Program for Banker’s Algorithm | Set 1 (Safety Algorithm)

3.2

Prerequisite: Banker’s Algorithm

The banker’s algorithm is a resource allocation and deadlock avoidance algorithm that tests for safety by simulating the allocation for predetermined maximum possible amounts of all resources, then makes an “s-state” check to test for possible activities, before deciding whether allocation should be allowed to continue.

Following Data structures are used to implement the Banker’s Algorithm:

Let ‘n’ be the number of processes in the system and ‘m’ be the number of resources types.

Available :

  • It is a 1-d array of size ‘m’ indicating the number of available resources of each type.
  • Available[ j ] = k means there are ‘k’ instances of resource type Rj

Max :

  • It is a 2-d array of size ‘n*m’ that defines the maximum demand of each process in a system.
  • Max[ i, j ] = k means process Pi may request at most ‘k’ instances of resource type Rj.

Allocation :

  • It is a 2-d array of size ‘n*m’ that defines the number of resources of each type currently allocated to each process.
  • Allocation[ i, j ] = k means process Pi is currently allocated ‘k’ instances of resource type Rj

Need :

  • It is a 2-d array of size ‘n*m’ that indicates the remaining resource need of each process.
  • Need [ i, j ] = k means process Pi currently allocated ‘k’ instances of resource type Rj
  • Need [ i, j ] = Max [ i, j ] – Allocation [ i, j ]

Allocationi specifies the resources currently allocated to process Pi and Needi specifies the additional resources that process Pi may still request to complete its task.

Banker’s algorithm consist of Safety algorithm and Resource request algorithm

Safety Algorithm

safety algorithm

Safe sequence is the sequence in which the processes can be safely executed.

In this post, implementation of Safety algorithm of Banker’s Algorithm is done.

// C++ program to illustrate Banker's Algorithm
#include<iostream>
using namespace std;

// Number of processes
const int P = 5;

// Number of resources
const int R = 3;

// Function to find the need of each process
void calculateNeed(int need[P][R], int maxm[P][R],
                   int allot[P][R])
{
    // Calculating Need of each P
    for (int i = 0 ; i < P ; i++)
        for (int j = 0 ; j < R ; j++)

            // Need of instance = maxm instance -
            //                    allocated instance
            need[i][j] = maxm[i][j] - allot[i][j];
}

// Function to find the system is in safe state or not
bool isSafe(int processes[], int avail[], int maxm[][R],
            int allot[][R])
{
    int need[P][R];

    // Function to calculate need matrix
    calculateNeed(need, maxm, allot);

    // Mark all processes as infinish
    bool finish[P] = {0};

    // To store safe sequence
    int safeSeq[P];

    // Make a copy of available resources
    int work[R];
    for (int i = 0; i < R ; i++)
        work[i] = avail[i];

    // While all processes are not finished
    // or system is not in safe state.
    int count = 0;
    while (count < P)
    {
        // Find a process which is not finish and
        // whose needs can be satisfied with current
        // work[] resources.
        bool found = false;
        for (int p = 0; p < P; p++)
        {
            // First check if a process is finished,
            // if no, go for next condition
            if (finish[p] == 0)
            {
                // Check if for all resources of
                // current P need is less
                // than work
                int j;
                for (j = 0; j < R; j++)
                    if (need[p][j] > work[j])
                        break;

                // If all needs of p were satisfied.
                if (j == R)
                {
                    // Add the allocated resources of
                    // current P to the available/work
                    // resources i.e.free the resources
                    for (int k = 0 ; k < R ; k++)
                        work[k] += allot[p][k];

                    // Add this process to safe sequence.
                    safeSeq[count++] = p;

                    // Mark this p as finished
                    finish[p] = 1;

                    found = true;
                }
            }
        }

        // If we could not find a next process in safe
        // sequence.
        if (found == false)
        {
            cout << "System is not in safe state";
            return false;
        }
    }

    // If system is in safe state then
    // safe sequence will be as below
    cout << "System is in safe state.\nSafe"
         " sequence is: ";
    for (int i = 0; i < P ; i++)
        cout << safeSeq[i] << " ";

    return true;
}

// Driver code
int main()
{
    int processes[] = {0, 1, 2, 3, 4};

    // Available instances of resources
    int avail[] = {3, 3, 2};

    // Maximum R that can be allocated
    // to processes
    int maxm[][R] = {{7, 5, 3},
                     {3, 2, 2},
                     {9, 0, 2},
                     {2, 2, 2},
                     {4, 3, 3}};

    // Resources allocated to processes
    int allot[][R] = {{0, 1, 0},
                      {2, 0, 0},
                      {3, 0, 2},
                      {2, 1, 1},
                      {0, 0, 2}};

    // Check system is in safe state or not
    isSafe(processes, avail, maxm, allot);

    return 0;
}

Output:

System is in safe state.
Safe sequence is: 1 3 4 0 2

Illustration :

Considering a system with five processes P0 through P4 and three resources types A, B, C. Resource type A has 10 instances, B has 5 instances and type C has 7 instances. Suppose at time t0 following snapshot of the system has been taken:

We must determine whether the new system state is safe. To do so, we need to execute Safety algorithm on the above given allocation chart.
banker's algorithm

Following is the resource allocation graph:
Bankers
Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement

This article is contributed by Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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