pointers, dynamic memory allocation, calloc

pointers, dynamic memory allocation, calloc

• 1.
  1 Pointers and Dynamic MemoryAllocations
• 2.
  Content Introduction to pointers Declarationof pointer Accessing a variable through pointer Chain of pointers Pointer expression Pointers and Array Pointer to function Pointer to structure Trouble with pointers Memory allocation Dynamic Memory Allocation Refrences 2
• 3.
  3 Computer use theirmemory for storing instructions of the programs. Pointers are more efficient in handling arrays and data types It allows C to support dynamic memory management It reduces length and complexity of programs Pointer Declaration
• 4.
  4 Pointer is avariable which can hold the address of a memory location The value stored in a pointer type variable is interpreted as an address Pointer
• 5.
  5 It contain addressthat belongs to separate data type It can be declared just like any other variables It takes the following form: data_type *pointer_name; The above statement tells the compiler three things above the variables pointer_name Pointer Declaration
• 6.
  6 1. The asterisk(*)tells that the variables pt_name is a pointer variable. 2. pt_name needs a memory location. 3. pt_name points to a variable of type data type. Example: Pointer declaration Interpretation Int *rollnumber; Create a pointer variables rollnumber capable of pointing to an integer type variable or capable of holding the address of an integer type variable Char *name; Create a pointer variable name capable of pointing to a character type variable or capable of holding the address of a character type Float *salary; Create a pointer variable salary capable of pointing to a float type variable or capable of holding the address of a float type variable
• 7.
  7 The process ofassigning the address of a variables to a pointer variable called initialization As pointer out earlier, all uninitialized pointers will have same unknown values that will be interpreted as memory address It takes the following form: datatype*pointer_name = & ordinary_variabl e_name Initialization of Pointer variables
• 8.
  8 Example: #include<stdio.h> #include<conio.h> void main() { int x,*p; printf("Enter a number: "); scanf("%d",&x); p = &x; printf("n value of x=%d",*p); printf("n Address of x using pointer = %u",p); printf("n Address of x using & pointer = %u",&x); getch(); }
• 9.
  9 Unary operator alsoknown as indirect operator * (asterick) used to access the value of variable using pointer. data_type *p=& ordinary_variable_name eg :- int quantity ,*p , n quantity =123; p=&quantity; n=*p; #include<stdio.h> #include<conio.h> void main() { int x, *p; printf("Enter a number: "); scanf("%d",&x); p = &x; printf("n value of x=%d",*p); printf("n Address of x using pointer = %u",p); printf("n Address of x using & pointer = %u",&x); getch(); } Accessing a variable through its pointer
• 10.
  10 • Chain ofpointer can be created by pointing a pointer to another pointer. P₂ P₁ variable • Here pointer variable P contains the address of the pointer variable P , which ₂ ₁ points to the location that contains the desire value . • also known as multiple indirections operators. Address 2 Address 1 value Chain of pointers
• 11.
  11 Example void main() { int x,*p1,**p2; x=100; p1=&x; p2=p1; printf(“%d”,**p2); }
• 12.
  12 • Pointer variablescan be used in expression. • C allows us to add integers to or subtract integers from pointer , as well as allows to subtract one from another. like , p1+p2 , p1-p2 etc • Two pointers can also be compared using the relational operators . like , p1>p2 , p1==p2 etc • However pointer cannot be use in multiple or division and also cannot be added. Pointer expression
• 13.
  13 Syntax: data_type *pointer_name[size] Example:- void main() { int*p[3] int a=2, b=3, c=4; p[0]=&a; p[1]=&b; p[2]=&c; for(i=0;i<3;i++) { printf (“value =%d”,*p[i]) } } Pointers and Array → ARRAY OF POINTER
• 14.
  14  Array nameitself is a pointer that points to it’s zeroth element Example:- int a[5] the array name a → &a[0]  When the pointer a is incremented by 1 it gives address of oneth element and so on i.e. a+1→ &a[1] a+2→ &a[2] a+i→ &a[i] Thus *(a+i)→ a[i] Example :- void main() { int a[2],i; printf(“input array elements:”); for (i=0;i<5;i++) { scanf(“%d”,a+1); } printf(”n array elements are:”); for (i=0;i<5;i++) { printf(“%d t”,*(a+i)); } } Relation between Array and Pointers
• 15.
