Deadlock Detection in C

Deadlock is a critical issue in concurrent programming where two or more processes become stuck in a state where each is waiting for the other to release a resource, causing a halt in program execution. Detecting and resolving deadlocks is crucial for maintaining the efficiency and reliability of a system.

What is a Deadlock?

A deadlock occurs when a set of processes are unable to proceed because each process is waiting for a resource that is being held by another process in the set. The following conditions must be met simultaneously for a deadlock to occur:

• Mutual Exclusion: At least one resource must be held in a non-shareable mode.
• Hold and Wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.
• No Pre-emption: Resources cannot be forcibly removed from the processes holding them.
• Circular Wait: A set of processes are waiting for each other in a circular chain.

Deadlock Detection Algorithms

Numerous algorithms exist for Detecting Deadlocks, with the Resource Allocation Graph (RAG) algorithm being one of the most prevalent. It is particularly effective in systems where there is only a single instance of each resource type.

Implementing Deadlock Detection in C

Here is a fundamental implementation of Deadlock Detection in C, utilizing a Resource Allocation Graph. This example examines the existence of a circular wait condition, a telltale sign of a deadlock.

Step-by-Step Implementation

1. Define the Data Structures:

• Processes and resources
• Allocation and request matrices
• An array to track which resources are currently available

2. Initialize the Data Structures:

• Set up the initial allocation of resources
• Define the resources requested by each process

3. Check for Deadlocks:

Use Depth-First Search (DFS) to detect cycles in the Resource Allocation Graph.

#include <stdio.h>
#include <stdbool.h>
#define NUM_PROCESSES 5
#define NUM_RESOURCES 3
// Function to perform DFS and check for cycles
bool dfs(int process, bool visited[], bool stack[], int allocation[][NUM_RESOURCES], int request[][NUM_RESOURCES], int available[]) {
if(!visited[process]) {
visited[process] = true;
stack[process] = true;
for(int r = 0; r < NUM_RESOURCES; r++) {
if(request[process][r] > available[r]) {
continue;
}
for(int p = 0; p < NUM_PROCESSES; p++) {
if(allocation[p][r] && !visited[p] && dfs(p, visited, stack, allocation, request, available)) {
return true;
} else if(stack[p]) {
return true;
}
}
}
}
stack[process] = false;
return false;
}
// Function to check for deadlocks
bool checkDeadlock(int allocation[][NUM_RESOURCES], int request[][NUM_RESOURCES], int available[]) {
bool visited[NUM_PROCESSES] = {false};
bool stack[NUM_PROCESSES] = {false};
for(int i = 0; i < NUM_PROCESSES; i++) {
if(dfs(i, visited, stack, allocation, request, available)) {
return true;
}
}
return false;
}
int main() {
int allocation[NUM_PROCESSES][NUM_RESOURCES] = {
{0, 1, 0},
{2, 0, 0},
{3, 0, 3},
{2, 1, 1},
{0, 0, 2}
};
int request[NUM_PROCESSES][NUM_RESOURCES] = {
{0, 0, 0},
{2, 0, 2},
{0, 0, 0},
{1, 0, 0},
{0, 0, 2}
};
int available[NUM_RESOURCES] = {0, 0, 0};
if(checkDeadlock(allocation, request, available)) {
printf(“Deadlock detected!\n”);
} else {
printf(“No deadlock detected.\n”);
}
return 0;
}

Explanation:

1. Data Structures:

• Allocation: Tracks the number of resources of each type currently allocated to each process.
• Request: Tracks the resources each process is currently requesting.
• Available: Tracks the number of available resources of each type.

2. Depth-First Search (DFS):

• The dfs function performs a DFS on the Resource Allocation Graph.
• If a cycle is detected during the DFS, it indicates a deadlock.

3. Deadlock Detection:

• The checkDeadlock function iterates through all processes, performing DFS from each one to detect cycles.
• If a cycle is found, it prints that a deadlock is detected; otherwise, it prints that no deadlock is detected.

Conclusion

Deadlock detection is an essential component of concurrent programming. Utilizing algorithms such as the Resource Allocation Graph and techniques like Depth-First Search enables effective deadlock detection and resolution. The given C program serves as a fundamental example that can be expanded to accommodate more intricate situations, including multiple resource instances and dynamic allocation requests. Grasping and applying these methods can greatly enhance the resilience of your concurrent applications.