First Fit algorithm in Memory Management using Linked List
Last Updated :
15 Jul, 2025
First Fit Algorithm for Memory Management: The first memory partition which is sufficient to accommodate the process is allocated.
We have already discussed first fit algorithm using arrays in this article. However, here we are going to look into another approach using a linked list where the deletion of allocated nodes is also possible.
Examples:
Input: blockSize[] = {100, 500, 200}
processSize[] = {417, 112, 426, 95}
Output:
Block of size 426 can't be allocated
Tag Block ID Size
0 1 417
1 2 112
2 0 95
After deleting block with tag id 0.
Tag Block ID Size
1 2 112
2 0 95
3 1 426
Approach: The idea is to use the memory block with a unique tag id. Each process of different sizes are given block id, which signifies to which memory block they belong to, and unique tag id to delete particular process to free up space. Create a free list of given memory block sizes and allocated list of processes.
Create allocated list:
Create an allocated list of given process sizes by finding the first memory block with sufficient size to allocate memory from. If the memory block is not found, then simply print it. Otherwise, create a node and add it to the allocated linked list.
Delete process:
Each process is given a unique tag id. Delete the process node from the allocated linked list to free up some space for other processes. After deleting, use the block id of the deleted node to increase the memory block size in the free list.
Below is the implementation of the approach:
C++
// C++ implementation of the First
// sit memory management algorithm
// using linked list
#include <bits/stdc++.h>
using namespace std;
// Two global counters
int g = 0, k = 0;
// Structure for free list
struct free {
int tag;
int size;
struct free* next;
}* free_head = NULL, *prev_free = NULL;
// Structure for allocated list
struct alloc {
int block_id;
int tag;
int size;
struct alloc* next;
}* alloc_head = NULL, *prev_alloc = NULL;
// Function to create free
// list with given sizes
void create_free(int c)
{
struct free* p
= (struct free*)malloc(sizeof(struct free));
p->size = c;
p->tag = g;
p->next = NULL;
if (free_head == NULL)
free_head = p;
else
prev_free->next = p;
prev_free = p;
g++;
}
// Function to print free list which
// prints free blocks of given sizes
void print_free()
{
struct free* p = free_head;
cout << "Tag\tSize\n";
while (p != NULL) {
cout << p->tag << "\t" << p->size << "\n";
p = p->next;
}
}
// Function to print allocated list which
// prints allocated blocks and their block ids
void print_alloc()
{
struct alloc* p = alloc_head;
cout << "Tag\tBlock ID\tSize\n";
while (p != NULL) {
cout << p->tag << "\t " << p->block_id << "\t\t"
<< p->size << "\n";
p = p->next;
}
}
// Function to allocate memory to
// blocks as per First fit algorithm
void create_alloc(int c)
{
// create node for process of given size
struct alloc* q
= (struct alloc*)malloc(sizeof(struct alloc));
q->size = c;
q->tag = k;
q->next = NULL;
struct free* p = free_head;
// Iterate to find first memory
// block with appropriate size
while (p != NULL) {
if (q->size <= p->size)
break;
p = p->next;
}
// Node found to allocate
if (p != NULL) {
// Adding node to allocated list
q->block_id = p->tag;
p->size -= q->size;
if (alloc_head == NULL)
alloc_head = q;
else {
prev_alloc = alloc_head;
while (prev_alloc->next != NULL)
prev_alloc = prev_alloc->next;
prev_alloc->next = q;
}
k++;
}
else // Node found to allocate space from
cout << "Block of size " << c
<< " can't be allocated\n";
}
// Function to delete node from
// allocated list to free some space
void delete_alloc(int t)
{
// Standard delete function
// of a linked list node
struct alloc *p = alloc_head, *q = NULL;
// First, find the node according
// to given tag id
while (p != NULL) {
if (p->tag == t)
break;
q = p;
p = p->next;
}
if (p == NULL)
cout << "Tag ID doesn't exist\n";
else if (p == alloc_head)
alloc_head = alloc_head->next;
else
q->next = p->next;
struct free* temp = free_head;
while (temp != NULL) {
if (temp->tag == p->block_id) {
temp->size += p->size;
break;
}
temp = temp->next;
}
}
// Driver Code
int main()
{
int blockSize[] = { 100, 500, 200 };
int processSize[] = { 417, 112, 426, 95 };
int m = sizeof(blockSize) / sizeof(blockSize[0]);
