Program for First Fit algorithm in Memory Management
Last Updated :
12 Mar, 2025
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Prerequisite : Partition Allocation Methods
In the first fit, the partition is allocated which is first sufficient from the top of Main Memory.
Example :
Input : blockSize[] = {100, 500, 200, 300, 600};
processSize[] = {212, 417, 112, 426};
Output:
Process No. Process Size Block no.
1 212 2
2 417 5
3 112 2
4 426 Not Allocated
- Its advantage is that it is the fastest search as it searches only the first block i.e. enough to assign a process.
- It may have problems of not allowing processes to take space even if it was possible to allocate. Consider the above example, process number 4 (of size 426) does not get memory. However it was possible to allocate memory if we had allocated using best fit allocation [block number 4 (of size 300) to process 1, block number 2 to process 2, block number 3 to process 3 and block number 5 to process 4].
Implementation:
1- Input memory blocks with size and processes with size.
2- Initialize all memory blocks as free.
3- Start by picking each process and check if it can
be assigned to current block.
4- If size-of-process <= size-of-block if yes then
assign and check for next process.
5- If not then keep checking the further blocks.
Below is an implementation of above steps.
// C++ implementation of First - Fit algorithm
#include<bits/stdc++.h>
using namespace std;
// Function to allocate memory to
// blocks as per First fit algorithm
void firstFit(int blockSize[], int m,
int processSize[], int n)
{
// Stores block id of the
// block allocated to a process
int allocation[n];
// Initially no block is assigned to any process
memset(allocation, -1, sizeof(allocation));
// pick each process and find suitable blocks
// according to its size ad assign to it
for (int i = 0; i < n; i++)
{
for (int j = 0; j < m; j++)
{
if (blockSize[j] >= processSize[i])
{
// allocate block j to p[i] process
allocation[i] = j;
// Reduce available memory in this block.
blockSize[j] -= processSize[i];
break;
}
}
}
cout << "\nProcess No.\tProcess Size\tBlock no.\n";
for (int i = 0; i < n; i++)
{
cout << " " << i+1 << "\t\t"
<< processSize[i] << "\t\t";
if (allocation[i] != -1)
cout << allocation[i] + 1;
else
cout << "Not Allocated";
cout << endl;
}
}
// Driver code
int main()
{
int blockSize[] = {100, 500, 200, 300, 600};
int processSize[] = {212, 417, 112, 426};
int m = sizeof(blockSize) / sizeof(blockSize[0]);
int n = sizeof(processSize) / sizeof(processSize[0]);
firstFit(blockSize, m, processSize, n);
return 0 ;
}
// C implementation of First - Fit algorithm
#include<stdio.h>
// Function to allocate memory to
// blocks as per First fit algorithm
void firstFit(int blockSize[], int m, int processSize[], int n)
{
int i, j;
// Stores block id of the
// block allocated to a process
int allocation[n];
// Initially no block is assigned to any process
for(i = 0; i < n; i++)
{
allocation[i] = -1;
}
// pick each process and find suitable blocks
// according to its size ad assign to it
for (i = 0; i < n; i++) //here, n -> number of processes
{
for (j = 0; j < m; j++) //here, m -> number of blocks
{
if (blockSize[j] >= processSize[i])
{
// allocating block j to the ith process
allocation[i] = j;
// Reduce available memory in this block.
blockSize[j] -= processSize[i];
break; //go to the next process in the queue
}
}
}
printf("\nProcess No.\tProcess Size\tBlock no.\n");
for (int i = 0; i < n; i++)
{
printf(" %i\t\t\t", i+1);
printf("%i\t\t\t\t", processSize[i]);
if (allocation[i] != -1)
printf("%i", allocation[i] + 1);
else
printf("Not Allocated");
printf("\n");
}
}
// Driver code
int main()
{
int m; //number of blocks in the memory
int n; //number of processes in the input queue
int blockSize[] = {100, 500, 200, 300, 600};
int processSize[] = {212, 417, 112, 426};
m = sizeof(blockSize) / sizeof(blockSize[0]);
n = sizeof(processSize) / sizeof(processSize[0]);
firstFit(blockSize, m, processSize, n);
return 0 ;
}
// Java implementation of First - Fit algorithm
// Java implementation of First - Fit algorithm
class GFG
{
// Method to allocate memory to
// blocks as per First fit algorithm
static void firstFit(int blockSize[], int m,
int processSize[], int n)
{
// Stores block id of the
// block allocated to a process
int allocation[] = new int[n];
// Initially no block is assigned to any process
for (int i = 0; i < allocation.length; i++)
allocation[i] = -1;
// pick each process and find suitable blocks
// according to its size ad assign to it
for (int i = 0; i < n; i++)
{
for (int j = 0; j < m; j++)
{
if (blockSize[j] >= processSize[i])
{
// allocate block j to p[i] process
allocation[i] = j;
// Reduce available memory in this block.
