Advanced Use of Pointers

CS2023 Winter 2004

Outcomes: Advanced use of Pointers

� “C for Java Programmers”, Chapter 8, section 8.11, 8.15

� Other textbooks on C on reserve

� After the conclusion of this section you should be able to

� Allocate and deallocate blocks of memory dynamically on the head

� Use pointers to functions in order to pass a function as a parameter to another function

� Use generic pointers to write more general functions

Dynamic Memory Allocation

� C's data structures normally fixed in size

� C supports dynamic memory allocation to allocate storage during execution

� needed for dynamic arrays, lists, strings, ...

� Dynamically allocated memory stored on the heap

Recall: Memory Management

� Dynamically allocated variables

� Memory allocated and destroyed at run time, under control of programmer!

� No guarantee that first variable to be destroyed is last created

� This area of memory can have holes and is called the heap

Recall: Memory Management

� Usually heap and stack begin at opposite ends of the program's memory, and grow towards each other

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� � � � ��� �

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Dynamic Memory Allocation

�� � � � ! Allocates a block of memory, but doesn't initialize it

! � � � � ! Allocates a block of memory and clears it

" Note: arguments of �� � � � ! and ! � � � � ! of type # $&% ')( *

, which is type returned by sizeof operator (normally + , # $ - , ' . � � ,- )

Dynamic Memory AllocationTwo primary methods of allocating memory:

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/ 0 12 3 8 6 7 7 08 9 : 1&; <)= > ? <@ A <: > < 2 E 0 A F >HG

: 1&; <)= > ? <@ A <: > < 2 B 1&; < C&D

I 35J D

J K 4 6 7 7 0 8 9 : 1; < 0 L 9 I C C D 3 0 ?M 3

J K 8 6 7 7 0 8 9N G : 1&; < 0 L 9 I C C&D

You should always remember to check if a call to a memory allocation function was successful.

Dynamic Memory Allocation

O�P Q R�S T

U R V WX YZ [ Q Y \ Q Y] ^ Y P ^ OP Q R U

O _` ` S a bc X X YZ ` \ O�d ^ Y _ ` OP Q e e e a a fg h h e

^i O Q ` jk lm n o V l h g p j e T

R S a q r T

O�P Q R�s T

U R c WX YZ [ Q Y \ Q Y] ^ t OP Q \ R U

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^i O Q ` jk lm n o V l h g p j e T

R s a u T5

s

Dynamic Memory Allocation

v Note that malloc returns void*, whereas the left hand side of w x yz { { | } ~ � �&� � | � ~ � � � � �

is of type � � � �

. Some programmers use an explicit cast, but this is not required:

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v Always pass ��� � | � ~ ��� w � �

as a parameter to malloc, rather than the absolute value

� use yz { { | } ~ � �� � | � ~ � � � � �

instead of yz { { | } ~� �

Memory Deallocation

Memory should be deallocated once the task it was allocated for has been completed.

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Memory Deallocation

Always follow the call to

Î Ï Ï Ð Ñ

with

Ò Ó Ô Ô

Errors

Õ Memory deallocation using

Ö&× Ø Ø ÙÚ

should only be used if memory has been previously allocated with ÛÜ Ý Ý Þ ß ÙÚ

:

à�á â àäã å�æ ç

æ è é à ç

ê�ë ì ì í æ î ç

ï always remember where memory came from: heap or stack

ð Don't create garbage objects!

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ñ ò óô õ õ�ö ÷ ø�ù ú�û ü ö ý ø ú�þ ÿ � ���

� The first object created is now inaccessible

Errors

� Given two pointers � and �, the assignment � � does not copy the block of memory pointed to by into a block of memory pointed to by �

� Remember that after � �; � and � share the value; if you call

��� � � ��� �

this would also deallocate �, now you must not call

��� � � ���

Pointers to Blocks Containing Pointers

! A block containing three pointers to double objects. In order to access a single object, the code has to apply dereferencing twice

2.1

3.1

4.1

block

Pointers to Blocks Containing Pointers

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8 8 ?@ A A >BC C D C E

FHG I JK L MON K P QR ST N K U U V

K F J JW X G Y Z[ K \ L Y] X X G Y J^ _ `K a b G F J c G dW X b V V V

L L ef g g V

hi i j i k

l m l no pq rs t uwv x kno p q ry z { y z { t uwv x k

Pointers to Blocks Containing Pointers

The complete code to initialize the block:|�} ~ �� � ��� � � �� �� � � � � �

�� } � �� � � � � � � �� � � � �To free memory :

