C programming - Pointer and DMA

Achyut Devkota
Kathford International College
Pointer and DMA
5/21/2015 3:52 PM1 Prepared by Achyut Dev

Introduction
5/21/2015 3:52 PMPrepared by Achyut Dev2
 Within the computer’s memory, every stored data item
occupies one or more contiguous memory cells.
 Suppose v is a variable that represents some
particular data item.
 The compiler will automatically assign memory cells
for this data item.
 The data item can then be accessed if we know the
location (i.e., the address) of the first memory cell.
 let another variable ptrv
ptrv=&v
where ptrv is a pointer variable which point the memory
address of variable v.

Introduction
FFF1
FFF0
FFF2
FFF3
FFF4
FFF5
FFF6
Addres
s
DataVariable Name
int v=10;
int *ptrv
ptr=&v
v
ptrv
10
FFF3
 data item represented by v
(i.e., the data item stored in
v’s memory cells) can be
accessed by the expression
*ptrv.
* is a unary operator, called the
indirection operator

Example
Outpu
t

Example
Output

 Pointer variables can point to numeric or
character variables, arrays, functions or other
pointer variables.
ptrv=&v
 Also, a pointer variable can be assigned the value
of another pointer variable
ptra=ptrv

Pointer Declaration
Syntax
data_type *ptvar;
Example
float u,v;
float *pv;
 The first line declares u and v to be floating-point
variables.
 The second line declares pv to be a pointer
variable whose object is a floating-point quantity.Note that pv represents an address, not a floating-point
quantity.

Passing pointer to a function
 When an argument is passed by value, the data item
is copied to the function. Thus, any alteration made to
the data item within the function is not carried over
into the calling routine.
 When an argument is passed by reference, however
(i.e., when a pointer is passed to a function), the
address of a data item is passed to the function.
 The contents of that address can be accessed freely,
either within the function or within the calling routine.
 An array name is actually a pointer to the array,
Therefore, an array name is treated as a pointer when
it is passed to a function. No need of & sign in an
arguments.
See tutorial Sheet 04 for more
examples

Passing Array to a function
 It is possible to pass a portion of an array, rather
than an entire array, to a function.
 The address of the first array element to be
passed must be specified as an argument.
 The remainder of the array, starting with the
specified array element, will then be passed to
the function.

•The address of z[50] (i.e.,
&z[50]) is passed to the
function process.
• Hence, the last 50
elements of z (i.e., the
elements z [50] through z
[99]) will be available to
process.

Example
Write a function that count vowels, consonants, digit,
whitespace and other characters in a line of text
which is entered through the keyboard. Display
results of counts in main() function.
Calling function:
scanline(line, &vowels, &consonants, &digits, &whitespc,
&other);
Function Header
void scanline(char line[] , int *pv, int *pc,int *pd,int *pw,int *po)
OR
void scanline(char *line, int *pv, int *pc,in t*pd,int *pw,int
*po)
declare line as a pointer rather than an array

Example
Output

Example
Output
Ans:
unary operators are evaluated
from right to left. Thus address
is increase itself.

Pointer –scanf()
 The scanf function requires that the addresses
of the data items being entered into the
computer's memory be specified
 The ampersands provide a means for accessing
the addresses of ordinary single-valued variables.
scanf("%d %f %s",&number,&value,line);
 Ampersands are not required with array names,
since array names themselves represent
addresses.
 If the scanf function is used to enter a single
array element rather than an entire array, the
name of the array element must be preceded by
an ampersand
scanf("%d“,&array[10]);

Pointer and 1-D array
 Array name is a pointer to the first element in the
array.
 if x is a one dimensional array, then the address
of the first array element can be expressed as
either &x[0]or simply as x.
 the address of the second array element can be
written as either &x[1] or as (x+1), and so on.
 In general, the address of array element (i+1) can be
expressed as either &x[i]or as (x+i).
 Array has int, float, double .. data type.Different Data type occupies different memory size
Is there
any
proble
m ?
No problem-C compiler adjusts for this
automatically

 &x [i] and (x + i)both represent the address of the
ith element of x.
 it would seem reasonable that x[i]and *(x+i) both
represent the contents of that address, i.e., the
value of the ith element of x.

Example
Output:

What is the difference between x[i] and*(x+i)?
value of an
array element
value of the
memory area
whose
address is
that of the
array element
 It is not possible to assign
an arbitrary address to an
array name or to an array
element.
 expressions such as x, (x+i)
and &x [i] cannot appear on
the left side of an
assignment statement.
Can we write
++x ?

Example
Output:

 Note : that the address of one array element
cannot be assigned to some other array element.
&line[2] = &line[l]; Error
ptrl=&line[l];
line[2]=*ptrl;
OR
ptrl=line+1;
*(line+2)=*ptrl;
What about these ?
We can do


Problem 1
 Write a program that takes a one-dimensional
array of ‘n’ elements. Pass it to a function that
sorts them in ascending order.

Problem 2
 Write a function that takes an array of size n and
finds the largest adjacent sum and display it in
main function. For example, if given array is {1.2.-
3.-1.6}, then called function must return 5.

Memory allocation
 Two types:
1. Compile time memory allocation or design time
money allocation or static memory allocation.
1. No of memory space is preserved.
2. Disadvantage
1. If the user’s data is less than the reserved bytes, there is
wastage of memory bytes.
2. If the user’s data exceeds the reserved bytes, then
memory overflow occurs.
eg. int array[20]; char line[100];
2. Run time memory allocation or Dynamic
memory allocation.

