Pointers

Addresses in Memory Everything in memory has an address. C allows us to obtain the address that a variable is stored at. scanf() is an example using the address of a variable scanf("%d", &year);

Addresses in Memory Preceding a variable name by an ampersand, known as the address operator, will return its address: #include <stdio.h> int main(void) { int x; /* notice the format specifier in printf() for an address is %p */ printf("The address for the memory allocated to x is %p\n", &x); return 0; }

hexadecimal (or called base-16) 0123456789ABCDEF The address for the memory allocated to x is 0012FF60 in the previous example. For 32 bits system the 0012FF60 hexadecimal will be converted to 32 bits binary (one hex character represented by 4 bits).

Pointer A pointer is a variable whose value is a memory address. Note the type of a pointer indicates the type of variable which it points to. E.g., int * (called pointer-to-int type) should be initialized to point to a variable of type int. Similarly, we have char*, float *, …… Given a pointer variable, assignment can be done by using: int * ptr = &pooh; /*assigns pooh’s address to ptr, we say ptr points to pooh*/ ptr = &bah; /*make ptr point to some other variables*/

#include<stdio.h> int main(void) { int num = 3, num1 = 5; int* numptr; /* numptr is a pointer */ printf("content of num is %d\n", num); printf("address of num is %p\n", &num); printf("address of num1 is %p\n", &num1); numptr = &num; /* initialize numptr with the address of num */ printf("content of numptr is %p\n", numptr); numptr = &num1; return 0; }

#include<stdio.h> int main(void) { int num = 3, num1 = 5; int* numptr; /* numptr is a pointer */ printf("content of num is %d\n", num); printf("address of num is %p\n", &num); printf("address of num1 is %p\n", &num1); numptr = &num; /* initialize numptr with the address of num */ printf("content of numptr is %p\n", numptr); numptr = &num1; return 0; } Output: content of num is 3 address of num is 0012FF60 address of num1 is 0012FF54 content of numptr is 0012FF60 content of numptr is 0012FF54 Press any key to continue . . .

Why pointers C was developed when computers were much less powerful The raw ability to work with particular memory locations was obviously a useful option to have. Programming microcontrollers still need this. Optimize a program to run faster or use less memory that it would otherwise.

#include<stdio.h> int main(void) { int arr[3] = {2, 4, 6}, i; for(i = 0; i<3; i++) printf("The address of %d element is %p.\n", i, &arr[i]); return 0; } The address of 0 element is 0032FE08. The address of 1 element is 0032FE0C. The address of 2 element is 0032FE10.

Indirection operator Pointers allow us to modify content in memory by using indirection operator (or called dereference operator). Putting an asterisk before your pointer variable See the example in the next slide

What’s going here. #include <stdio What’s going here? #include <stdio.h> int main(void) { int bah = 10, val; int* ptr = &bah; val = *ptr; printf("The value of val is %d.\n", val); return 0; }

Pointers We can use pointers in much the same way we do the variables that they point to. #include <stdio.h> int main(void) { int a = 3, b = 3; /* a and b start with equal values */ int* bptr = &b; /* we’ll modify b using a pointer */ a *= 4; *bptr *= 4; printf("a is %d, b is %d\n", a, b); a--; (*bptr)--; /* parentheses are necessary here to override the order of precedence */ return 0; }

Output: a is 12, b is 12 a is 11, b is 11

Define multiple pointers together Wrong! If you use int * a, b to define two integer pointer variables. Instead, you should use int * a, *b; #include<stdio.h> int main(void) { int * a, *b;//it’s wrong if int *a,b int c = 10; b = &c; a = &c; printf("b: %p and a: %p\n", b, a); return 0; }

Pointers to Pointers Pointers can contain the address of another pointer. int main(void) { int num = 5; int* numptr = &num; int** ptr2 = &numptr; /* notice the two asterisks */ printf("num is %d\n", num); printf("*numptr is %d\n", *numptr); printf("The content in numptr is %p.\n", numptr); printf("*ptr2 is %p\n", *ptr2); printf("**ptr2 is %d\n", **ptr2); return 0; }

