03-arrays-pointers (1).ppt

Arrays and Pointers in C
Alan L. Cox
alc@rice.edu

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
Cox Arrays and Pointers 2

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)

Array Representation
Homogeneous  Each element same size – s bytes
 An array of m data values is a sequence of ms bytes
 Indexing: 0th value at byte s0, 1st value at byte s1, …
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];

Array Representation
char c1;
int a[3];
char c2;
int i;
c1
a[0]
a[1]
a[2]
i
0x1000
0x1004
0x1008
0x100C
0x1014
c2
0x1010
Could be optimized by
making these adjacent,
and reducing padding
(by default, not)
Array aligned by
size of elements

Array Sizes
What is
sizeof(array[3])?
sizeof(array)?
int array[10];
4
40
returns the size of
an object in bytes

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

Variable-Length Arrays
int
function(int n)
{
int array[n];
…
New C99 feature: Variable-length arrays
defined within functions
Global arrays must still have fixed (constant) length

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

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”.

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...
Assignment
pointer = ptr Stores pointer in another variable

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

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?

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?

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

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>

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…

Pass-by-Reference
void
set_x_and_y(int *x, int *y)
{
*x = 1001;
*y = 1002;
}
void
f(void)
{
int a = 1;
int b = 2;
set_x_and_y(&a, &b);
}
1
2
a
b
x
y
1001
1002

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 explicitly
pass the size
Passing arrays:

Arrays and Pointers
int
foo(int array[],
unsigned int size)
{
…
printf(“%dn”, sizeof(array));
}
int
main(void)
{
int a[10], b[5];
… foo(a, 10)… foo(b, 5) …
printf(“%dn”, sizeof(a));
}
What does this print?
What does this print?
8
40
... because array is really
a pointer

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

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 l
o r d !n
length
H e l l o , w l
o r d !nterminator
Pascal, Java, …
C, …
C terminator: ’0’

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 an
array or pointer
Check for
terminator
array access
to pointer!
What is the size
of the array???

Pointer to Pointer (char **argv)
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)

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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