Pointers

CS157 Pointers

The best (and worst) thing about C

Best?

Very Powerful
- Needed for any complex data structures
- Needed for dynamic memory management
The “Sharpest” tool in your C toolkit.
- Makes a lot of things easier
- Faster, smaller, etc …

Worst?

If used incorrectly it can be bad.
- Hand + Power Drill = Hospital Visit
- Pointers + carelessness = segfault
Segmentation faults are common when you misuse pointers.
But, errors don’t always crash … sometimes they mess up your program later

What is a →pointer←, anyway?

It’s a type (int, float, pointer to int)
There is a pointer type for every type:
- A pointer to an int is int *
- A pointer to a float is float *
They can even point to other pointers!
- A pointer to a pointer to an int is int **
- A pointer to a pointer to a pointer to a pointer to an int is int ****
One or two stars are common; rarely more than that.

Declaring and Initializing

Declare and initialize in one step: int *p = &x;

-or-

Declare: int *p;
Initialize: p = &x;

Yeah, but what is a pointer?

int a = 123;
int *p = &a;

Physically, assuming a lives at location 7689:

      ┌────┐     ┌───┐
    p:│7689│   a:│123│
      └────┘     └───┘

Or, thinking the contents of p as a pointer rather than as a number:

      ┌──┐       ┌───┐
    p:│ ─┼────>a:│123│
      └──┘       └───┘

Yeah, but what is a pointer?

It’s a variable that points to another.

int val;    // regular int
int *ptr;   // pointer to an int

        ┌────┐        ┌────┐
    ptr:│ ?? │    val:│ ?? │
        └────┘        └────┘

Yeah, but what is a pointer?

It’s a variable that points to another.

int val;    // regular int
int *ptr;   // pointer to an int
ptr = &val; // ptr contains the address of val

        ┌──┐         ┌────┐
    ptr:│ ─┼────>val:│ ?? │
        └──┘         └────┘

Yeah, but what is a pointer?

It’s a variable that points to another.

int val;    // regular int
int *ptr;   // pointer to an int
ptr = &val; // ptr contains the address of val
*ptr = 42;  // Whatever ptr refers to becomes 42

        ┌──┐         ┌────┐
    ptr:│ ─┼────>val:│ 42 │
        └──┘         └────┘

Some Examples

      ┌──┐     ┌───┐      ┌──┐
      │ ─┼────>│123│<─────┼─ │
      └──┘     └───┘      └──┘
       p         x          q

int x, *p = &x, *q = &x;
x=4;           printf("x=%d\n", x);
x=x+3;         printf("x=%d\n", x);
*p = 42;       printf("x=%d\n", x);
*q = 8;        printf("x=%d\n", x);
*&x = *q + *p; printf("x=%d\n", x);
x = *q**p;     printf("x=%d\n", x);

x=4
x=7
x=42
x=8
x=16
x=256

And this is useful because …

Let’s say we have a program which has two integers i and j
We want to be able to call a function to sort them such that i has the smaller, and j has the larger value.
How do we swap values in a function without using pointers?
We can’t!

What about this?

void swap(int *a, int *b);
int main() {
    int i=2, j=5;
    swap(&i, &j);
    return 0;
}

      ┌──┐       ┌───┐
    a:│ ─┼────>i:│ 2 │
      └──┘       └───┘
      ┌──┐       ┌───┐
    b:│ ─┼────>j:│ 5 │
      └──┘       └───┘

The swap routine

By using pointers we can make ONE more generic swap routine, using “Pass by reference” instead of “Pass by value”

void swap(int *a, int *b) {
    int tmp = *a;
    *a = *b;
    *b = tmp;
}
int main() {
    int washington=1, lincoln=16;
    swap(&washington, &lincoln);
    printf("%d %d\n", washington, lincoln);
}

16 1

When do we need Pointers?

When passing an array, it automatically passes by reference, so anything we do in the function changes the original—no need to send the address for changing elements

NULL

NULL is a pointer to NOTHING!
ptr = NULL;
This means that ptr isn’t pointing to anything useful.
For example, strchr returns NULL when it can't find the character.

Pointer Arithmetic

pointer + int gives you a pointer
- It moves the pointer forward
  - How much depends on the type of the pointer
pointer - int gives you a pointer
- It moves the pointer backward
pointer - pointer gives you a int
- Gives you the difference between the memory cells, in the proper units
These don't work:
- pointer+pointer, pointer/pointer, etc.

