The Banker's Algorithm is a fundamental deadlock avoidance algorithm used in operating systems for safe resource allocation to processes. The algorithm determines whether resources can be safely granted to a process without causing the system to enter a deadlock state. This comprehensive guide explores the algorithm's working principles, implementation, characteristics, and practical examples.
Publication Date: 22 August 2025
Reading Time: 5 minutes
Source: NxtWave
The Banker's Algorithm is a resource allocation and deadlock prevention algorithm useful when allocating resources to several processes in an operating system while keeping the system safe. The algorithm tracks three critical aspects:
When a process requests resource allocation, the algorithm checks the system to ensure granting the request will not lead the system into a deadlock state by verifying if it can eventually attain a safe state where all processes finish. If the system cannot reach a safe state, the request is declined, thus avoiding potential deadlock.
The Banker's Algorithm derives its name from the analogy of a bank managing resources, particularly loans. This model considers a bank with numerous customers who deposit specific amounts. Loans are given out by deducting the loan amounts from the total deposits. However, to prevent an unstable financial situation from developing, the banker must be ready to meet all deposit demands from all depositors, even when they all request their money simultaneously. Otherwise, a deadlock would arise, and neither the bank nor the customers could move further.
Similarly, the Banker's Algorithm checks whether allocating resources to a process is safe in the operating system. The system tests whether it can still fulfill all process requirements without causing a deadlock. Resource allocation must be both efficient and secure.
The Banker's Algorithm exhibits four key characteristics:
A requesting process must wait for requested resources when they cannot be safely allocated immediately.
The algorithm attempts to ensure resources are allocated in a way that avoids deadlock while maintaining system efficiency.
The algorithm assures quick release for reusing resources when processes are done, allowing resources to be returned to the available pool.
The Banker's Algorithm is dynamic in handling requests for allocation and releases, adapting to changing system states in real-time.
The Banker's Algorithm can be understood through a practical analogy: Consider a library with a limited collection of books where each student requires a certain number of books. The lending of books each student is requesting is checked to ensure that it does not cause the system to become an unsafe state.
The algorithm performs the following verification:
Like how bank or classroom resources are efficiently managed, the Banker's Algorithm ensures that resource allocation will not lead to an unsafe state from where deadlock arises.
Implementing the Banker's Algorithm requires three fundamental components:
The number of resources initially allocated to each process. It carries out a record of the number of each resource instance currently assigned to each process.
The maximum number of resources that each process may require during its execution. It may be used to predict future resource requests for each process.
The total number of instances for every resource type in the system that were unallocated before allocation.
The Banker's Algorithm can allocate resources safely without entering a deadlock state using these three components.
Implementing the Banker's Algorithm in an operating system adopts several key data structures for administrating and tracking resources and processes:
A vector of length "m" that tracks the number of available instances for each resource type in the system.
Format: If Available[j] = k, there are k instances of resource type Rj available.
A matrix of size [n × m] expressing the maximum number of resources each process can demand.
Format: If Max[i][j] = k, process Pi can request up to k instances of resource Rj.
A matrix of size [n × m] representing the number of resources allocated to each process.
Format: If Allocation[i][j] = k, process Pi has been allocated k of resource Rj.
A matrix of size [n × m] that provides the number of resources still needed for each process.
Format: If Need[i][j] = k, process Pi still needs k more instances of resource Rj to complete.
Calculation: Need = Maximum Demand - Allocation
A vector of length n, where each entry is a Boolean value (TRUE or FALSE).
Purpose: Shows if the process has finished executing and released all its resources. If Finish[i] = TRUE, process Pi is said to have completed execution with the resources freed.
A system is said to be in a secure state when there is a sequence of processes such that by each process obtaining required resources, finishing, and then releasing resources that might be needed by other methods, such other processes have a possibility of finishing. In a safe state, no deadlock can occur in the system.
It is called unsafe when resources may still be allocated to processes, allowing them to proceed but with no guarantees that all will be completed without causing deadlocks. In this case, a deadlock occurs, and the process may not finish.
The Banker's Algorithm process involves five key steps:
Mark active processes as running or blocked.
Active := Running ∪ Blocked
for k = 1 to r:
New_request[k] := Requested_resources[requesting_process, k]
Change the resource allocation state to include the most recent request.
