Dynamic allocation

Dynamic Memory
Allocation
Submitted to:
Anupam Shrama
Assistant Professor ,CSE
Submitted by: (GROUP-14)
SUNNY CHAKRABORTY-1915132
SUDHIR-1915131
SHIVANSHU-1915125
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Problem with Arrays
 Sometimes
Amount of data cannot be predicted beforehand
Number of data items keeps changing during
program execution
 Example: Seach for an element in an array of N
elements
 One solution: find the maximum possible value of N
and allocate an array of N elements
Wasteful of memory space, as N may be much
smaller in some executions
Example: maximum value of N may be 10,000, but a
particular run may need to search only among 100
elements
Using array of size 10,000 always wastes memory
in most cases

Better Solution
Dynamic memory allocation
Know how much memory is needed after the
program is run
Example: ask the user to enter from keyboard
Dynamically allocate only the amount of memory
needed
C provides functions to dynamically allocate
memory
malloc, calloc, realloc
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Memory Allocation Functions
 malloc
Allocates requested number of bytes and returns a
pointer to the first byte of the allocated space
 calloc
Allocates space for an array of elements, initializes
them to zero and then returns a pointer to the memory.
 free
Frees previously allocated space.
 realloc
Modifies the size of previously allocated space.
 We will only do malloc and free

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Allocating a Block of Memory
A block of memory can be allocated using the
function malloc
 Reserves a block of memory of specified size and returns a pointer of type void
 The return pointer can be type-casted to any pointer type
General format:
type *p;
p = (type *) malloc (byte_size);

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Example
p = (int *) malloc(100 * sizeof(int));
 A memory space equivalent to 100 times the size of an int
bytes is reserved
 The address of the first byte of the allocated memory is
assigned to the pointer p of type int
p
400 bytes of space

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Contd.
 cptr = (char *) malloc (20);
Allocates 20 bytes of space for the pointer cptr of type char
 sptr = (struct stud *) malloc(10*sizeof(struct
stud));
Allocates space for a structure array of 10 elements. sptr points
to a structure element of type struct stud
Always use sizeof operator to find number of bytes for a
data type, as it can vary from machine to machine

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Points to Note
 malloc always allocates a block of contiguous bytes
 The allocation can fail if sufficient contiguous memory
space is not available
 If it fails, malloc returns NULL
if ((p = (int *) malloc(100 * sizeof(int))) == NULL)
{
printf (“n Memory cannot be allocated”);
exit();
}

Using the malloc’d Array
 Once the memory is allocated, it can be used with
pointers, or with array notation
 Example:
int *p, n, i;
scanf(“%d”, &n);
p = (int *) malloc (n * sizeof(int));
for (i=0; i<n; ++i)
scanf(“%d”, &p[i]);
The n integers allocated can be accessed as *p, *(p+1),
*(p+2),…, *(p+n-1) or just as p[0], p[1], p[2], …,p[n-1]
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Example
printf("Input heights for %d
students n",N);
for (i=0; i<N; i++)
scanf ("%f", &height[i]);
for(i=0;i<N;i++)
sum += height[i];
avg = sum / (float) N;
printf("Average height = %f
n",
avg);
free (height);
return 0;
}
int main()
{
int i,N;
float *height;
float sum=0,avg;
printf("Input no. of
studentsn");
scanf("%d", &N);
height = (float *)
malloc(N *
sizeof(float));

malloc( )-ing array of structures
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typedef struct{
char name[20];
int roll;
float SGPA[8], CGPA;
} person;
void main()
{
person *student;
int i,j,n;
scanf("%d", &n);
student = (person *)malloc(n*sizeof(person));
for (i=0; i<n; i++) {
scanf("%s", student[i].name);
scanf("%d", &student[i].roll);
for(j=0;j<8;j++) scanf("%f", &student[i].SGPA[j]);
scanf("%f", &student[i].CGPA);
}
}

Dynamic Arrays of pointers
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int main()
{
char word[20], **w; /* “**w” is a pointer to a pointer array */
int i, n;
scanf("%d",&n);
w = (char **) malloc (n * sizeof(char *));
for (i=0; i<n; ++i) {
scanf("%s", word);
w[i] = (char *) malloc ((strlen(word)+1)*sizeof(char));
strcpy (w[i], word) ;
}
for (i=0; i<n; i++) printf("w[%d] = %s n",i, w[i]);
return 0;
}
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India
Australia
Kenya
NewZealand
SriLanka
w[0] = India
w[1] = Australia
w[2] = Kenya
w[3] = NewZealand
w[4] = SriLanka
Output

How this will look like
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w 0
1
2
3
4
I n d i a 0
S r i L a n k a 0
A u s t r a l I a 0
K e n y a 0
N e w Z e a l a n d 0

free()14
The C library function void free(void *ptr) deallocates the memory
previously allocated by a call to calloc, malloc, or realloc.

Dynamic allocation

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