  15 Pointer and string Stringsare character arrays for string syntax char str[6]=”hello” → null character at the end of string but in pointer to string char *str = ”hello” char *cards[ 4 ] = {"Hearts", "Diamonds", "Clubs",”spades” }; → here it represents that the strings are not in the array, only pointers to the strings are in the array
• 16.
  16  Call by referencemethod is used  The address of data items is passed to the function  Data items can be altered globally from within the function Pointer to a Function
• 17.
  17 Example: #include<stdio.h> void swap (int*p,int*q); int main() { int x,y; x=50; y=70; printf("nValue of x and y before swap are : %dt%d",x,y); swap(&x,&y); printf ("Values of x and y after swap are: %dt%d",x,y); return 0; } void swap(int*p, int*q) { int temp; temp=*p; *p=*q; *q=temp; } Values of x and y before swap are : 50 70 Values of x and y after swap are : 70 50
• 18.
  18  pointers pointing toa struct are known as "Structure Pointers"  address of a structure variable is assigned to the pointer variable  pointer variables which stores address of structure must be declared as pointer to structure Pointer to structure
• 19.
  19 //Program to passthe address of a structure variable to a function #include <stdio.h> #include <conio.h> struct employee { char name [20]; int id; float salary; }; void main() { struct employee e1 ={"John",201,10000.50}; struct employee *e; e=&e1; printf("n%st%dt%.2f",e->name,e->id,e->salary); getch(); }
• 20.
  20 Trouble with Pointers Compiler may not detect the error and produce unnecessary results  Debugging becomes difficult  Some common errors int *p, m=100; p=m; p=&m; printf(“%d”,p);
• 21.
  21 Types of memoryallocation Fixed in size Automatic memory allocation space is automatically determined E.g: int a =10; (2 bytes space is occupied in memory) Static Memory Allocation (SMA) Fixed in size Done at compile time only Cannot take data more than the allocated size E.g: char mem_alloc[12]; int a[5]={1,6,9,4,11};
• 22.
  22 • Often realworld problems meaning that, required storage space changes over time • Size can be changed • Done at run time • We can allocate (create) and deallocate (free/delete) storage space whenever needed • We can always have exact amount of storage space (no more, no less) • There are 4 library functions that come into action for the DMA: • malloc() • calloc() • realloc() • free() Dynamic Memory Allocation (DMA)
• 23.
  23 • Syntax: m=(cast-type*)malloc(size-in-bytes) • Reservesa (single) block of memory of specified size in bytes • Returns a pointer of void type to the first byte of the space which is allocated malloc() E.g: a=(int*)malloc(10*sizeof(int));
• 24.
  24 #include<stdio.h> #include<conio.h> #include<stdlib.h> void main() { int *a,i; floatnum; printf("Enter size for memory allocation: "); scanf("%f",&num); a=(int*)malloc(sizeof(num)); printf("Allocated memory is: "); for(i = 0; i < num; ++i) { printf("%ut",a+i); } free(a); getch(); }
• 25.
  25 calloc() • Syntax: m=(cast-type*)calloc(n,element-size) • Reservesmultiple block of memory each of same size and sets all bytes to zero • The difference in malloc and calloc is that malloc does not set the memory to zero where as calloc sets allocated memory to zero.
• 26.
  26 #include<stdio.h> #include<conio.h> #include<stdlib.h> void main() { int *a,i; floatnum; printf("Enter size for memory allocation: "); scanf("%f",&num); a=(int*)calloc(num,sizeof(int)); printf("Allocated memory is: "); for(i = 0; i < num; ++i) { printf("%ut",a+i); } free(a); getch(); }
• 27.
  27 realloc() • Syntax: m= (int*)malloc(size from the user); ------------------- m=(cast-type*)realloc(n, new-size) • Used to increase or decrease the allocated memory size using malloc() or calloc() functions • It adjusts the old memory region if the new size is smaller than the size of old memory • If the new size is larger than the existing memory size, it increases the size by copying the contents of old memory region to new memory region
• 28.
  28 #include <stdlib.h> void main() { int*ptr, i , n1, n2; printf("Enter size of array: "); scanf("%d", &n1); ptr = (int*) malloc(n1*sizeof(int)); printf("Address of previously allocated memory: "); for(i = 0; i < n1; ++i) { printf("%ut",ptr + i); } printf("nEnter new size of array: ");
• 29.
  29 References: Websites: • google.com • codecadamy.com •tutorialspoint.com External sources: Text book: Programming in ANSIC (E-Balagurusamy)