Arrays and Pointers in C Each element same size ... A basic type in C int *ptr; The variable...

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Arrays and Pointers in C Alan L. Cox [email protected]

Transcript of Arrays and Pointers in C Each element same size ... A basic type in C int *ptr; The variable...

Page 1: Arrays and Pointers in C Each element same size ... A basic type in C int *ptr; The variable “ptr” stores a pointer to an “int”. ... Arrays and Pointers in C

Arrays and Pointers in C

Alan L. [email protected]

Page 2: Arrays and Pointers in C Each element same size ... A basic type in C int *ptr; The variable “ptr” stores a pointer to an “int”. ... Arrays and Pointers in C

Objectives

Be able to use arrays, pointers, and strings in C programs

Be able to explain the representation of these data types at the machine level, including their similarities and differences

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Arrays in C

No bounds checking!Allowed – usually causes no obvious error array[10] may overwrite b

Unlike Java, array size in declaration

int array[10];

int b;

array[0] = 3;

array[9] = 4;

array[10] = 5;

array[-1] = 6;

Compare: C: int array[10];

Java: int[] array = new int[10];

All elements of same type – homogenous

First element (index 0)Last element (index size - 1)

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Array Representation

Homogeneous Each element same size – s bytes An array of m data values is a sequence of ms bytes Indexing: 0th value at byte s0, 1st value at byte s1, …

m and s are not part of representation Unlike in some other languages s known by compiler – usually irrelevant to programmer m often known by compiler – if not, must be saved by

programmer

a[0]

a[1]

a[2]

0x1000

0x1004

0x1008

int a[3];

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Array Representation

char c1;int a[3];char c2;int i;

c1

a[0]

a[1]

a[2]

i

0x1000

0x1004

0x1008

0x100C

0x1014

c20x1010

Could be optimized by making these

adjacent, and reducing padding (by default, not)

Array aligned bysize of elements

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Array Sizes

What is

sizeof(array[3])?

sizeof(array)?

int array[10];

4

40

returns the size of an object in bytes

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Multi-Dimensional Arrays

int matrix[2][3];

matrix[1][0] = 17;

matrix[0][0]

matrix[0][1]

matrix[0][2]

0x1000

0x1004

0x1008

matrix[1][0]

matrix[1][1]

matrix[1][2]

0x100C

0x1010

0x1014

Recall: no bounds checking

What happens when you write:

matrix[0][3] = 42;

“Row Major”Organization

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Variable-Length Arrays

int

function(int n)

{

int array[n];

…

New C99 feature: Variable-length arraysdefined within functions

Global arrays must still have fixed (constant) length

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Memory Addresses

Storage cells are typically viewed as being byte-sized

Usually the smallest addressable unit of memory• Few machines can directly address bits individually

Such addresses are sometimes called byte-addresses

Memory is often accessed as words Usually a word is the largest unit of memory

access by a single machine instruction• CLEAR’s word size is 8 bytes (= sizeof(long))

A word-address is simply the byte-address of the word’s first byte

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Pointers

Special case of bounded-size natural numbers Maximum memory limited by processor word-size 232 bytes = 4GB, 264 bytes = 16 exabytes

A pointer is just another kind of value A basic type in C

int *ptr;

The variable “ptr” stores a pointer to an “int”.

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Pointer Operations in C

Creation& variable Returns variable’s memory address

Dereference* pointer Returns contents stored at address

Indirect assignment* pointer = val Stores value at address

Of course, still have...

Assignmentpointer = ptr Stores pointer in another variable

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Using Pointers

int i1;

int i2;

int *ptr1;

int *ptr2;

i1 = 1;

i2 = 2;

ptr1 = &i1;

ptr2 = ptr1;

*ptr1 = 3;

i2 = *ptr2;

i1:

i2:

ptr1:

0x1000

0x1004

0x1008

…

ptr2:

…

0x100C

0x1010

0x1014

1

2

0x1000

0x1000

3

3

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Using Pointers (cont.)

Type check warning: int_ptr2 is not an int

int1 becomes 8

int int1 = 1036; /* some data to point to */int int2 = 8;

int *int_ptr1 = &int1; /* get addresses of data */int *int_ptr2 = &int2;

*int_ptr1 = int_ptr2;

*int_ptr1 = int2;

What happens?

