C Programming - Refresher - Part III

Embedded C – Part III
Pointers
Team Emertxe

Pointers : Sharp Knives
Handle with Care!

Pointers : Why
● To have C as a low level language being a high level language.
● To have the dynamic allocation mechanism.
● To achieve the similar results as of ”pass by variable”
parameter passing mechanism in function, by passing the reference.
● Returning more than one value in a function.

Rule #1
Pointer as a integer variable
Example
Syntax
dataType *pointer_name;
Pictorial Representation
10
a
100
100
200
p
DIY:
Declare the float pointer & assign the address of float variable

Rule #2
Referencing & Dereferencing
Example
DIY:
Print the value of double using the pointer.
Variable Address
Referencing
De-Referencing
&
*

Rule #3
Type of a pointer
All pointers are of same size.
l
Pointer of type t t Pointer (t *) A variable which contains an≡ ≡ ≡
address,which when dereferenced becomes a variable of type t

Rule #4
Value of a pointer
Example
10
a
100
100
200
p
Pointing means Containing
l
Pointer pointing to a variable ≡
l
Pointer contains the address of the
variable

Rule #5
NULL pointer
Example
NULL
200
p
Not pointing to anywhere
l
Pointer Value of zero Null Addr NULL≡ ≡
pointer Pointing to nothing≡

Segmentation fault
A segmentation fault occurs when a program attempts to access a memory location
that it is not allowed to access, or attempts to access a memory location in a way
that is not allowed.
Example
Fault occurs, while attempting to write to a read-only
location, or to overwrite part of the operating system

Bus error
A bus error is a fault raised by hardware, notifying an operating system (OS) that a
process is trying to access memory that the CPU cannot physically address: an
invalid address for the address bus, hence the name.
Example
DIY: Write a similar code which creates bus error

Rule #6:
Arithmetic Operations with Pointers & Arrays
l
value(p + i) value(p) + value(i) * sizeof(*p)≡
l
Array Collection of variables vs Constant pointer variable→
l
short sa[10];
l
&sa Address of the array variable→
l
sa[0] First element→
l
&sa[0] Address of the first array element→
l
sa Constant pointer variable→
l
Arrays vs Pointers
l
Commutative use
l
(a + i) i + a &a[i] &i[a]≡ ≡ ≡
l
*(a + i) *(i + a) a[i] i[a]≡ ≡ ≡
l
constant vs variable

Rule #7:
Static & Dynamic Allocation
l
Static Allocation Named Allocation -≡
l
Compiler’s responsibility to manage it – Done internally by compiler,
l
when variables are defined
l
Dynamic Allocation Unnamed Allocation -≡
l
User’s responsibility to manage it – Done using malloc & free
l
Differences at program segment level
l
Defining variables (data & stack segmant) vs Getting & giving it from
l
the heap segment using malloc & free
l
int x, int *xp, *ip;
l
xp = &x;
l
ip = (int*)(malloc(sizeof(int)));

Pointers :
Simple data Types
l
'A'
100 101 102 104 105 . . .
100
chcptr
200
'A'
100 101 102 104 105 . . .
100
chcptr
200
100 101 102 104 105 . . .
100
iiptr
200

Pointers :
Compound data Types
Example: Arrays & Strings (1D arrays)
int age[ 5 ] = {10, 20, 30, 40, 50};
int *p = age;
Memory Allocation
10 20 30 40 50
age[0]
age[4]
age[3]
age[2]
age[1]
100 104 108 112 116

DIY : Write a program to add all the elements of the array.
100
p
age[i] ≡ *(age + i)
i[age] ≡ *(i + age)
≡
≡
age[2] = *(age + 2) = *(age + 2 * sizeof(int))
= *(age + 2 * 4)
= *(100 + 2 * 4)
= *(108)
= 30 = *(p + 2) = p[2]

Pointers :
Compound data Types
int a[ 3 ][ 2 ] = {10, 20, 30, 40, 50, 60};
int ( * p) [ 2 ] = a;
Memory Allocation
p
10 20
30 40
50 60

Pointers :
Compound data Types
int a[ 3 ][ 2 ] = {10, 20, 30, 40, 50, 60};
int ( * p) [ 2 ] = a;

DIY : Write a program to print all the elements of the

2D array.
a[2][1] = *(*(age + 2) + 1) = *(*(a + 2 * sizeof(1D array)) + 1 * sizeof(int))
= *(*(a + 2 * 8) + 1 * 4)
= *(*(100 + 2 * 8) + 4)
= *(*(108) + 4)
= *(108 + 4)
= *(112)
= 40 = p[2][1]
In general :
a[i][j] ≡ *(a[i] + j) ≡ *(*(a + i) + j) ≡ (*(a + i))[j] ≡ j[a[i]] ≡ j[i[a]] ≡ j[*(a + i)]