Dynamic Memory allocation
 An array name is actually a pointer to the first element
within the array
 it should be possible to define the array as a pointer
variable rather than as a conventional array
 A conventional array definition results in a fixed block
of memory being reserved at the beginning of
program execution, whereas this does not occur if the
array is represented in terms of a pointer variable.
 Therefore, the use of a pointer variable to represent
an array requires some type of initial memory
assignment before the array elements are processed.
This is known as dynamic memory allocation

 Suppose x is one dimensional array
we can write
int *x;
rather than
int x[10];
To assign sufficient memory
for x, we can make use of
the library function malloc,
as follows.
x =(int*)malloc(l0*sizeof(int));
more…
x =(int*)calloc(l0,sizeof(int));
x =(int*)realloc(x,20*sizeof(int));
x is not automatically assigned a
memory block when it is defined
as a pointer variable
a block of memory large enough to
store 10 integer quantities will be
reserved in advance when x is
defined as an array

 If the declaration is to include the assignment
of initial values, however, then x must be
defined as an array rather than a pointer
variable.
For example,
int x[10]=(1,2,3,4,5,6,7,8,9,10);
or
int x[]=(1,2,3,4,5,6,7,8,9,10);

Example
void sort_ascending(int*,int*);
void main()
{
int i,e,*a;
printf("Enter no of elements of
matrix:");
scanf("%d",&e);
a=(int*)malloc(e*sizeof(int));
printf("Enter Elements of Matrix ");
for (i=0;i<e; i++)
{ printf("A[%d] : ",i);
scanf("%d",&a[i]);
}
sort_ascending(a,&e);
printf("nnAscending order :n");
for (i=0;i<e;i++)
printf("%d ",a[i]);
getch();
}
void sort_ascending(int *a, int*x)
{
int i,j,temp;
for(i=0;i<*x-1;i++)
{
for(j=i+1; j<*x; j++)
{
if(*(a+i)>*(a+j))
{
temp=*(a+i);
*(a+i)=*(a+j);
*(a+j)=temp;
}
}
}
}

Why DML ?
 An important advantage of dynamic memory
allocation is the ability to reserve as much
memory as may be required during program
execution, and then release this memory when it
is no longer needed.
 Moreover, this process may be repeated many
times during execution of a program.

Pointer operation
 an integer value can be added to or subtracted
from a pointer variable
 for example, that px is a pointer variable that
represents the address of some variable x. We
can write expressions such as ++px, --px, (px+ 3
),(px+i),and (px-i),where i is an integer variable.

Pointer and Multi-dimentional
array
 A two-dimensional array is actually a collection of
one-dimensional arrays.
 Therefore, we can define a two-dimensional array
as a pointer to a group of contiguous one-
dimensional arrays.
Array declaration
data-type(*ptvar)[expression2];
for higher D-array
data-type(*ptvar)[expression2][expression3]
. . . [expression n];
These parentheses must be
present.
Without parentheses
we would be defining
an array of pointers
rather than a pointer to
a group of arrays

Example
int (*x)[20]; x is defined to be a pointer to a group
of contiguous, one-dimensional, 20-
element integer arrays.
Thus, x points to the first 20-element array, which is
actually the first row (i.e., row 0) of the original two-
dimensional array. Similarly, ( x + 1) points to the second
20-element array, which is the second row (row 1) of the
original two dimensional array, and so on.
For 3-d array
float (*t)[20][30]
Similar for higher d-
array
…

2-D pointer
 we can write
x[2][5]
as
(*(x+2)+5)
 (x + 2) is a pointer to row 2
 ( x + 2) is actually a pointer
to the first element in row 2
 (* (x + 2) + 5) is a pointer
to element 5 in row 2

Problem
 Write three different function a) readmatrix()-
to read elements of array b) processmatrix() -
to perform certain calculation and c)
showmatrix () - to display result and find
i) Sum of two matrix
ii) Multiplication

Problem
Function Declaration
Function definition
Calling Function

Array of Pointer
 A multidimensional array can be expressed in
terms of an array of pointers rather than a pointer
to a group of contiguous arrays.
For two dimensional array
data-type*array[expression1];
rather then
data-type array[expression1][expression2];
for higher d-array
data-type*array[expression1][expression2)...
[expression n-1];
array name and its
preceding asterisk
are not enclosed
in parentheses

Array of Pointer
data-type*array[expression1];
 a right-to-left rule first associates the pairs of
square brackets with array, defining the named
object as an array.
 The preceding asterisk then establishes that the
array will contain pointers.
 Note that the last (i.e., the rightmost) expression
is omitted when defining an array of pointers,
 whereas the first (i.e., the leftmost) expression is
omitted when defining a pointer to a group of
arrays.

Array of Pointer
Fig show the array of
pointer : int*x[l0];
x[2][5], can be accessed
by writing : *(x[2]+5)
x[2] is a
pointer to the
first element
in row
( x[2J+5) points to
element 5 (actually, the
sixth element) within row
2.

Calculate the sum of the elements in two tables of
integersFunction Declaration
Function definition
Calling Function
Dynamic memory allocation

More…
int*p(int a); : indicates a function that accepts an
integer argument, and returns a pointer to
an integer.
int(*p)(int a);: indicates a pointer to a function
that accepts an integer argument and returns an
integer.
int*(*p)(int(*a)[]);
(*p)(...)indicates a pointer to a function.
int*(*p)(...)indicates a pointer to a function
that returns a pointer to an integer.
int(*a)[]represents a pointer to an array of
integers.
represents a pointer to a function that accepts a pointer to
an array of integers as an argument, and returns a pointer to
an integer.

Class work
 WAP to multiply two matrix using following
concept for passing array to a function that
multiply two matrix.
i) Array of a pointer
ii) Pointer of an array (use DMA to allocate
appropriate memory)

C programming - Pointer and DMA

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