Pointer Arithmetic Example

int data[] = {11,22,33,44,55,66,77,88};
int *p;
p = data;     printf("%d\n", *p);
p += 4;       printf("%d\n", *p);
p--;          printf("%d\n", *p);
p = &data[2]; printf("%d\n", *p);

Subscripting

a[i] is equivalent to *(a+i)
what is the difference between a[i] and i[a]?
Is i[a] legal?

double a[] = {1.2, 3.4, 5.6, 7.8};
printf("%.2f\n", a[2]);
printf("%.2f\n", *(a+2));
printf("%.2f\n", *(2+a));
printf("%.2f\n", 2[a]);

5.60
5.60
5.60
5.60

Array Name is Address

An array name is a pointer!
- It points to the beginning of the cells.
- These are the same: a &a[0]

double a[] = {1.2, 3.4, 5.6, 7.8};
double *p;
p = a;     printf("%.3f\n", *p);
p = &a[0]; printf("%.3f\n", *p);

1.200
1.200

Arrays are pointers

void p3chars(char *p) {
    printf("%c%c%c\n", *p, *(p+1), *(p+2));
}

int main() {
    char buf[] = "abcdefghij";
    p3chars(&buf[0]); // abc
    p3chars(&buf[2]); // cde
    p3chars(buf);     // abc
    return 0;
}

a␜␜3␜␜b␜␜4␜␜c
c␜␜3␜␜d␜␜4␜␜e
a␜␜3␜␜b␜␜4␜␜c

Pointer/Array Notation Correspondence

    char word[100];
    char * wptr = word;  // or char* wptr;  or char *wptr;

`wptr`	`word`	`&word[0]`
`(wptr+n)`	`word+n`	`&word[n]`

`*wptr`	`*word`	`word[0]`
`*(wptr+n)`	`*(word+n)`	`word[n]`

Strings

Strings are char arrays, but arrays are just pointers, so a string is just a pointer to char(s)

String functions

strcpy(char *dest, char *src): copy a string from src to dest
strcat(char *dest, char *src): copy src onto the end of dest
int strcasecmp(const char *s1, const char *s2): less standard: compare s1 to s2, ignoring case

More String Functions

strncpy(char *dest, char *src, int max): copy up to max chars from src to dest
strncat(char *dest, char *src, int max): copy up to max chars from src onto the end of dest
int strncasecmp(const char *s1, const char *s2, int max): compare up to max chars from s1 to s2, ignoring case

String functions

strchr(char *s, char c): Returns a pointer to the first occurrence of the character c in the string s, or NULL if it isn’t there
strstr(char *haystack, char *needle): Returns a pointer to the first occurrence of the string needle in the string haystack, or NULL if it isn’t there!

const char *p = strchr("My dog has fleas", 'g');
const char *q = strstr("My dog has fleas", "has");
printf("p=%s\n", p);
printf("q=%s\n", q);

p=g has fleas
q=has fleas

String Ambiguity

const char *p = "elephant";
const char *q = "elephant";
if (p == q)
    puts("Same");
else 
    puts("Different");

Same

What does it print? It depends on your compiler.
It’s comparing addresses, not string contents.
String constants with the same characters may be stored in the same location, or they might not. It’s up to your compiler.

Returning values

Pointers can also be used to return multiple values, instead of just one.

void func(int *a, int *b, int *c) {
    *a = 1; *b = 4; *c = 9;
}

int main() {
    int number=6, thingy=7, value=8;
    func(&number, &thingy, &value);
    printf("%d %d %d\n", number, thingy, value);
    return 0;
}

1 4 9

Returning multiple values

A function that takes two numbers and returns the quotient and the remainder!

void  long_division(int *quotient_ptr, int *remainder_ptr,
                    int dividend, int  divisor) {
    *quotient_ptr = dividend / divisor;
    *remainder_ptr = dividend % divisor;
}

int main() {
    int quot, rem;
    long_division(&quot, &rem, 40, 3);
    printf("quotient is %d, remainder is %d\n", quot, rem);
    return 0;
}

quotient is 13, remainder is 1

PROBLEMS!

int miles, nums[25];
int *a = &45;         // invalid — attempt to take address of a value
int *b = &(miles+10); // invalid — attempt to take address of a value
int *c = &miles + 10; // semi-valid — 10 is added to address of miles
int *d = &nums;       // invalid — nums is an address, so no address of the address!
int *e = nums;        // valid

PROBLEM! faults

NULL pointers are not to be read from, or written to:

int *p = NULL;
printf("Read from NULL: %d\n", *p);

SIGSEGV: Segmentation fault

int *p = NULL;
puts("Let’s write to a NULL pointer!");
*p = 0;

SIGSEGV: Segmentation fault

CS157: Intro to C, Part II

Spring 2018

Pointers

Pointers

The best (and worst) thing about C

Best?

Worst?

What is a →pointer←, anyway?

Declaring and Initializing

Yeah, but what is a pointer?

Yeah, but what is a pointer?

Yeah, but what is a pointer?

Yeah, but what is a pointer?

Some Examples

And this is useful because …

What about this?

The swap routine

When do we need Pointers?

NULL

Pointer Arithmetic

Pointer Arithmetic Example

Subscripting

Array Name is Address

Arrays are pointers

Pointer/Array Notation Correspondence

Strings

String functions

More String Functions

String functions

String Ambiguity

Returning values

Returning multiple values

PROBLEMS!

PROBLEM! faults