Deadlock Avoidance in Operating Systems: Preventing Deadlocks Through Resource Management
Learn how deadlock avoidance prevents deadlocks by carefully managing resource allocation. This tutorial explains the concept of safe and unsafe states, explores resource allocation graphs, and demonstrates how deadlock avoidance algorithms ensure that resource requests are granted only if they maintain a safe system state.
Deadlock Avoidance
Understanding Deadlock Avoidance
Deadlock avoidance is a strategy that prevents deadlocks by carefully managing resource allocation. Before granting a process's request for a resource, the system checks if granting the request would lead to a deadlock situation. If it would, the request is denied. The system constantly monitors its state to ensure it remains in a "safe" state.
To use deadlock avoidance, each process must inform the operating system about the maximum number of resources of each type it might ever need. The avoidance algorithm then uses this information to decide whether to grant resource requests.
Safe and Unsafe States
A system's resource allocation state is defined by:
- Available resources: How many instances of each resource type are currently unused.
- Allocated resources: How many instances of each resource type are currently assigned to processes.
- Max resource needs: The maximum number of each resource type each process might request.
A safe state is one where there's a sequence of processes that can complete their execution without causing a deadlock. Each process in this sequence can acquire all its remaining needed resources, complete its task, and release its resources, allowing the next process in the sequence to do the same. If there's no such sequence, the system is in an unsafe state. This doesn't automatically mean a deadlock will occur, but it increases the risk.
Example: Safe and Unsafe States
Let's look at an example with four processes (A, B, C, D) and four resource types. The tables below illustrate a system's resource allocation state.
Resources Assigned
| Process | Type 1 | Type 2 | Type 3 | Type 4 |
|---|---|---|---|---|
| A | 3 | 0 | 2 | 2 |
| B | 0 | 0 | 1 | 1 |
| C | 1 | 1 | 1 | 0 |
| D | 2 | 1 | 4 | 0 |
Resources Still Needed
| Process | Type 1 | Type 2 | Type 3 | Type 4 |
|---|---|---|---|---|
| A | 1 | 1 | 0 | 0 |
| B | 0 | 1 | 1 | 2 |
| C | 1 | 2 | 1 | 0 |
| D | 2 | 1 | 1 | 2 |
E (Total Resources): (7, 6, 8, 4)
P (Allocated Resources): (6, 2, 8, 3)
A (Available Resources): (1, 4, 0, 1)
Whether this state is safe or unsafe would require further analysis using a deadlock avoidance algorithm. The core idea is that resource requests are only granted if the resulting state remains safe.
C Code Example: Declaring a Character Variable
Syntax
char ch = 'a';
Example Output
Output
She said "Hello!" to me.
Next Topic: Resource Allocation Graph