4 - Introduction to Pointers in C

<aside> 💡 C was designed to give programmers “low-level” access to memory and expose the underlying memory model

</aside>

// This program demonstrates how to use the address operator

#include "cs136.h"

int g = 42;

int main(void) {
	printf("the value of g is    %d\\n", g);
	printf("the address of g is  %p\\n", &g);

	// to define a pointer, place a star (*) before the identifier
	int *pg = &g;
}

Aliasing occurs when the same memory address can be accessed from more than one pointer variable

C Inputs & Pointers

// read_sum() reads ints from input (until failure)
//   and returns their sum
// effects: read input

int read_sum(void) {
	int sum = 0;
	int n = 0;
	while (scanf("%d", &n) == 1) {
		sum += n;
	}
	return sum;
}

Use scanf(” %c”, &c) to omit whitespace

Const pointers

*int p;	p can point at any mutable integer, you can modify the int (via *p)
*const int p;	p can point at any integer, you can NOT modify the integer (via *p)
*int const p = &i;**	p always points at the integer i, i must be mutable and can be modified
*const int const p = &i;**	p always points at the integer i, you can not modify i (via *p)

Documenting Side Effects

As a recap, we now have a fourth side effect that a function may have:

Produce output
Read input
Mutate a global variable
Mutate a variable through a pointer parameter

(Make sure you remember do document them during the exam!)

https://online.cs.uwaterloo.ca/assets/courseware/v1/38297a5bb5b49d85c7a18e0cc4acca70/asset-v1:UW+CS136+2023_01+type@asset+block/04-pointers-show.pdf