int n = sizeof(processSize) / sizeof(processSize[0]);
for (int i = 0; i < m; i++)
create_free(blockSize[i]);
for (int i = 0; i < n; i++)
create_alloc(processSize[i]);
print_alloc();
// Block of tag id 0 deleted
// to free space for block of size 426
delete_alloc(0);
create_alloc(426);
cout << "After deleting block"
<< " with tag id 0.\n";
print_alloc();
}
// Java implementation of the First
// sit memory management algorithm
// using linked list
public class GFG {
// Two global counters
static int g = 0, k = 0;
// Structure for free list
static class free {
int tag;
int size;
free next;
}
static free free_head = null;
static free prev_free = null;
// Structure for allocated list
static class alloc {
int block_id;
int tag;
int size;
alloc next;
}
static alloc alloc_head = null;
static alloc prev_alloc = null;
// Function to create free
// list with given sizes
static void create_free(int c)
{
free p = new free();
p.size = c;
p.tag = g;
p.next = null;
if (free_head == null)
free_head = p;
else
prev_free.next = p;
prev_free = p;
g++;
}
// Function to print free list which
// prints free blocks of given sizes
static void print_free()
{
free p = free_head;
System.out.println("Tag\tSize");
while (p != null) {
System.out.println(p.tag + "\t" + p.size);
p = p.next;
}
}
// Function to print allocated list which
// prints allocated blocks and their block ids
static void print_alloc()
{
alloc p = alloc_head;
System.out.println("Tag\tBlock ID\tSize");
while (p != null) {
System.out.println(p.tag + "\t " + p.block_id
+ "\t\t" + p.size);
p = p.next;
}
}
// Function to allocate memory to
// blocks as per First fit algorithm
static void create_alloc(int c)
{
// create node for process of given size
alloc q = new alloc();
q.size = c;
q.tag = k;
q.next = null;
free p = free_head;
// Iterate to find first memory
// block with appropriate size
while (p != null) {
if (q.size <= p.size)
break;
p = p.next;
}
// Node found to allocate
if (p != null) {
// Adding node to allocated list
q.block_id = p.tag;
p.size -= q.size;
if (alloc_head == null)
alloc_head = q;
else {
prev_alloc = alloc_head;
while (prev_alloc.next != null)
prev_alloc = prev_alloc.next;
prev_alloc.next = q;
}
k++;
}
else // Node found to allocate space from
System.out.println("Block of size " + c
+ " can't be allocated");
}
// Function to delete node from
// allocated list to free some space
static void delete_alloc(int t)
{
// Standard delete function
// of a linked list node
alloc p = alloc_head, q = null;
// First, find the node according
// to given tag id
while (p != null) {
if (p.tag == t)
break;
q = p;
p = p.next;
}
if (p == null)
System.out.println("Tag ID doesn't exist");
else if (p == alloc_head)
alloc_head = alloc_head.next;
else
q.next = p.next;
free temp = free_head;
while (temp != null) {
if (temp.tag == p.block_id) {
temp.size += p.size;
break;
}
temp = temp.next;
}
}
// Driver Code
public static void main(String[] args)
{
int blockSize[] = { 100, 500, 200 };
int processSize[] = { 417, 112, 426, 95 };
int m = blockSize.length;
int n = processSize.length;
for (int i = 0; i < m; i++)
create_free(blockSize[i]);
for (int i = 0; i < n; i++)
create_alloc(processSize[i]);
print_alloc();
// Block of tag id 0 deleted
// to free space for block of size 426
delete_alloc(0);
create_alloc(426);
System.out.println("After deleting block"
+ " with tag id 0.");
print_alloc();
}
}
// This code is contributed by Lovely Jain
# Python3 implementation of the First
# sit memory management algorithm
# using linked list
# Two global counters
g = 0; k = 0
# Structure for free list
class free:
def __init__(self):
self.tag=-1
self.size=0
self.next=None
free_head = None; prev_free = None
# Structure for allocated list
class alloc:
def __init__(self):
self.block_id=-1
self.tag=-1
self.size=0
self.next=None
alloc_head = None;prev_alloc = None
# Function to create free
# list with given sizes
def create_free(c):
global g,prev_free,free_head
p = free()
p.size = c
p.tag = g
p.next = None
if free_head is None:
free_head = p
else:
prev_free.next = p
prev_free = p
g+=1
# Function to print free list which
# prints free blocks of given sizes
def print_free():
p = free_head
print("Tag\tSize")
while (p != None) :
print("{}\t{}".format(p.tag,p.size))
p = p.next
