struct union

8/11/2019 Struct Union

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Structures and Unions in C

Scott Rixner

[email protected]


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)b*ectives

+e able to use com,ound data structures in,rograms

+e able to use com,ound data structures as

(unction arguments either by value or byre(erence

+e able to do sim,le bit-vector mani,ulations

Cox / Rixner Structures and Unions 3


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Structures

Com,ound data

A date isan int month andan int day and

an int year

Unlike /ava0 C doesn tautomatically de(ine (unctions(or initiali2ing and ,rinting 3

struct ADate { int month; int day; int year;};

struct ADate date;

date.month = 9;date.day = 1;date.year = 2005;


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Structure Re,resentation 4 Si2e

sizeof(struct 5sum o( sizeof( (ield6 alignment ,adding

7rocessor- and com,iler-s,eci(ic

!2!1 "# $" AD D"c1 c2 ipadding

struct %har%har&nt { char c1; char c2; int i;} foo;

foo.c1 = 'a';foo.c2 = ' ';foo.i = 0)D"AD$""#;

x86 uses little-endian representation


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Ty,ede(

8echanism (or creating ne9 ty,e names:e9 names are an alias (or some other ty,e;m,roves clarity o( the ,rogram

ty*edef +on, int!- t;ty*edef struct ADate { int month; int day; int year;

} Date;

int!- t i = 100000000000;Date d = {9/ 1/ 2005};

Overload existing type

names for clarity andportability

Simplify complex typenames


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Constants

Allo9 consistent use o( the same constantthroughout the ,rogram;m,roves clarity o( the ,rogramReduces likelihood o( sim,le errors


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Arrays o( Structures

Date irthdays 7#8&"7D ;

oo+chec irthday(Date today

{ int i;

for (i = 0; i 7#8&"7D ; i33 { if ((today.month == irthdays i .month :: (today.day == irthdays i .day return (true ; }

return (fa+se ;}

ConstantArray declaration

Array index, thenstructure field


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7ointers to Structures

Datecreate date1(int month/ int day/ int year{ Date d;

d.month = month; d.day = day; d.year = year;

return (d ;}

oidcreate date2(Date month = month; d >day = day; d >year = year;}

Copies date

Pass-by-reference

Date today;

today = create date1(1/ 2?/ 201- ;create date2(:today/ 1/ 2?/ 201- ;


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7ointers to Structures $cont.&

oid

create date2(Date month = month; d >day = day; d >year = year;}

oidfun @ith dates( oid{ Date today; create date2(:today/ 2/ 5/ 201- ;}

today.month

today.day

today.year

x!

x! "

x! #

month 2

day 5

year 201-

x$ A

x$ A"

x$ A#

d 0)1000x$ %#

2

5

201-


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7ointers to Structures $cont.&

Date day = day; d >year = year;

return (d ;}

&hat is d pointing to'('(

)more on this later*


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Cox Structures and Unions 12

Abstraction in C

struct @id,et;

struct @id,et


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Collections o( +ools $+it =ectors&

+yte0 9ord0 ... can re,resent many booleans)ne ,er bit0 e.g.0 00100101 5 (alse0 (alse0 true0 ...0 true

+it-9ise o,erations+it-9ise A:> 00100101 : 10111100 == 00100100+it-9ise )R 00100101 10111100 == 10111101+it-9ise :)T E 00100101 == 11011010+it-9ise ?)R 00100101 F 10111100 == 10011001


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),erations on +it =ectors

const unsi,ned int +o@ three its mas = 0)G;unsi,ned int it ec = 0)15;

000 0111001 0101

Al+ays use C s unsi,ned types for bit vectors

A mask indicates +hich bit positions +e are interested in

000 0101 == 001 0101 : 000 0111

im*ortant its = it ec : +o@ three its mas ;

Selecting bits

.esult / '


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000 0111001 0101

it ec = +o@ three its mas ;

Setting bits

.esult / '

001 0111 == 001 0101 000 0111


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000 0111001 0101

it ec := E+o@ three its mas ;

Clearing bits

.esult / '

001 0000 == 001 0101 : E000 0111


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+it-(ield Structures

S,ecial syntax ,acksstructure values moretightly

Similar to bit vectors0 but

arguably easier to read:onetheless0 bit vectorsare more commonlyused.

7added to be an integralnumber o( 9ords7lacement is com,iler-s,eci(ic.

1 1 0 1 1 0 f1 f2 f?

struct #+a,s { int f1 ?; unsi,ned int f2 1; unsi,ned int f? 2;} my f+a,s;

my f+a,s.f1 = 2;my f+a,s.f2 = 1;my f+a,s.f? = 2;


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Unions

Choices

An element isan int i ora char c

sizeof(union 5maximum o( sizeof( (ield

"# $" AD D"c

i

padding

union An"+t { int i; char c;} e+t1/ e+t2;

e+t1.i = -;e+t2.c = 'a';e+t2.i = 0)D"AD$""#;


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Unions

A union value doesn t kno9 9hich case itcontains

union An"+t { int i;

char c;} e+t1/ e+t2;

e+t1.i = -;e+t2.c = 'a';e+t2.i = 0)D"AD$""#;

if (e+t1 currently has a char

0o+ should your program 1eeptrac1 +hether e+t1 , e+t2 hold

an int or a char '

?

?2asic ans+er Another variable

holds that info


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Tagged Unions

Tag every value 9ith its case

;.e.0 ,air the ty,e in(o together 9ith the union;m,licit in /ava0 Scheme0 8L0 3

3num must be external to struct,so constants are globally visible4

Struct field must be named4

enum Hnion Ia, {& &7I/ & %JA8};struct Ia,,edHnion { enum Hnion Ia, ta,; union { int i; char c; } data;};


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:ext Time

8emory Allocation

struct union

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