blockSize[j] -= processSize[i];
break;
}
}
}
System.out.println("\nProcess No.\tProcess Size\tBlock no.");
for (int i = 0; i < n; i++)
{
System.out.print(" " + (i+1) + "\t\t" +
processSize[i] + "\t\t");
if (allocation[i] != -1)
System.out.print(allocation[i] + 1);
else
System.out.print("Not Allocated");
System.out.println();
}
}
// Driver Code
public static void main(String[] args)
{
int blockSize[] = {100, 500, 200, 300, 600};
int processSize[] = {212, 417, 112, 426};
int m = blockSize.length;
int n = processSize.length;
firstFit(blockSize, m, processSize, n);
}
}
# Python3 implementation of First-Fit algorithm
# Function to allocate memory to
# blocks as per First fit algorithm
def firstFit(blockSize, m, processSize, n):
# Stores block id of the
# block allocated to a process
allocation = [-1] * n
# Initially no block is assigned to any process
# pick each process and find suitable blocks
# according to its size ad assign to it
for i in range(n):
for j in range(m):
if blockSize[j] >= processSize[i]:
# allocate block j to p[i] process
allocation[i] = j
# Reduce available memory in this block.
blockSize[j] -= processSize[i]
break
print(" Process No. Process Size Block no.")
for i in range(n):
print(" ", i + 1, " ", processSize[i],
" ", end = " ")
if allocation[i] != -1:
print(allocation[i] + 1)
else:
print("Not Allocated")
# Driver code
if __name__ == '__main__':
blockSize = [100, 500, 200, 300, 600]
processSize = [212, 417, 112, 426]
m = len(blockSize)
n = len(processSize)
firstFit(blockSize, m, processSize, n)
# This code is contributed by PranchalK
// C# implementation of First - Fit algorithm
using System;
class GFG
{
// Method to allocate memory to
// blocks as per First fit algorithm
static void firstFit(int []blockSize, int m,
int []processSize, int n)
{
// Stores block id of the block
// allocated to a process
int []allocation = new int[n];
// Initially no block is assigned to any process
for (int i = 0; i < allocation.Length; i++)
allocation[i] = -1;
// pick each process and find suitable blocks
// according to its size ad assign to it
for (int i = 0; i < n; i++)
{
for (int j = 0; j < m; j++)
{
if (blockSize[j] >= processSize[i])
{
// allocate block j to p[i] process
allocation[i] = j;
// Reduce available memory in this block.
blockSize[j] -= processSize[i];
break;
}
}
}
Console.WriteLine("\nProcess No.\tProcess Size\tBlock no.");
for (int i = 0; i < n; i++)
{
Console.Write(" " + (i+1) + "\t\t" +
processSize[i] + "\t\t");
if (allocation[i] != -1)
Console.Write(allocation[i] + 1);
else
Console.Write("Not Allocated");
Console.WriteLine();
}
}
// Driver Code
public static void Main()
{
int []blockSize = {100, 500, 200, 300, 600};
int []processSize = {212, 417, 112, 426};
int m = blockSize.Length;
int n = processSize.Length;
firstFit(blockSize, m, processSize, n);
}
}
// This code is contributed by nitin mittal.
// Javascript implementation
<script>
const firstFit = (blockSize,m,processSize,n) => {
// Stores block id of the
// block allocated to a process
const allocation = [];
// Initially no block is assigned to any process
for (let i = 0; i < allocation.length; i++)
allocation[i] = -1;
// pick each process and find suitable blocks
// according to its size ad assign to it
for (let i = 0; i < n; i++)
{
for (let j = 0; j < m; j++)
{
if (blockSize[j] >= processSize[i])
{
// allocate block j to p[i] process
allocation[i] = j;
// Reduce available memory in this block.
blockSize[j] -= processSize[i];
break;
}
}
}
document.write("\nProcess No.\tProcess Size\tBlock no.");
for (let i = 0; i < n; i++)
{
let s = "Not Allocated"
if (allocation[i] > -1)
document.write(" " + (i+1) + "\t\t\t\t" +
processSize[i] + "\t\t\t\t"+ (allocation[i] + 1));
else
document.write(" " + (i+1) + "\t\t\t\t" +
processSize[i] + "\t\t\t\tNot Allocated");
}
}
const blockSize = [100, 500, 200, 300, 600];
const processSize = [212, 417, 112, 426];
let m = blockSize.length;
let n = processSize.length;
firstFit(blockSize, m, processSize, n);
// This code is contributed by ashishsingh13122000.
</script>
Output :
Process No. Process Size Block no.
1 212 2
2 417 5
3 112 2
4 426 Not Allocated
Time complexity of First Fit algorithm is O(n*m), where n is the number of processes and m is the number of memory blocks. The outer for loop runs for n times and the inner for loop runs for m times.
Auxiliary Space of First Fit algorithm is O(n), where n is the number of processes. The allocation array is used to store the block number allocated to the process, which takes a space of O(n).
