1/15/2016Course material created by D. Woit 1 CPS 393 Introduction to Unix and C START OF WEEK 11...

05/03/23 Course material created by D. Woit 1

CPS 393Introduction to Unix and C

START OF WEEK 11 (C-5)


Functions and Structures • We can pass a structure to a

function: func1(woit);• call by value as always (not

modified)• func1 makes a *copy* of car woit

• /*Source: struct5.c*/• #include <stdio.h>• struct complex {• int r;• int i;• };• void WriteSum(struct complex c1,

struct complex c2);

• int main(void) {• struct complex A1, A2;• A1.r = 4; A1.i = 7;• A2.r = 3; A2.i = -1;• WriteSum(A1,A2);• return 0;• }• • void WriteSum(struct

complex c1, struct complex c2) {

• printf("%d + %di\n", (c1.r)+(c2.r), (c1.i)+(c2.i));

• }


Returning a structure to a calling program• /*Source: struct6.c*/• #include <stdio.h>• struct complex {• int r;• int i;• } ;• struct complex ReadIt(void);• int main(void) {• struct complex X;• X=ReadIt();• printf("%d + %di\n",X.r, X.i);• return 0;• }

• struct complex ReadIt() {• struct complex temp;• scanf("%d %d", &temp.r,

&temp.i);• return(temp);• }

• data from temp *copied* into storage set aside for X


HMWKRewrite the above programs using typedefsfor complex

numbers.• Put functions WriteSum and MakeComplex, (along with

anything else necessary)into a file called cmplx.c that can be compiled separately. Write a header file cmplx.h

• A program similar to this should then work (try it):• #include "cmplx.h"• void main(void) {• complex X,Y;• X=MakeComplex(1,2);• Y=MakeComplex(3,4);• WriteSum(X,Y);• }


Pointers to Structures • In the example below

pointer p point to the structure c1.

• With pointer, structure can be accessed in two ways:

• struct complex {• int r;• int i;• } c1, *p;• p=&c1;• (*p).r = 4; //or• p->r = 4;

• We can pass pointers to structures to functions instead of actual structure

• 1. to change value of structure inside function

• 2. saves execution time since structures could be hundreds of bytes long while a ptr is usually 4 bytes (if just pass structure, a *copy*of it must be made--time consuming)


Example of pointer to structures• Note ReadIt and WriteIt print using different syntax

because • ReadIt has POINTER to complexi, but WriteIt has complex.• /*Source: struct8.c*/• #include <stdio.h>• struct complex {• int r;• int i;• };• void ReadIt(struct complex *c);• void WriteIt(struct complex p);


Struct8.c• int main(void) {• struct complex A1, A2;• ReadIt(&A1); • WriteIt(A1);• ReadIt(&A2);• WriteIt(A2);• return 0;• }

• void ReadIt(struct complex *c) {

• printf("Enter complex: ");• scanf("%d %d",&(c->r),

&(c->i));• printf("Read %d + %di\

n",c->r, c->i);• }• void WriteIt(struct

complex c) {• printf("Write: %d +

%di\n", c.r, c.i);• }


Struct9.c• /*Source: struct9.c*/• #include <stdio.h>• struct complex {• int r;• int i;• };• void ScalarMult(int i, struct complex *p);• void PrintComplex(struct complex *p);• int main(void) {• struct complex C={4,-2};• ScalarMult(3,&C);• PrintComplex(&C);• return 0;• }


Struct9.c• void ScalarMult(int m, struct complex *p) {• //pointer is required since mutating p• p->r = p->r * m;• p->i = p->i * m;• }• void PrintComplex(struct complex *c){• //pointer unnecessary since not mutating p• //printf("%d + %di\n", c->r, c->i);• printf("%d + %di\n", (*c).r, (*c).i);• }


HMWK• rewrite the above program using typedefs

for complex.• Write ConjugateComplex which turns a

complex number into its conjugate (imaginary part multiplied by -1).

• Write a function called NegComplex turns a complex number

• into its negative (both imaginary and real parts multiplied by -1.