Simulated_allocation := Allocated_resources
for k = 1 to r:
// Compute projected allocation state
Simulated_allocation[requesting_process, k] := Simulated_allocation[requesting_process, k] + New_request[k]
Verify if such resource allocation exceeds the available resources; if not, it proceeds.
Feasible := true
for k = 1 to r:
// Check if projected allocation state is feasible
if Total_resources[k] < Simulated_total_alloc[k]:
Feasible := false
The algorithm removes active processes that can finish upon accessing the available resources.
if Feasible = true:
while set Active contains a process P1 such that:
for all k, Total_resources[k] - Simulated_total_alloc[k] >= Max_need[P1, k] - Simulated_allocation[P1, k]:
Delete P1 from Active
for k = 1 to r:
Simulated_total_alloc[k] := Simulated_total_alloc[k] - Simulated_allocation[P1, k]
If all processes can be finished, the request will be granted, and resources will be re-updated accordingly.
if set Active is empty:
for k = 1 to r:
Requested_resources[requesting_process, k] := 0
for k = 1 to r:
Allocated_resources[requesting_process, k] := Allocated_resources[requesting_process, k] + New_request[k]
Total_alloc[k] := Total_alloc[k] + New_request[k]
There are two types of banker's algorithms:
The Safety Algorithm determines whether the system is in a safe state.
The algorithm initializes vectors Work and Finish.
Initialize: Work = Available
Finish[i] = false; for i = 1, 2, 3, 4....n
It searches for processes not finished yet for available resources.
Find an i such that both:
2.1 Finish[i] = false
2.2 Needi <= Work
if no such i exists, go to step (4)
The processes are finished after available resources are updated.
Work = Work + Allocation[i]
Finish[i] = true
Go to step (2)
It ensures a safe state if all processes are successfully finished.
If Finish[i] = true for all i, then the system is in a safe state
The Resource-Request Algorithm handles individual resource requests from processes.
This requires a check to see if the request for resources by a process is valid.
If Requesti <= Needi, go to step (2);
The process request for resources does not exceed the maximum claim.
If Requesti <= Available, go to step (3);
If requested resources are unavailable in the system, the process waits until they are released.
Available = Available - Requesti
Allocationi = Allocationi + Requesti
Needi = Needi - Requesti
This section demonstrates a complete example of the Banker's Algorithm with solution.
We have the following tables for processes, allocations, maximum needs, and available resources:
| Process | Allocation (A, B, C) | Max (A, B, C) | Available (A, B, C) |
|---|---|---|---|
| P0 | (1, 2, 5) | (4, 3, 2) | (3, 2, 1) |
| P1 | (2, 1, 2) | (4, 3, 2) | |
| P2 | (0, 1, 9) | (6, 2, 3) | |
| P3 | (2, 0, 8) | (6, 4, 4) | |
| P4 | (1, 1, 2) | (2, 2, 5) |
The Need matrix is calculated using the formula:
Need[i] = Max[i] - Allocation[i]
Calculation for each process:
| Process | Need (A, B, C) |
|---|---|
| P0 | (4-1, 3-2, 2-5) = (3, 1, -3) |
| P1 | (4-2, 3-1, 2-2) = (2, 2, 0) |
| P2 | (6-0, 2-1, 3-9) = (6, 1, -6) |
| P3 | (6-2, 4-0, 4-8) = (4, 4, -4) |
| P4 | (2-1, 2-1, 5-2) = (1, 1, 3) |
To determine if the system is in a safe state, we follow the Safety Algorithm.
Process P0: Need = (3, 1, -3), but this contains a negative value, which is invalid. Therefore, P0 cannot finish.
Process P1: Need = (2, 2, 0) and Work = (3, 2, 1). Since Need ≤ Work, P1 can finish.
Process P2: Need = (6, 1, -6), but this contains a negative value, so P2 cannot finish.
Process P3: Need = (4, 4, -4), which also contains a negative value, so P3 cannot finish.
Process P4: Need = (1, 1, 3) and Work = (5, 3, 3). Since Need ≤ Work, P4 can finish.
Now, we have the updated Work = (6, 4, 5). We check again for other processes:
Since P1 and P4 can finish, we have a safe sequence: <P1, P4>
This section provides a complete C++ implementation of the Banker's Algorithm.