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ÓÔ Ô ÕÔ

Ö�× Ø ÙÚ Û ÜÞÝ Ú ß àá âã Ý Ú ä ä å

Ú Ö Ù Ùæ ç × è éê Ú ë Û èì ç ç × è ÙÚ ä íïî ðÚ ñ ò × Ö Ùó × ôæ ç ò å å å Û Ûõö ÷ ÷ åøù ùú ù

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&' ' ( '

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+�5 6 7D 8 9< D = 8 , < D B B C

23 4 B 8 EF 5 G HI , J I D J <

Pointers to Functions

K A pointer to a function determines the prototype of this function, but it does not specify its implementation:

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L�M N P Q R OT UV W X Y SZ [P \ QV M ^ N L U M ] Y N V ] M L�M _ ` ` ` P [

a You can assign an existing function to the pointer as long as both have identical parameter lists and return types:

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e�u v en

w You can call the function

xy z

through the pointer

{}| :

~�� � ~ � ��� �� � � ���

Functions as Parameters

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¢ � � � ¢ ¦   ¥ ­ ¡

°��� ��� �� �� �²± © « ® � © « ® � ®« ® © � � � � © � ¡

Functions as Parameters

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¶ ´º ¹ »¼  ·�¼ à ¿ ¶ ´º ¹ »¼ ½Ä ÅÆ ½ ¶ ´º ¹ »¼ Æ Æ Ç

È

is called:– a virtual function; its implementation is not known to ÉËÊ Ì}Í ÎÊ ÉËÏ ÐÑ

but will be provided when ÒËÓ Ô}Õ ÖÓ ÒË× ØÙ

is called.

– a callback function, because it calls back the function supplied by the client.

Generic Search

C does not support polymorphic programming, but it can besimulated using generic pointers (i.e. Ú Û ÜÝ Þ

). A function prototype may specify that a block of memory

and the value it is looking for are not typed:

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ëê ß ì í ë äï õã ñ ß�à á é í æê ÿ ü ä å ã ö é æê à â á ëê ß ì í ñ

æê à â á ëê ß ì í ö ö�÷

Implementation of Generic Search (incorrect)

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� � � �� � � � � � ! " " �

� � � �� � # �

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(

Implementation of Generic Search

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, )�< - =+ , )�< - ; 54 )6 7 5 .9 >- ; , )< - =+ - 9 ? )< - ;)�* + 3 7 04 @A . / - B 3 04 * , + 54 )6 7 ; 0 4 * , + 54 )6 7 B B C

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/ - + >/ * J D

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- 9 ? )< - B

) E 3 04 @A ./ - 3 A ; 5 .9 >- B B

/ - + >/ * M D

/ - + >/ * J D

N

Application of Generic Search

The client's responsibilities:

O�P Q RS TU V RS P W Q XS Y Z[ \ ]_^ ` RS P W Q XS Y Z[ \ ]_a b c

d \ Q Y dP ] ^ e e ] a f

gO�P Q RS TU V RS P W Q hS O X ]^ ` RS P W Q hS O X ]_a b c

d \ Q Y dP ] V XS Y Z[ \ ] b ^ e e ] V XS Y Z [ \ ] b a f

g

Note that this callback is sufficient for search, but not for sort.

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{ | w uo t w }~ �� � �

| s� � n w j ���

| s� � n w � � � �� � � �

o t r o �

o u� � � � �q n n s p � }~ �� j y o � w s u � | s � � n w � � � � � �� � � �

w �o r � � �~ ��

� k~ � � �� � �

u s x � o � � � o � }~ �� � o � � � ij o t o r o q no � w j i

o u � y p q t u � �� n u ��� � �� o   � ¡ � � � ¢

u x w w � � � �

w �o r � � �~ ��

� k~ � � �� � �

£

l xo t r u � �� u ¤q y � y st w s u r z w �q n� w y ¥t � �

�� y w q x p z¦ w t � �� } ~ �� � � �� y o � w s u � | s � � n w �� p s �l �

� � � § � � ¨ � t s r � � �

NAME qsort – sorts an array

SYNOPSIS #include <stdlib.h>

void qsort(void *base, size_t nmemb, size_t size, int(*compar)(const void *, const void *));

DESCRIPTION The qsort() function sorts an array with nmemb elements of size size. The base argument points to the start of the array.

The contents of the array are sorted in ascending order according to a comparison function pointed to by compar, which is called with two arguments that point to the objects being compared.

The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respec- tively less than, equal to, or greater than the second. If two members compare as equal, their order in the sorted array is undefined.