# Function to print allocated list which
# prints allocated blocks and their block ids
def print_alloc():
p = alloc_head
print("Tag\tBlock ID\tSize")
while (p is not None) :
print("{}\t{}\t{}\t".format(p.tag,p.block_id,p.size))
p = p.next
# Function to allocate memory to
# blocks as per First fit algorithm
def create_alloc(c):
global k,alloc_head
# create node for process of given size
q = alloc()
q.size = c
q.tag = k
q.next = None
p = free_head
# Iterate to find first memory
# block with appropriate size
while (p != None) :
if (q.size <= p.size):
break
p = p.next
# Node found to allocate
if (p != None) :
# Adding node to allocated list
q.block_id = p.tag
p.size -= q.size
if (alloc_head == None):
alloc_head = q
else :
prev_alloc = alloc_head
while (prev_alloc.next != None):
prev_alloc = prev_alloc.next
prev_alloc.next = q
k+=1
else: # Node found to allocate space from
print("Block of size {} can't be allocated".format(c))
# Function to delete node from
# allocated list to free some space
def delete_alloc(t):
global alloc_head
# Standard delete function
# of a linked list node
p = alloc_head; q = None
# First, find the node according
# to given tag id
while (p != None) :
if (p.tag == t):
break
q = p
p = p.next
if (p == None):
print("Tag ID doesn't exist")
elif (p == alloc_head):
alloc_head = alloc_head.next
else:
q.next = p.next
temp = free_head
while (temp != None) :
if (temp.tag == p.block_id) :
temp.size += p.size
break
temp = temp.next
# Driver Code
if __name__ == '__main__':
blockSize = [100, 500, 200]
processSize = [417, 112, 426, 95]
m = len(blockSize)
n = len(processSize)
for i in range(m):
create_free(blockSize[i])
for i in range(n):
create_alloc(processSize[i])
print_alloc()
# Block of tag id 0 deleted
# to free space for block of size 426
delete_alloc(0)
create_alloc(426)
print("After deleting block with tag id 0.")
print_alloc()
// C# implementation of the First
// sit memory management algorithm
// using linked list
using System;
public class MainClass {
// Two global counters
public static int g = 0, k = 0;
public class Free {
// Structure for free list
public int tag;
public int size;
public Free next;
}
public static Free free_head = null, prev_free = null;
// Structure for allocated list
public class Alloc {
public int block_id;
public int tag;
public int size;
public Alloc next;
}
public static Alloc alloc_head = null, prev_alloc
= null;
// Function to create free
// list with given sizes
public static void CreateFree(int c)
{
Free p = new Free();
p.size = c;
p.tag = g;
p.next = null;
if (free_head == null)
free_head = p;
else
prev_free.next = p;
prev_free = p;
g++;
}
// Function to print free list which
// prints free blocks of given sizes
public static void PrintFree()
{
Free p = free_head;
Console.WriteLine("Tag\tSize");
while (p != null) {
Console.WriteLine(p.tag + "\t" + p.size);
p = p.next;
}
}
// Function to print allocated list which
// prints allocated blocks and their block ids
public static void PrintAlloc()
{
// create node for process of given size
Alloc p = alloc_head;
Console.WriteLine("Tag\tBlock ID\tSize");
while (p != null) {
// Iterate to find first memory
// block with appropriate size
Console.WriteLine(p.tag + "\t " + p.block_id
+ "\t\t" + p.size);
p = p.next;
}
}
public static void CreateAlloc(int c)
{
Alloc q = new Alloc();
q.size = c;
q.tag = k;
q.next = null;
Free p = free_head;
while (p != null) {
if (q.size <= p.size)
break;
p = p.next;
}
if (p != null) {
// Adding node to allocated list
q.block_id = p.tag;
p.size -= q.size;
if (alloc_head == null)
alloc_head = q;
else {
prev_alloc = alloc_head;
while (prev_alloc.next != null)
prev_alloc = prev_alloc.next;
prev_alloc.next = q;
}
k++;
}
else // Node found to allocate space from
Console.WriteLine("Block of size " + c
+ " can't be allocated");
}
// Function to delete node from
// allocated list to free some space
public static void DeleteAlloc(int t)
{
// Standard delete function
// of a linked list node
Alloc p = alloc_head, q = null;
while (p != null) {
// First, find the node according
// to given tag id
if (p.tag == t)
break;
q = p;
p = p.next;
}
if (p == null)
Console.WriteLine("Tag ID doesn't exist");
else if (p == alloc_head)
alloc_head = alloc_head.next;
else
q.next = p.next;
Free temp = free_head;
while (temp != null) {
