A semaphore is a synchronization mechanism used in computer science and operating system theory to control access to shared resources in a parallel or distributed system. Semaphores are particularly useful for avoiding race conditions and deadlocks.
Suppose we have a resource that can be used by multiple threads. A semaphore can protect this resource:
// PHP example using semaphores (pthreads extension required)
class SemaphoreExample {
private $semaphore;
public function __construct($initial) {
$this->semaphore = sem_get(ftok(__FILE__, 'a'), $initial);
}
public function wait() {
sem_acquire($this->semaphore);
}
public function signal() {
sem_release($this->semaphore);
}
}
// Main program
$sem = new SemaphoreExample(1); // Binary semaphore
$sem->wait(); // Enter critical section
// Access shared resource
$sem->signal(); // Leave critical sectionSemaphores are a powerful tool for making parallel programming safer and more controllable by helping to solve synchronization problems.
"Hold and Wait" is one of the four necessary conditions for a deadlock to occur in a system. This condition describes a situation where a process that already holds at least one resource is also waiting for additional resources that are held by other processes. This leads to a scenario where none of the processes can proceed because each is waiting for resources held by the others.
"Hold and Wait" occurs when:
Consider two processes P1P_1 and P2P_2 and two resources R1R_1 and R2R_2:
In this scenario, both processes are waiting for resources held by the other process, creating a deadlock.
To avoid "Hold and Wait" and thus prevent deadlocks, several strategies can be applied:
Resource Request Before Execution:
function requestAllResources($process, $resources) {
foreach ($resources as $resource) {
if (!requestResource($resource)) {
releaseAllResources($process, $resources);
return false;
}
}
return true;
}Resource Release Before New Requests:
function requestResourceSafely($process, $resource) {
releaseAllHeldResources($process);
return requestResource($resource);
}Priorities and Timestamps:
function requestResourceWithPriority($process, $resource, $priority) {
if (isHigherPriority($process, $resource, $priority)) {
return requestResource($resource);
} else {
// Wait or abort
return false;
}
}Banker's Algorithm:
"Hold and Wait" is a condition for deadlocks where processes hold resources while waiting for additional resources. By implementing appropriate resource allocation and management strategies, this condition can be avoided to ensure system stability and efficiency.
"Circular Wait" is one of the four necessary conditions for a deadlock to occur in a system. This condition describes a situation where a closed chain of two or more processes or threads exists, with each process waiting for a resource held by the next process in the chain.
A Circular Wait occurs when there is a chain of processes, where each process holds a resource and simultaneously waits for a resource held by another process in the chain. This leads to a cyclic dependency and ultimately a deadlock, as none of the processes can proceed until the other releases its resource.
Consider a chain of four processes P1,P2,P3,P4P_1, P_2, P_3, P_4 and four resources R1,R2,R3,R4R_1, R_2, R_3, R_4:
In this situation, none of the processes can proceed, as each is waiting for a resource held by another process in the chain, resulting in a deadlock.
To prevent Circular Wait and thus avoid deadlocks, various strategies can be applied:
Preventing Circular Wait is a crucial aspect of deadlock avoidance, contributing to the stable and efficient operation of systems.
A deadlock is a situation in computer science and computing where two or more processes or threads remain in a waiting state because each is waiting for a resource held by another process or thread. This results in none of the involved processes or threads being able to proceed, causing a complete halt of the affected parts of the system.
For a deadlock to occur, four conditions, known as Coffman conditions, must hold simultaneously:
A simple example of a deadlock is the classic problem involving two processes, each needing access to two resources:
Deadlocks are a significant issue in system and software development, especially in parallel and distributed processing, and require careful planning and control to avoid and manage them effectively.
A mutex (short for "mutual exclusion") is a synchronization mechanism in computer science and programming used to control concurrent access to shared resources by multiple threads or processes. A mutex ensures that only one thread or process can enter a critical section, which contains a shared resource, at a time.
Here are the essential properties and functionalities of mutexes:
Exclusive Access: A mutex allows only one thread or process to access a shared resource or critical section at a time. Other threads or processes must wait until the mutex is released.
Lock and Unlock: A mutex can be locked or unlocked. A thread that locks the mutex gains exclusive access to the resource. Once access is complete, the mutex must be unlocked to allow other threads to access the resource.
Blocking: If a thread tries to lock an already locked mutex, that thread will be blocked and put into a queue until the mutex is unlocked.
Deadlocks: Improper use of mutexes can lead to deadlocks, where two or more threads block each other by each waiting for a resource locked by the other thread. It's important to avoid deadlock scenarios in the design of multithreaded applications.
Here is a simple example of using a mutex in pseudocode:
mutex m = new mutex()
thread1 {
m.lock()
// Access shared resource
m.unlock()
}
thread2 {
m.lock()
// Access shared resource
m.unlock()
}In this example, both thread1 and thread2 lock the mutex m before accessing the shared resource and release it afterward. This ensures that the shared resource is never accessed by both threads simultaneously.
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