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Using Pointers (cont.)

Type check warning: *int_ptr2 is not an int *

Changes int_ptr1 – doesn’t change int1

int int1 = 1036; /* some data to point to */int int2 = 8;

int *int_ptr1 = &int1; /* get addresses of data */int *int_ptr2 = &int2;

int_ptr1 = *int_ptr2;

int_ptr1 = int_ptr2;

What happens?

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Pointer Arithmetic

pointer + number pointer – number

E.g., pointer + 1 adds 1 something to a pointer

char *p;char a;char b;

p = &a;p += 1;

int *p;int a;int b;

p = &a;p += 1;In each, p now points to b

(Assuming compiler doesn’t reorder variables in memory)

Adds 1*sizeof(char) to the memory address

Adds 1*sizeof(int) to the memory address

Pointer arithmetic should be used cautiously

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A Special Pointer in C

Special constant pointer NULL Points to no data Dereferencing illegal – causes segmentation fault

To define, include <stdlib.h> or <stdio.h>

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Generic Pointers

void *: a “pointer to anything”

Lose all information about what type of thing is pointed to

Reduces effectiveness of compiler’s type-checking Can’t use pointer arithmetic

void *p;int i;char c;p = &i;p = &c;putchar(*(char *)p);

type cast: tells the compiler to “change” an object’s type (for type checking purposes – does not modify the object in any way)

Dangerous! Sometimes necessary…

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Pass-by-Reference

voidset_x_and_y(int *x, int *y){

*x = 1001; *y = 1002;}

voidf(void){ int a = 1; int b = 2;

set_x_and_y(&a, &b);}

1

2

a

b

x

y

1001

1002

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Arrays and Pointers

Dirty “secret”:Array name a pointer to the initial (0th) array element

a[i] *(a + i)

An array is passed to a function as a pointer

The array size is lost!

Usually bad style to interchange arrays and pointers

Avoid pointer arithmetic!

Really int *array

int

foo(int array[],

unsigned int size)

{

… array[size - 1] …

}

int

main(void)

{

int a[10], b[5];

… foo(a, 10)… foo(b, 5) …

}

Must explicitlypass the size

Passing arrays:

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Arrays and Pointers

int

foo(int array[],

unsigned int size)

{

…

printf(“%d\n”, sizeof(array));

}

int

main(void)

{

int a[10], b[5];

… foo(a, 10)… foo(b, 5) …

printf(“%d\n”, sizeof(a));

}

What does this print?

What does this print?

8

40

... because array is reallya pointer

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Arrays and Pointers

int i;

int array[10];

for (i = 0; i < 10; i++)

{

array[i] = …;

}

int *p;

int array[10];

for (p = array; p < &array[10]; p++)

{

*p = …;

}

These two blocks of code are functionally equivalent

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Strings

In C, strings are just an array of characters Terminated with ‘\0’ character Arrays for bounded-length strings Pointer for constant strings (or unknown length)

char str1[15] = “Hello, world!\n”;char *str2 = “Hello, world!\n”;

H e l l o , w lo r d !\nlength

H e l l o , w lo r d !\nterminator

Pascal, Java, …

C, …

C terminator: ’\0’

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String length

Must calculate length:

Provided by standard C library: #include <string.h>

int

strlen(char str[])

{

int len = 0;

while (str[len] != ‘\0’)

len++;

return (len);

}

can pass anarray or pointer

Check for terminator

array access to pointer!

What is the size of the array???

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Pointer to Pointer (char **argv)

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Passing arguments to main:

int

main(int argc, char **argv)

{

...

}

an array/vector of char *

Recall when passing an array, a pointer to the first element is passed

size of the argv array/vector

Suppose you run the program this way

UNIX% ./program hello 1 2 3

argc == 5 (five strings on the command line)

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char **argv

argv[0]

argv[1]

argv[2]

0x1000

0x1008

0x1010

argv[3]

argv[4]

0x1018

0x1020

“./program”

“hello”

“1”

“2”

“3”

These are strings!!Not integers!

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Next Time

Structures and Unions


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