malloc detail

7/24/2019 malloc detail

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DynamicMemoryAllocation

Dynamicmemoryallocation

Howtoallocatememoryforvariables(esp.arrays/strings)

malloc(),

calloc(),

realloc(),

and

free()

CSE

251

Dr.

Charles

B.

Owen

ProgramminginC1


2/24

Why

dynamic

memory

allocation?

Usually,sofar,thearraysandstringswereusinghavefixedlength(i.e.,lengthisknownatcompiletime)

Example:

char

myStr[11];

allocates

memory

for

10

chars

printf(Enterastring:);

Whatiftheuserwantsto

gcharslongorifthelengthis

knownonl atruntime?

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Dr.

Charles

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Owen

ProgramminginC2


3/24

malloc()

malloc()isusedtorequestadditionalmemoryfromtheoperatingsystemduringprogramexecution

Syntax: malloc(numBytes)

Inputisthenumberofconsecutivebytestobeallocated

ReturnvalueisapointertothebeginningoftheblockofmemoryallocatedorNULLifmalloc fails

Tousemalloc(),youmust#include

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Charles

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ProgramminginC3


4/24

malloc()

c ar

c arP;

ec are

a

po nter

to

c ar

charP

=

charP

charP

contains

the

address

of

the

beginning

of

that

block.

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ProgramminginC4


5/24

free()

Thefunctionfree()returnsmemorytothememory

pool. Itfreesupmemory

Syntax:

free(ptr)

malloc()function

Tousefree(),youmust#include

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ProgramminginC5


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Example

Thisprogramallowstheuserto,

allocates

memory

for

the

string,

and

then

a lies

strin

o erations

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ProgramminginC6


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#include /*youneedthislibraryformalloc(),free(),exit()*/

#include

#include

/*

you

need

this

library

for

strchr(),

strlen()

*/int main()

{

char*charP,*q;

int maxlen;

printf("Enter

maximum

string

length:

");

" ",

getchar(); /*readsthenewlinecharacter*/

printf("Enter

the

string:

");

fgets(charP,

maxlen,

stdin);

if

((q

=

strchr(charP,

'\n'))

!=

NULL)

*q

=

'\0';printf("You'veenteredastring%soflength%d\n",charP,strlen(charP));

free(charP);

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ProgramminginC7 A


8/24

Whatitdoes:

if((q=strchr(charP,'\n'))!=NULL)

*q

=

'\0';

The

function

fgets returns

the

entire

line

input

by

the

user

including

the

newlineattheend(whentheuserhitreturn).Thefunctionstrchr(charP,\n)

.

the

result

in

the

variable

q

is

not

NULL,

the

if

statement

executes

the

line

of

code

to

replace

the

newline

with

a

null

termination

for

the

string.

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Charles

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ProgramminginC8


9/24

strchr

if

((q

=

strchr(charP,

'\n'))

!=

NULL)

*q='\0';

This

code

finds

the

first

occurance of

thecharacter\nwhichisthenewlinecharacter

.

notNULL),itsetsthatcharactertothestring

null

termination.

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Charles

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ProgramminginC9


10/24

Memoryleak

Ifmalloced memoryisnotfreeed,thentheOSwill

leakmemory

This

means

that

memory

is

allocated

to

the

program

but

not

returned

to

the

OS

when

it

is

finished

using

it

.

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ProgramminginC10


11/24

Exampleint main()

{

char

*charP,

r[80]; Memoryleakexampleint length;

while(1)

printf("Enter

the

maximum

string

length:

");

fgets(r,

80,

stdin);

sscanf(r,

"%d",

&length);

if((charP =malloc(length))==NULL){

printf("Out

of

memory.

Quitting\n");

exit 1

}

printf("Enterthestring:");

fgets(charP,length,stdin);

/*

free(charP);

*/}

}

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ProgramminginC11 B


12/24

Exampleint main()

{

char

*charP,

r[80];

Memoryleakexample

int length;

while(1)forastring.But,thatmemoryisneverfreed.