Dynamic Memory Allocation
l
In C functions for dynamic memory allocation functions are
l
declared in the header file <stdlib.h>.
l
In some implementations, it might also be provided
l
in <alloc.h> or <malloc.h>.
● malloc
● calloc
● realloc
● free

Malloc
l
The malloc function allocates a memory block of size size from dynamic
l
memory and returns pointer to that block if free space is available, other
l
wise it returns a null pointer.
l
Prototype
l
void *malloc(size_t size);

calloc
l
The calloc function returns the memory (all initialized to zero)
l
so may be handy to you if you want to make sure that the memory
l
is properly initialized.
l
calloc can be considered as to be internally implemented using
l
malloc (for allocating the memory dynamically) and later initialize
l
the memory block (with the function, say, memset()) to initialize it to zero.
l
Prototype
l
void *calloc(size_t n, size_t size);

Realloc
l
The function realloc has the following capabilities
l
1. To allocate some memory (if p is null, and size is non-zero,
l
then it is same as malloc(size)),
l
2. To extend the size of an existing dynamically allocated block
l
(if size is bigger than the existing size of the block pointed by p),
l
3. To shrink the size of an existing dynamically allocated block
l
(if size is smaller than the existing size of the block pointed by p),
l
4. To release memory (if size is 0 and p is not NULL
l
then it acts like free(p)).
l
Prototype
l
void *realloc(void *ptr, size_t size);

free
l
The free function assumes that the argument given is a pointer to the memory
l
that is to be freed and performs no heck to verify that memory has already
l
been allocated.
l
1. if free() is called on a null pointer, nothing happens.
l
2. if free() is called on pointer pointing to block other
l
than the one allocated by dynamic allocation, it will lead to
l
undefined behavior.
l
3. if free() is called with invalid argument that may collapse
l
the memory management mechanism.
l
4. if free() is not called on the dynamically allocated memory block
l
after its use, it will lead to memory leaks.
l
Prototype
l
void free(void *ptr);

2D Arrays
Each Dimension could be static or Dynamic
Various combinations for 2-D Arrays (2x2 = 4)
• C1: Both Static (Rectangular)
• C2: First Static, Second Dynamic
• C3: First Dynamic, Second Static
• C4: Both Dynamic
2-D Arrays using a Single Level Pointer

C1: Both static
Rectangular array
int rec [5][6];
Takes totally 5 * 6 * sizeof(int) bytes
Static
Static

C2: First static,
Second dynamic
 One dimension static, one dynamic (Mix of Rectangular & Ragged)
int *ra[5];
for( i = 0; i < 5; i++)
ra[i] = (int*) malloc( 6 * sizeof(int));
Total memory used : 5 * sizeof(int *) + 6 * 5 * sizeof(int) bytes
Static
Dynamic

C3: Second static,
First dynamic
One static, One dynamic
int (*ra)[6]; (Pointer to array of 6 integer)
ra = (int(*)[6]) malloc( 5 * sizeof(int[6]));
Total memory used : sizeof(int *) + 6 * 5 * sizeof(int) bytes
Static
ra

C4: Both dynamic
Ragged array
int **ra;
ra = (int **) malloc (5 * sizeof(int*));
for(i = 0; i < 5; i++)
ra[i] = (int*) malloc( 6 * sizeof(int));
Takes 5 * sizeof(int*) for first level of indirection
Total memory used : 1 * sizeof(int **) + 5 * sizeof(int *) + 5 * 6 *
sizeof(int) bytes
ra[0]
ra[1]
ra[2]
ra[3]
ra[4]
ra

Function pointers : Why
l
● Chunk of code that can be called independently and is standalone
● Independent code that can be used to iterate over a collection of
objects
● Event management which is essentially asynchronous where there
may be several objects that may be interested in ”Listening” such
an event
● ”Registering” a piece of code and calling it later when required.

Function Pointers:
Declaration
Syntax
return_type (*ptr_name)(type1, type2, type3, ...)
Example
float (*fp)( int );
Description:
fp is a pointer that can point to any function that returns a float value and accepts an int as
an argument.

Function Pointers:
As an argument

Function Pointers:
More examples
● The bsearch function in the standard header file <stdlib.h>
void *bsearch(void *key, void *base, size_t num, size_t width,
int (*compare)(void *elem1, void *elem2));
● The last parameter is a function pointer.
● It points to a function that can compare two elements (of the sorted array, pointed by
base) and return an int as a result.
● This serves as general method for the usage of function pointers. The bsearch function
does not know anything about the elements in the array and so it cannot decide how to
compare the elements in the array.
● To make a decision on this, we should have a separately function for it and pass it to
bsearch.
● Whenever bsearch needs to compare, it will call this function to do it. This is a simple
usage of function pointers as callback methods.

Function Pointers:
More examples
● Function pointers can be registered & can be called when the program exits
Output ?

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C Programming - Refresher - Part III

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C Programming - Refresher - Part III