The difference between Mutex and Semaphore in C# and usage scenarios
In C# multithreaded programming, the synchronization mechanism is the key to ensuring that resources are correctly shared among threads. Among them, Mutex and Semaphore are two commonly used synchronization tools, each with unique characteristics and applicable scenarios. This article will explore in detail the differences between Mutex and Semaphore and their respective usage scenarios.
Mutex (mutex)
Definition and Features：
Mutex (abbreviation of Mutual Exclusion) is a synchronous primitive used to control access to shared resources between multiple threads or processes. Its main feature is that only one thread can hold Mutex at the same time, thereby ensuring mutually exclusive access to critical sections.
Use scenarios：
- Protect critical zone: When multiple threads need to access shared resources (such as global variables, files, database connections, etc.), Mutex can be used to ensure that only one thread can access these resources at the same time, thereby avoiding data competition and resource conflicts.
- Cross-process synchronization: Mutex can not only synchronize between multiple threads in the same process, but also synchronize between different processes. This is useful for scenarios where applications need to ensure that a single instance runs or where different processes need to coordinate access to shared resources.
Things to note：
- Mutex must be released by the process that acquires the lock, otherwise it may cause a deadlock.
- When using Mutex, make sure to release Mutex after access is over, avoid holding Mutex for a long time to reduce thread waiting time and improve performance.
Semaphore
Definition and Features：
- Semaphore is a synchronization tool commonly used in concurrent programming to control access to shared resources. Unlike Mutex, Semaphore can have a count of more than 1, meaning it allows multiple threads to access the shared resource at the same time.
- The core principle of Semaphore is based on maintaining a certain number of "permits" that represent the number of threads that can access a specific resource at the same time.
Use scenarios：
- Resource pool management: Semaphore can be used to limit the size of resource pools, such as database connection pools, thread pools, etc. By setting the initial number of permits, it is possible to ensure that no resources exceeding the maximum limit are used simultaneously.
- Current limit control: When it is necessary to limit the number of concurrent accesses to certain resources or operations, such as limiting the maximum number of concurrent HTTP request processing, limiting the number of threads to access a file or data simultaneously, Semaphore is an effective tool.
- Producer-Consumer Model: Semaphore can be used to control synchronization between producers and consumers. For example, the production of one semaphore control term (number of vacancy), and the consumption of another semaphore control term (number of available terms).
Things to note：
- The number of licenses for Semaphore should be set according to actual needs to avoid resource overload or resource hunger.
- When using Semaphore, make sure that the license is released after the access is over, otherwise other threads may not be able to obtain the license and thus cannot access the resources.
summary:
Mutex and Semaphore are synchronization tools in C# for controlling thread access to shared resources, but they have different characteristics and applicable scenarios. Mutex is mainly used to protect critical areas and implement cross-process synchronization to ensure that only one thread can access shared resources at the same time; while Semaphore allows multiple threads to access shared resources at the same time, and controls the number of concurrent accesses by maintaining a certain number of licenses. When choosing to use Mutex or Semaphore, trade-offs should be made based on the specific application scenario and requirements.
The following are examples of using Mutex and Semaphore in C# to understand their differences and applicable scenarios more intuitively.
Mutex usage example
The following example shows how to use Mutex in C# to achieve mutex access to shared resources.
using System;
using ;
class Program
{
private static Mutex mutex = new Mutex();
static void Main(string[] args)
{
Thread[] threads = new Thread[3];
for (int i = 0; i < 3; i++)
{
threads[i] = new Thread(new ThreadStart(ThreadMethod));
threads[i].Name = "Thread-" + (i + 1);
}
for (int i = 0; i < 3; i++)
{
threads[i].Start();
}
();
}
public static void ThreadMethod()
{
($"{} is waiting for the mutex.");
(); // Get the lock try
{
($"{} has acquired the mutex.");
// Perform operations on shared resources for (int i = 1; i <= 5; i++)
{
($"{} is running iteration {i}.");
(100); // Simulate some work }
}
finally
{
($"{} is releasing the mutex.");
(); // Release the lock }
}
}In this example, we create three threads, each trying to get the same Mutex object. When a thread successfully acquires Mutex, it performs some operations (in this example, printing some information and simulating some work), and then releases Mutex. Other threads can only obtain Mutex and continue execution after it is released.
Semaphore usage example
The following example shows how to use Semaphore in C# to control the number of concurrent accesses to shared resources.
using System;
using ;
class Program
{
private static Semaphore semaphore = new Semaphore(2, 2); // Allow two threads to access at the same time
static void Main(string[] args)
{
Thread[] threads = new Thread[5];
for (int i = 0; i < 5; i++)
{
threads[i] = new Thread(new ThreadStart(ThreadMethod));
threads[i].Name = "Thread-" + (i + 1);
}
for (int i = 0; i < 5; i++)
{
threads[i].Start();
}
();
}
public static void ThreadMethod()
{
($"{} is waiting for the semaphore.");
(); // Wait for the semaphore try
{
($"{} has entered the critical section.");
// Perform operations on shared resources for (int i = 1; i <= 3; i++)
{
($"{} is running iteration {i}.");
(200); // Simulate some work }
}
finally
{
($"{} is leaving the critical section.");
(); // Release the semaphore }
}
}In this example, we create a Semaphore object with both the initial count and the maximum count set to 2, which means that at most two threads can access the critical section at the same time. We created five threads, each trying to get the Semaphore. When the count of Semaphore is greater than 0, the thread will successfully acquire the Semaphore and enter the critical area to perform operations; when the count of Semaphore is 0, the thread will be blocked and wait for other threads to release the Semaphore. In this example, we can see that although five threads try to access the shared resource at the same time, due to Semaphore limitations, only two threads are able to enter the critical section at the same time.
Through these examples, we can understand the difference between Mutex and Semaphore and applicable scenarios more clearly. Mutex is mainly used to protect critical areas to ensure that only one thread can access shared resources at the same time; while Semaphore allows multiple threads to access shared resources at the same time, but will limit the number of concurrent accesses.
Summarize
The above is personal experience. I hope you can give you a reference and I hope you can support me more.