Hence,wehaveamemoryleak.

printf("Enter

the

maximum

string

length:

");

fgets(r,

80,

stdin);

sscanf(r,

"%d",

&length);

if((charP =malloc(length))==NULL){

printf("Out

of

memory.

Quitting\n");

exit 1

}

printf("Enterthestring:");

fgets(charP,length,stdin);

/*

free(charP);

*/}

}

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Owen

ProgramminginC12


13/24

mallocwithoutfreeing while(1)

{

}

charP

If

you

dont

free

the

allocated

memory,

previous

block

is

still

ours

according

to

the

OS,

but

we

can

no

longer

find

it

(no

pointer

to

it).

That

block

is

an

orphan!Itslikeyouboughtahouse,butthenlostthe

. ,

you

cant

use

it

because

you

cant

find

it.

CSE

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ProgramminginC13


14/24

Alwaysfreewhatyoumalloc

Youareresponsibleforyourmemory,youmust

allocateitandfreeit.

Unlikeotherlanguages,itisalluptoyou!

If

you

dont

free

it,

your

program

grows

larger

and

eventuallyrunsoutofmemory!

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ProgramminginC14


15/24

mallocallocatesbytes

Ifyouwantacharacterarraythatstores10

characters(including\0):

char*p=malloc(10);

youwan oa oca es orage or n s or ou es

orfloats),youcantdothis:

* * *

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ProgramminginC15


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allocateint anddoublearray

int *intP;

double*doubleP;

//

Allocate

space

for

10

integers

*

//

Allocate

space

for

10

doublesdoubleP =malloc(10*sizeof(double));

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OwenProgramminginC16 C


17/24

allocateint anddoublearray

int *intP;

double*doubleP;

//

Allocate

space

for

10

integers

*

Allocates

40

bytessizeof(int)=4

//

Allocate

space

for

10

doublesdoubleP =malloc(10*sizeof(double));

Allocates80bytes

sizeof(double)

=

8

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OwenProgramminginC17


18/24

realloc

realloctakesapointertoallocatedmemoryand

reallocatesthememorytoalargersize

ifitcanmaketheoldblockbigger,great

ifnot,itwillgetanother,largerblock,copytheold

,

returnapointertothenew

intP =realloc(intP,2*sizeof(intP));

intP may

be

different

after

a

realloc!

CSE

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Dr.

Charles

B.



19/24

int main

()

{

double*dblPtr;

Step1:Prompttheusertoenter

the

number

of

random

numbers

to

generate

ow y,

u ;

printf("How

many

random

numbers

to

generate:");

howMany=ProcessInput(stdin);

dblPtr =

malloc(howMany *

sizeof(double));

if

(dblPtr ==

NULL)

{

" " Anexam le ro ram

o y o o o , x ;

exit(1);

}

for(int i=0;i


20/24

Step2:createadynamicarrayto

store

the

random

numbers

int main

()

{

double*dblPtr;

ow y,

u ;

printf("Howmanyrandomnumberstogenerate:");


dblPtr =

malloc(howMany *

sizeof(double));

if

(dblPtr ==

NULL)

{

" "

In

this

example

we

have

testedtobesuremalloc , ;

exit(1);

}

succeeded.Ifnot,weare

out

of

memory.

for(int i=0;i


21/24

Step3:generatetherandom

numbersandprintthem

int main

()

{

double*dblPtr;

ow y,

u ;

printf("Howmanyrandomnumberstogenerate:");


dblPtr =

malloc(howMany *

sizeof(double));

if

(dblPtr ==

NULL){

" " , ;

exit(1);

}

for

(int i=0;i


22/24

dblPtr =realloc(dblPtr,2*howMany); Step4:doublethesizeofthearrayusing

realloc

for(int i=howMany;i


23/24

int ProcessIn ut FILE

*f{

int val; Safedataentr ustlikelast

char

in[100];fgets(in,100,f);

week

sscan(in,%d,&val);

return

val;}

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PrintArray

voidPrintArray(double*ptr,int cnt)

{

printf("Printing

Array

Values\n");for double* = t r < tr+cnt ++

printf("Valis:%lf\n",*p);

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OwenProgramminginC24 1

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