Output num is 5 *numptr is 5 The content in numptr is 0012FF60

Comparing Pointers Note the difference between comparing the contents of pointers and the variables that pointers point to. To compare the addresses stored in pointers, use if(numptr == valptr) To compare the values of the variables that pointers point to, use if(*numptr == *valptr)

Pointers and Functions Previously, we made function calls like this: int x = 3; int y; y = do_something(x); In this case, a copy of the variable’s value are passed to the function in a process called pass by value.

Pointers and Functions Passing pointers to a function will allow us to change the value of the original variable, called pass by reference, We do this by passing the variable’s address to the function. We already know passing array to a function is pass by reference. What’s the association between array and pointer?????

Pointers and Functions #include<stdio.h> void interchange(int * u, int * v); int main(void) { int x = 5, y = 10; printf("Originally x = %d and y = %d.\n", x,y); interchange(&x, &y); printf("Now x = %d and y = %d.\n", x,y); return 0; } void interchange(int * u, int * v) int temp; temp = *u; *u = *v; *v = temp;

Pointers and Functions #include <stdio.h> void tripleNum(int* aptr); int main(void) { int num = 8; int* numptr = &num; printf("before the function call, num is %d\n", num); tripleNum(numptr); /*or simply use &num*/ printf("after the function call, num is %d\n", num); } void tripleNum(int* aptr) /* pass by reference */ *aptr = 3 * *aptr; /* first asterisk is for multiplication, second is to dereference the pointer */

Array review We have already learned how to work with arrays using subscript notation. Example: float myData[] = {3.5, 4.0, 9.34}; myData[0] += 2; printf("myData[0] is %f\n", myData[0]);

Pointers and Arrays The array name evaluates to the address of the first element in the array. int data[] = {5, 6, 7}; int* dataptr = data; /* notice that we don’t use the ampersand here */ int* firstptr = &data[0]; /* we use & here since data[0] evaluates to a number */ printf("*dataptr is %d, *firstptr is %d\n", *dataptr, *firstptr);

Pointer Arithmetic Since arrays consist of contiguous memory locations, we can increment (or decrement) the addresses to move through the array. int data[] = {5, 6, 7}; int i; for (i = 0; i < 3; i++) printf("the value at address %p is %d\n", (data + i), *(data + i));

Pointer Arithmetic We can use pointer arithmetic with pointers to non-array int data[] = {5, 6, 7}; int* dataptr = data; printf("pointer address = %p, next address = %p\n", dataptr, dataptr + 1);

Pointer Arithmetic #include <stdio.h> int main(void) { char chararray[] = {68, 97, 114, 105, 110}; /* 1 byte each */ int intarray[] = {10, 11, 12, 13, 14}; /* 4 bytes each */ int i; printf("chararray intarray\n"); printf("-------------------\n"); for(i = 0; i < 5; i++) printf("%p, %p\n", (chararray + i), (intarray + i)); return 0; }

chararray intarray ------------------- 0012FF58, 0012FF3C 0012FF59, 0012FF40 0012FF5A, 0012FF44 0012FF5B, 0012FF48 0012FF5C, 0012FF4C Press any key to continue . . .

Array of pointers We can have an array whose members are pointers, in this example pointers-to-int. int* data[3]; int i; int x = 5; int y = 89; int z = 34; data[0] = &x; data[1] = &y; data[2] = &z; for(i = 0; i < 3; i++) printf("%d\n", *data[i]);

#include<stdio.h> #include<string.h> int main() { printf("The address of the string is %p.\n", "apple"); printf("The length of the string is %d.\n", strlen("Hello")); return 0; } Output: The address of the string is 00415744. The length of the string is 5. Press any key to continue . . .