#include <bits/stdc++.h>
using namespace std;
int main() {
int n = 5, m = 3;
vector<vector<int>> Max = {{6, 5, 3}, {4, 2, 1}, {5, 1, 2}, {1, 0, 2}, {5, 2, 3}};
vector<vector<int>> Alloc = {{1, 0, 2}, {3, 1, 0}, {0, 1, 1}, {1, 0, 1}, {1, 0, 2}};
vector<int> Total = {7, 5, 7}; // Total available resources
vector<int> Avail(m); // Available resources after allocation
for (int j = 0; j < m; j++) {
int s = 0;
for (int i = 0; i < n; i++) {
s += Alloc[i][j];
}
Avail[j] = Total[j] - s;
}
vector<int> Exec(n, 0);
vector<vector<int>> Need(n, vector<int>(m));
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
Need[i][j] = Max[i][j] - Alloc[i][j];
}
}
int k, c = 0;
vector<int> seq;
while (c < n) {
for (int i = 0; i < n; i++) {
if (Exec[i] == 0) {
int mark = 0;
for (int j = 0; j < m; j++) {
if (Need[i][j] > Avail[j]) {
mark = 1;
break;
}
}
if (mark == 0) {
seq.push_back(i + 1);
for (k = 0; k < m; k++) {
Avail[k] += Alloc[i][k];
}
Exec[i] = 1;
}
}
}
c++;
}
cout << "The safe sequence of execution is ";
for (int i = 0; i < n; i++) {
if (i == n - 1)
cout << " P" << seq[i] << "\n";
else
cout << " P" << seq[i] << " ->";
}
}
The above C++ code implements the Banker's Algorithm to determine a safe sequence of process execution. It computes the Need, Available, and Allocation matrices and then iterates through processes to check whether their resource needs can be satisfied. If so, that process runs, and resources are updated. Finally, the safe sequence is printed.
The safe sequence of execution is P1 -> P3 -> P4 -> P2 -> P5
This section demonstrates a scenario where the system enters an unsafe state.
Consider a system with 3 processes (P1, P2, P3) with 6 instances of Resources. The state of the system is defined by the following conditions:
Step 1: Available Resources
1 instance of resource available.
Step 2: Current Allocation
Step 3: Maximum Demand
Maximum resources each process may request:
Step 4: Calculate Need for Each Process
Need = Maximum Demand - Allocation
| Process | Maximum Demand | Allocation | Need |
|---|---|---|---|
| P1 | 4 | 2 | 2 |
| P2 | 5 | 3 | 2 |
| P3 | 3 | 1 | 2 |
In this example:
The system has only one instance of the resource, and each process needs two more resources in order to complete. Even in the presence of resources, there is no guarantee that all processes will reach termination without a deadlock.
The Banker's Algorithm offers several key advantages:
The algorithm detects a deadlock and prevents it as it allocates resources safely.
The algorithm caters to process requests by checking safe or unsafe allocation of resources.
One advantage of this algorithm is that it allows processes to declare their maximum resource requirements in advance.
The safety algorithm ensures that resources are allocated to avoid leaving the system unsafe.
The Banker's Algorithm has several limitations:
The processes must declare the maximum resource requirements before and cannot be changed during execution.
All processes must declare their maximum requirements before execution begins.
It starts with a fixed number of processes; at no time during execution can another process be entered.
The algorithm has a certain finite period within which resource allocation must be done; if unsuccessful, there is a maximum time limit (e.g., one year).
The Banker's Algorithm is a simple but competent way of resource allocation and deadlock avoidance in an operating system. The Banker's Algorithm will always keep a system in a safe state. It has some overheads, and there are restrictions on its use. It suits those systems in which resource requirements are known, and deadlock avoidance is essential. Other methods may prove more effective for real-time systems with highly variable resource requirements.
The Banker's Algorithm is an example of a deadlock avoidance algorithm. It ensures that resources are allotted so the system can safely avoid entering a deadlock state by checking for potential safe sequences in resource allocation.
The Banker's Algorithm manages resource allocation and avoids deadlock. It determines whether a requested resource can be safely allocated to a process without leaving the system unsafe so that all processes can finish in time.
The deadlocks in an operating system are the set of strategies to detect, avoid, and even resolve the deadlocks. The standard algorithms for deadlock avoidance include the Banker's algorithm, while the resource allocation graph and wait-for graphs are used for deadlock detection.
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