Pointers

A pointer is just a variable that stores a memory address instead of a value. That's it, everything else about pointers is a consequence of that one idea.

#include <stdio.h>

int main(void) {
    int xp = 10;
    int *xpPtr = &xp;   // xpPtr holds the address of xp

    printf("value: %d\n", xp);       // 10
    printf("address: %p\n", (void*)&xp);
    printf("via pointer: %d\n", *xpPtr); // 10, dereferenced

    *xpPtr = 20;    // writes through the pointer
    printf("now: %d\n", xp);         // 20
    return 0;
}

• &xp is the address-of operator, "give me the memory address where xp lives".
• int *xpPtr declares xpPtr as "a pointer to an int".
• *xpPtr is the dereference operator used on an existing pointer, "go to the address this pointer holds, and give me (or set) the value there".

Why bother?

Without pointers, C functions can only work with copies of the arguments you pass in:

#include <stdio.h>

void doubleIt(int n) {
    n = n * 2;   // only changes the local copy
}

void doubleItProperly(int *n) {
    *n = *n * 2; // changes the caller's variable
}

int main(void) {
    int x = 5;
    doubleIt(x);
    printf("%d\n", x);          // still 5
    doubleItProperly(&x);
    printf("%d\n", x);          // now 10
    return 0;
}

Passing a pointer lets a function reach back into the caller's memory and modify it directly, this is how C simulates "pass by reference" since it only ever passes values (and an address is just a value).