C Pointers

A pointer is a variable that holds an address

#include <stdio.h>

int main() {
    int i = 10;
    printf("The value of i is %d\n", i);
    printf("The address of i is %p\n", (void *) &i);
};

Passing pointers as arguments

#include <stdio.h>

void increment(int *p) {
    *p = *p + 1;
}

int main() {
    int i = 10;
    int *j = &i;

    printf("i is %d\n", i);
    printf("j is also %d\n", *j);
    increment(j);
    printf("i is %d\n",i );
};

Pointers and Arrays

When a pointer is assigned to an array, it points to the first element of the array.

#include <stdio.h>

int main() {
    int a[5] = {11,22,33,44,55};
    int *p;

    p = a;
    printf("%d\n", *p);
    // 11
    printf("%d\n", *(p+1));
    // 22
};

Array/Pointer Equivalence in function calls

void test(char *s){
    printf("%c", *s);
}
int main(void){
    char s[]="Antelopes";
    char *t = "Wombats";

    test(s); //works
    test(t); //works, too
}

Changing arrays in functions

#include <stdio.h>

void double_array(int *a, int len){
    for (int i = 0; i< len;i++){
        a[i] *= 2;
    }
}

int main() {
    int x[5] = {1,2,3,4,5};
    double_array(x,5);

    for (int i = 0; i< 5; i++){
        printf("%d\n", x[i]);
    }

};

Void pointer

Sometimes it’s useful to have a pointer to a thing that we don’t know the type of.
Example use case:

void *memcpy(void *s1, void *s2, size_t n);

This function copies n bytes of memory starting from address s2 into the memory starting at address s1. With the void * type, we can use this function to copy any type.

char s[] = "Goats!";
char t[100];
memcpy(t,s,7); //Copy 7 bytes including the NUL terminator;

int a[] = {11,22,33};
int b[3];
memcpy(b,a,3*sizeof(int));
printf("%d\n", b[1]); // 22

A more practical example:

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    char *name;
    int leg_count;
} animal;

int compar(const void *elem1, const void *elem2){
    const animal *animal1 = elem1;
    const animal *animal2 = elem2;

    if (animal1->leg_count > animal2->leg_count){
        return 1;
    }
    if (animal1->leg_count < animal2->leg_count){
        return -1;
    }
    return 0;
};

int main(){
    animal a[4] = {
            {.name="Dog", .leg_count=4},
            {.name="Monkey", .leg_count=2},
            {.name="Antelope", .leg_count=4},
            {.name="Snake", .leg_count=0}
        };

    qsort(a,4,sizeof(animal), compar);

    for (int i = 0; i < 4 ;i++){
        printf("%d: %s\n", a[i].leg_count, a[i].name);
    }
}

Pointers to Pointers

When we use pointer as an argument in function, only changes in where this pointer is pointing at

#include <stdio.h>
#include <stdlib.h>

void change_pointer(int *p)
{
    printf("[Begin of func]: \n\tValue of p = %d\n\tAddress of p = %p\n", *p, p);
    // 200;

    int *b;
    b = malloc(sizeof(int));
    *b = 100;

    p = b;
    // 100
    printf("[End of func]: \n\tValue of p = %d\n\tAddress of p = %p\n", *p, p);
}

int main(void)
{
    int x = 200;
    int *p = &x;
    printf("-------- CALLING FUNCTION ----------\n");
    change_pointer(p);
    printf("-------- **************** ----------\n");
    printf("[Main]: \n\tValue of p = %d\n\tAddress of p = %p\n", *p, p);
    // p still points to address of x
    // 200;
}

Pointers to Functions

#include <stdio.h>
#include <stdlib.h>

int add(int num1, int num2)
{
    return num1 + num2;
}

int subtract(int num1, int num2)
{
    return num1 - num2;
}

typedef int (*fptrOperation)(int,int);


int compute(fptrOperation operation, int num1, int num2)
{
    return operation(num1, num2);
}

int main(void)
{
    int a = 10;
    int b = 20;
    printf("Result is: %d\n", compute(add, a, b));
}