C's Time and Date Functions use Structures • use structures to hold info• defined in structure tm in <time.h>• struct tm {• int tm_sec; /*seconds, 0-59*/• int tm_min; /*minutes, 0-59*/• int tm_hour; /*hours, 0-23*/• int tm_mday; /*day of month, 0-31*/• int tm_mon; /*months since Jan, 0-11*/• int tm_year; /*years since 1900*/• int tm_wday; /*days since Sunday 0-6*/• int tm_yday; /*days since Jan 1, 0-365*/• int tm_isdst; /*daylight savings time flag*/• };• struct tm *localtime(time_t *time);• time_t time(time_t *time1);


Example tm1.c• /*Source: tm1.c• Purpose: to display

current system time and date

• */• #include <stdio.h>• #include <time.h>• void main(void) {• struct tm *systime;• time_t t;

• t = time(NULL);• systime = localtime(&t);• printf("Time is %d:%d:%d\

n",• systime->tm_hour,

systime->tm_min,• systime->tm_sec);• printf("Date is %d/%d/%d\

n",• systime->tm_mon+1,

systime->tm_mday,• systime-

>tm_year+1900);• }


System structures• C has many other structures to give

programmer access to system information. e.g., dirent.h lets you access the

• linux filesystem (files, dirs, their perms, etc.)


Nested Structures • struct evaluation {• int tests; /*number of

tests*/• int exam; /*non-0=yes,

0=no*/• int assigns; /*number of*/• int labs; /*number of*/• };

• struct class {• char CourseNum[10];

/*e.g., CPS393 */• char CourseName[80];

/*e.g., Intro to C & UNIX */• struct evaluation eval;

/*eval info*/• } c1, c2[10];

• c1.eval.assigns = 3;• c2[0].eval.labs=6;


Dynamic Memory Allocation • In C we can allocate and free storage at run-time (DMA)• v.s. compile-time as we have done so far• calloc(NumItems, ItemSize);• NumItems: number of items of storage requested• ItemSize: size of each item (in bytes)• calloc returns void (generic) pointers.• must *cast* to desired type • (ptr to NumItems x ItemSize bytes of memory)• storage is initialized to zeros


Dynamic Memory Allocation

• malloc(Size);• Size: number of bytes• storage not initialized• e.g., to reserve an area for n integers

05/03/23Course material created by D. Woit 17

• /*source: trycalloc.c */• #include <stdio.h>• #include <stdlib.h>• int main(void) {• int n; /*num elts of future

array*/• int *p, *tmp; /*pointers to the

future array*/• scanf("%d",&n);• int i;• p = (int *) calloc (n, sizeof(int));• if (p == NULL) {• perror("calloc");• exit(0);}• tmp = p;• /* tmp is used to point to

elements while p point to the beginning of the array */

• /*load array with 0,1,2,...(n-1) */• for (i=0; i<n; i++)• *tmp++ = i;• //tmp[i]=i; //same• for (i=0; i<n; i++)• printf("p[%d] is %d\n",i,p[i]);• /* or• tmp = p;• for (i=0; i<n; i++)• printf("p[%d] is %d\n",i,*tmp+

+);• */• exit(1);• }


Releasing the memory• When memory no longer needed, free it for later use.• When pgm ends, all dynamically allocated memory is

freed, but if no longer needed *during* pgm execution, could free it immediately to release storage

• A long-running program, like the linux operating system itself must always free its memory, or eventually lots of unused, but unavailable memory around (full kidneys)

• free(p);

• Can dynamically allocate strings similarly.• Since char is always 1 byte didn't do sizeof(char)


Stralloc.c

• /*source: stralloc.c*/• #include <stdio.h>• #include <stdlib.h>• #define MAX 10• int main(void){• char *A;• int max_int=0;

• //need to add error-checking• printf("enter max string length:

");• scanf("%d",&max_int);• while ((getchar())!='\n');• A=(char

*)malloc(max_int+1); //room for null char

• printf("enter string: ");• fgets(A,max_int,stdin);• printf("Third char is: %c\

n",*(A+2));• exit(0);• }


HMWK• read about malloc and calloc (text, man pages).• What do they return if no storage left to allocate• (i.e., if they fail)?• Write a program that reads an input file such as:• 4• 1 5• 2 -4• -3 9• 8 2• The first integer, call it n, is the number of • complex numbers to follow. The n complex numbers• are then listed. Your program must dynamically• allocate enough memory to store the array of n• complex numbers.


HMWK cont• Complex numbers must be stored• in a structure. (So you have an array of • structures.) Then your program should loop through• the array from end to front, and print out the• complex numbers in it. E.g., your output should• look like• 8 + 2i• -3 + 9i• 2 + -4i• 1 + 5i• on stdout.

1/15/2016Course material created by D. Woit 1 CPS 393 Introduction to Unix and C START OF WEEK 11...

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Transcript of 1/15/2016Course material created by D. Woit 1 CPS 393 Introduction to Unix and C START OF WEEK 11...