if (temp.tag == p.block_id) {
temp.size += p.size;
break;
}
temp = temp.next;
}
}
// Driver Code
public static void Main()
{
int[] blockSize = { 100, 500, 200 };
int[] processSize = { 417, 112, 426, 95 };
int m = blockSize.Length;
int n = processSize.Length;
for (int i = 0; i < m; i++)
CreateFree(blockSize[i]);
for (int i = 0; i < n; i++)
CreateAlloc(processSize[i]);
PrintAlloc();
// Block of tag id 0 deleted
// to free space for block of size 426
DeleteAlloc(0);
CreateAlloc(426);
Console.WriteLine(
"After deleting block with tag id 0.");
PrintAlloc();
}
}
//Javascript Equivalent
// Two global counters
let g = 0; let k = 0
// Structure for free list
class Free {
constructor() {
this.tag = -1;
this.size = 0;
this.next = null;
}
}
let freeHead = null; let prevFree = null;
// Structure for allocated list
class Alloc {
constructor() {
this.blockId = -1;
this.tag = -1;
this.size = 0;
this.next = null;
}
}
let allocHead = null; let prevAlloc = null;
// Function to create free
// list with given sizes
function createFree(c) {
let p = new Free();
p.size = c;
p.tag = g;
p.next = null;
if (freeHead === null) {
freeHead = p;
} else {
prevFree.next = p;
}
prevFree = p;
g+=1;
}
// Function to print free list which
// prints free blocks of given sizes
function printFree() {
let p = freeHead;
console.log("Tag\tSize");
while (p !== null) {
console.log(`${p.tag}\t${p.size}`);
p = p.next;
}
}
// Function to print allocated list which
// prints allocated blocks and their block ids
function printAlloc() {
let p = allocHead;
console.log("Tag\tBlock ID\tSize");
while (p !== null) {
console.log(`${p.tag}\t${p.blockId}\t${p.size}\t`);
p = p.next;
}
}
// Function to allocate memory to
// blocks as per First fit algorithm
function createAlloc(c) {
let q = new Alloc();
q.size = c;
q.tag = k;
q.next = null;
let p = freeHead;
// Iterate to find first memory
// block with appropriate size
while (p !== null) {
if (q.size <= p.size) {
break;
}
p = p.next;
}
// Node found to allocate
if (p !== null) {
// Adding node to allocated list
q.blockId = p.tag;
p.size -= q.size;
if (allocHead === null) {
allocHead = q;
} else {
prevAlloc = allocHead;
while (prevAlloc.next !== null) {
prevAlloc = prevAlloc.next;
}
prevAlloc.next = q;
}
k+=1;
} else { // Node found to allocate space from
console.log(`Block of size ${c} can't be allocated`);
}
}
// Function to delete node from
// allocated list to free some space
function deleteAlloc(t) {
// Standard delete function
// of a linked list node
let p = allocHead; let q = null;
// First, find the node according
// to given tag id
while (p !== null) {
if (p.tag === t) {
break;
}
q = p;
p = p.next;
}
if (p === null) {
console.log("Tag ID doesn't exist");
} else if (p === allocHead) {
allocHead = allocHead.next;
} else {
q.next = p.next;
}
let temp = freeHead;
while (temp !== null) {
if (temp.tag === p.blockId) {
temp.size += p.size;
break;
}
temp = temp.next;
}
}
// Driver Code
function main() {
let blockSize = [100, 500, 200];
let processSize = [417, 112, 426, 95];
let m = blockSize.length;
let n = processSize.length;
for (let i = 0; i < m; i++) {
createFree(blockSize[i]);
}
for (let i = 0; i < n; i++) {
createAlloc(processSize[i]);
}
printAlloc();
// Block of tag id 0 deleted
// to free space for block of size 426
deleteAlloc(0);
createAlloc(426);
console.log("After deleting block with tag id 0.");
printAlloc();
}
main();
Output: Block of size 426 can't be allocated
Tag Block ID Size
0 1 417
1 2 112
2 0 95
After deleting block with tag id 0.
Tag Block ID Size
1 2 112
2 0 95
3 1 426
Time complexity of the First Fit memory management algorithm is O(n), where n is the number of memory blocks. When a process is to be allocated, it will traverse the whole list of free blocks and check for the first block which is capable of accommodating the process. Hence, the time complexity is O(n).
Auxiliary Space complexity of the First Fit memory management algorithm is O(n), where n is the number of memory blocks. This is because the algorithm requires two linked lists for storing the free and allocated blocks. The free list stores the details of free blocks whereas the allocated list stores the details of allocated blocks. Hence, the space complexity is O(n).
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