First Fit algorithm in Memory Management using Linked List

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++

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// 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++;
}
  
// Fuction 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();
}

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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

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