamically allocated memory

High address

Low address

Code (Read-only)Uninitialized/Initialized data (No eXecute)

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Memory basics

C++ (as well as C) provides mechanismsfor dynamic memory allocation

ask for memory as we need it

let it go when we don't

possible to write memory-efficient code(small memory footprint)

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Memory basics

The catch(es):

Somewhat complex mechanism fordoing so

Manual (i.e. programmer-initiated)memory management

Coding mistakes leads to ugly, hard-to-

find, hard-to-replicate bugs No automatic garbage collection

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One scenario

Loading an image from disk for display

locate the file and format (jpg/tiff/bmp/png...)

allocate enough memory to hold the image

copy the image from disk to memory

process

deallocate the memory previously allocated

Theallocate-use-deallocatecyclemust be maintained

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Graphically...

find image

Main memory

Secondarymemory

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Graphically...

find imagecalculate size

Main memory

Secondarymemory

645KBytes

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Graphically...

find imagecalculate sizeallocate memory

dynamicallyallocate

Main memory

Secondarymemory

645KBytes

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Graphically...

find imagecalculate sizeallocate memory

Main memory

Secondarymemory

645KBytes

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Simple workflow, but...

Task for the programmer to do all that

unlike Java, where the compiler/interpretertakes care of it

No automatic garbage collection Programmers must clean up themselves

Memory allocation requests might fail

what if not enough free memory?

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Pointers

holds the address of a data object or afunction/method

handler to dynamically allocated memory

pass arguments as reference

we've seen this last class

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Pointers

declaration

int *i;

char *c;

and so on.

refers to objects of one data type only

i.e. the same pointer cannot refer to ints and

chars

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Pointers

simple usage:

int number = 10;

int *i = &number;

*i is a pointer to an int

i contains the address ofnumber

*i contains the value stored in number i.e. *i has the value stored at the address pointed

to by i

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Pointers

stray pointer:

int *i;

i points to what?

undetermined, random address

Stray or uninitialized pointer.

using a pointer without initialization has

undefined results in other words, big bad bug.

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Bad Pointer

int *i;

int num = 10;

...

*i = 100; // Where is this going to???

This program:

will compile,will run, and maybe will not even crash depending on what i points to, it might bring down

your program entirely

always initialize with either a valid address or NULL

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Allocation

usage:

int *i = new int;

*i is a pointer to an int, but also we have:

dynamically allocated space for one int

using the C++ new operator

new provides dynamic storage allocation

find storage in free store (or heap)

returns a pointer to the object created

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More about new

new can fail

if it does, then either returns NULL or throwsan exception (std::bad_alloc)

can be used to initialize the memory: int *i = new int(10);

note the bracket type

can be used to allocate arrays: int *i = new int[10];

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new example

char* pCharArray;

try {

pCharArray = new char[256];

// sample usagestrcpy(pCharArray, "Array of characters");

}

catch( std::bad_alloc e )

{

std::cerr

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Deallocation

an object created with new exists untiloperator delete is used to deallocate

will not be implicitly called for an object

created with a new must explicitly deallocate

otherwise will create memory holes

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Deallocation

new and delete must come in pairs

use correct matches

use delete() with new() (individual objects)

use delete[]() with new[]() (arrays)

if not, the result is undefined

deleting a NULL pointer has no effect.

delete does not assign NULL to a pointer

accessing a deleted pointer hasundefined effects

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delete example

char* pCharArray = 0;

try {


}catch( std::bad_alloc e )

{

std::cerr

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memory holes?

void SomeFunction() {

char* pCharArray = 0;

try {


}catch( std::bad_alloc e ){

std::cerr

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memoryF.A.Q.

why new and not malloc?

support for objects and type-safe pointers

double delete?

causes prorgam to crash

mixing new/malloc?

bad idea.

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