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**R**_{j}

**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
**P**may request at most_{i}**‘k’**instances of resource type**R**_{j.}

**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
**P**is currently allocated_{i}**‘k’**instances of resource type**R**_{j}

**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
**P**currently allocated_{i}**‘k’**instances of resource type**R**_{j} - Need [ i, j ] = Max [ i, j ] – Allocation [ i, j ]

Allocation_{i} specifies the resources currently allocated to process P_{i} and Need_{i} specifies the additional resources that process P_{i} may still request to complete its task.

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

**Safety Algorithm**

The algorithm for finding out whether or not a system is in a safe state can be described as follows:

- Let Work and Finish be vectors of length ‘m’ and ‘n’ respectively.

Initialize: Work= Available

Finish [i]=false; for i=1,2,……,n - Find an i such that both

a) Finish [i]=false

b) Need_i<=work

if no such i exists goto step (4) - Work=Work + Allocation_i

Finish[i]= true

goto step(2) - If Finish[i]=true for all i,

then the system is in safe state.

**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.

Following is the resource allocation graph:

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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