Pointers

#include <stdio.h>

int main(void) {
    int count = 2;
    int *ptr;          /* declare pointer (uninitialized — danger!) */
    ptr = &count;     /* make ptr point to count */

    /* Three ways to see what's happening */
    printf("count = %d\n",  count);      /* 2: value of count */
    printf("*ptr  = %d\n",  *ptr);       /* 2: value AT address ptr holds */
    printf("&count = %p\n", (void*)&count); /* address of count */
    printf("ptr    = %p\n", (void*)ptr);    /* same address (ptr == &count) */

    /* Modify count through the pointer */
    *ptr = 99;
    printf("After *ptr=99, count = %d\n", count);  /* 99 */

    /* Pointer arithmetic */
    int arr[] = {10, 20, 30, 40};
    int *p = arr;       /* points to arr[0] = 10 */
    printf("%d %d %d\n", *p, *(p+1), *(p+2));  /* 10 20 30 */

    /* sizeof any pointer is 8 on 64-bit */
    printf("sizeof(int *) = %zu\n", sizeof(int *));   /* 8 */
    printf("sizeof(char*) = %zu\n", sizeof(char *));  /* 8 */

    return 0;
}

#include <stdio.h>

/* Pass by value — does NOT work */
void swap_fail(int a, int b) {
    int tmp = a;
    a = b;
    b = tmp;
    /* a and b are local copies — caller's variables unchanged */
}

/* Pass by pointer — WORKS */
void swap(int *a, int *b) {
    int tmp = *a;   /* save value at a's address */
    *a = *b;        /* write b's value to a's address */
    *b = tmp;       /* write saved value to b's address */
}

int main(void) {
    int x = 5, y = 10;

    swap_fail(x, y);
    printf("After swap_fail: x=%d, y=%d\n", x, y);  /* still 5, 10 */

    swap(&x, &y);   /* pass addresses so swap can modify originals */
    printf("After swap:      x=%d, y=%d\n", x, y);  /* 10, 5 */

    return 0;
}

#include <stdio.h>

int main(void) {
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;           /* points to first element */
    int *end = arr + 5;     /* one past the last element */

    /* Pointer + n advances by n * sizeof(int) bytes */
    printf("p+0: %d\n", *(p + 0));  /* 10 */
    printf("p+2: %d\n", *(p + 2));  /* 30 */
    printf("p+4: %d\n", *(p + 4));  /* 50 */

    /* Pointer subtraction gives number of elements between */
    int *p2 = &arr[3];
    printf("p2 - p = %td\n", p2 - p);  /* 3 elements apart */

    /* Iterate with pointer increment */
    for (int *it = arr; it < end; it++) {
        printf("%d ", *it);
    }
    printf("\n");

    /* NULL check before dereference */
    int *q = NULL;
    if (q != NULL) {
        printf("%d\n", *q);  /* safe — only runs if q is valid */
    } else {
        printf("q is NULL, not dereferencing\n");
    }

    return 0;
}