1. Day 04 Introduction to C

2. Functions – pointers as arguments
#include <stdio.h>
int SumAndInc(int *pa, int *pb,int* pc);
int
main(int argc, char *argv[]) {
int a=4, b=5, c=6;
int *ptr = &b;
int total = SumAndInc(&a,ptr,&c);
/* call to the function */
printf(“The sum of 4 and 5 is %d and c is %p\n”,total,c); }
SumAndInc(int *pa, int *pb,int *pc ){/* pointers as arguments */
*pc = *pc+1; /* return a pointee value */
/*NOT *(pc+1)*/
return (*pa+*pb); /* return by value */

3. In main() a 4
4000 b 5
4004 c 6
4008
ptr
4004
4012 pa
4000
6000 pb
4004
6004 pc
4008
6008
In function

4. In main() after the function call4
4000 b 5
4004 c 7
4008
ptr
4004
4012
In main() after the function call

5. What’s wrong with this ? #include <stdio.h>
void DoSomething(int *ptr);
int
main(int argc, char *argv[]) {
int *p;
DoSomething(p);
printf(“%d”, *p); /* will this work ? */
return 0; }
void
DoSomething(int *ptr){ /* passed and returned by reference */
int temp= 5+3;
ptr = &(temp);
/* compiles correctly, but gives incorrect output */

6. p ?
4000
In main()
ptr
6000
temp
6004
In the function ? 8
6004

7. p ?
4000
In main() after the function call

8. Functions: Pass by value
int main(){ int p, q, r; p = 5; q = 6; r = Sum_1(p, q + 1); return 0; }
void Sum_1(int a, int b){ int c; c = a + b; return c; } a b c
4000
4004
4008 p q r 5 6
2000
2004
2008 12 12

9. Problem 2 – pass pointer by value
int main(){ int *x,*y; int p = 5; int q = 6; x = &p; y = &q; Sum_1(p, y); return 0; }
void Sum_1(int a, int *b){ int input; printf(“Enter an integer:”); scanf("%d", &input); *b = a + input; }
Example15.c a b
input
4000
4004
4008 x y p q 5
2000
2004
2008
2012 15 6 20

10. Functions - Passing and returning arrays
#include <stdio.h>
void init_array( int array[], int size ) ;
int
main(int argc, char *argv[] ) {
int list[5];
init_array( list, 5);
for (i = 0; i < 5; i++)
printf(“next:%d”, list[i]); }
void init_array(int array[], int size) { /* why size ? */
/* arrays ALWAYS passed by reference */
int i;
for (i = 0; i < size; i++)
array[i] = 0;

11. Passing/Returning a structure
/* pass struct by value */
void DisplayYear_1( struct birthday mybday ) {
printf(“I was born in %d\n”, mybday.year);
} /* - inefficient: why ? */
/* pass pointer to struct */
void DisplayYear_2( struct birthday *pmybday ) {
printf(“I was born in %d\n”, pmybday->year);
/* Note: ‘->’, not ‘.’, after a struct pointer*/ }
/* return struct by value */
struct birthday GetBday(void){
struct birthday newbday;
newbday.year=1971; /* ‘.’ after a struct */
return newbday;
} /* - also inefficient: why ? */

12. Example: Array and pointer
Statically allocated data
int a[5]; int *p; int i = 0; for(i=0; i < 5; i++) a[i] = 3 + i; p= a; p = (int *)malloc(3 * sizeof(int));
a[0]
a[1]
a[2]
a[3]
a[4] p
2000
2004
2008
2012
2016
2020
2000
8000
p[0]
p[1]
p[2]
8000
8004
8008
Heap (space for dynamically allocated data)

13. Problem 3: pointer to a struct
void print_student_age_1(Student any){ printf("Age is %d\n", any.age); any.gpa = 2.0; } void print_student_age_2(Student * any){ printf("Age is %d\n", any->age); any->gpa = 2.0; void print_student_age_3(const Student * any){
int main(){ Student john; Student * john_ptr = &john; john.age = 20; john.gpa = 3.5; print_student_age_1(john); printf("gpa is %.1f\n", john.gpa); print_student_age_2(john_ptr); return 0; }
any.age
any.gpa 20
4000
4008
Example16.c
3.5
2.0
john.age
john.gpa
john_ptr 20
2000
2004
2008
3.5
2.0
any
2000
4000

14. Problem 4: malloc and structs
Student * many = (Student *)malloc(3 * sizeof(Student)); many[0].age = 20; many[0].gpa = 3.5; //void print_student_info( Student class [ ], int count ); OR //void print_student_info( Student * class, int count ); print_student_info(___________________, 3 );
Example17.c
many
8000
2000
many[0].age
many[0].gpa
many[1].age
many[1].gpa
many[2].age
many[2].gpa 23
3.4 22
3.6 21
4.0
8000
8004
8008
8012
8016
8020

15. Input/Output statements
fprintf(stdout,”….”,…); - buffered output
Equivalent to printf(“….”,…)
fscanf(stdin,…);
Equivalent to scanf(…)
fprintf(stderr,”…”,…); - un-buffered output
Use for error messages.
perror(…);
Use to print messages when system calls fail.

16. Storage classes Allocate memory only when function is executed
Automatic (default for local variables)
Allocate memory only when function is executed
e.g. auto int i;
Register
Direct compiler to place variable in a register
e.g. register counter = 1;
Static
Allocate memory as soon as program execution begins
Scope is local to the function that declares the variable.
Value is retained and space is de-allocated only when program (not function) quits.
e.g. static int i;

17. Storage classes - contd
Extern
Default for function names.
For a variable shared by two or more files:
int i; //global variable in file 1
extern int i; //global in files 2,3 … x
For a function shared by 2 or more files, place a function prototype at the beginning of the files.

18. Program with multiple files
#include <stdio.h>
#include “mypgm.h”
void
Myproc() {
int mydata=g_data * 2;
. . . /* some code */ }
#include <stdio.h>
#include “mypgm.h”
int g_data=5;
int
main() {
Myproc(); return 0; }
mypgm.c
main.c
main.c
Library headers
Standard
User-defined
void Myproc();
extern int g_data;
mypgm.h

19. To compile gcc main.c mypgm.c –o run or gcc main.c –c -o main.o
gcc mypgm.c –c -o mypgm.o
gcc mypgm.o main.o –o run
Can also use a makefile.

20. enum - enumerated data types
#include <stdio.h>
enum month{
JANUARY, /* like #define JANUARY 0 */
FEBRUARY, /* like #define FEBRUARY 1 */
MARCH /* … */ };
In main:
enum month birthMonth;
If(birthMonth = = JANUARY){…}
/* alternatively, …. */
JANUARY=1, /* like #define JANUARY 1 */
MARCH=3, /* like #define MARCH 3 */
FEBRUARY=2, /* … */
Note: if you use the #define, the value of JANUARY will not be visible in the debugger. An enumerated data type’s value will be.

21. Before you go….
Always initialize anything before using it (especially pointers)
Don’t use pointers after freeing them
Don’t return a function’s local variables by reference
No exceptions – so check for errors everywhere
An array is also a pointer, but its value is immutable.