Скачать или смотреть Understanding Global Semaphores for Synchronization: Fixing the Counter Issue in C

Learn how to properly implement global semaphores for synchronization in C and why your counter may not increment correctly when using forks in your code.
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Understanding Global Semaphores for Synchronization: Fixing the Counter Issue in C

When working with concurrent programming in C, particularly when processes are creating and manipulating shared resources, it can be tricky to ensure that these resources are accessed safely and correctly. A common problem that arises is the improper use of semaphores, particularly when trying to synchronize access to shared variables between processes. This guide will address a specific scenario in which a counter variable does not yield the expected result due to common pitfalls in semaphore usage and inter-process communication.

The Problem: Why Doesn't the Counter Sum Up Correctly?

Let's take a look at a code snippet where a counter is incremented by two different processes concurrently. The goal is to ensure that the counter ends up being equal to 2000 after both processes have finished executing. However, in this scenario, the code fails to achieve this result, leaving the output misleading. The code that was initially given can be summarized as follows:

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As you can see, iCounter is declared as a local variable in the main function, making it non-shareable between processes created by fork(). This leads us to our first issue: when two processes are trying to increment the same iCounter, they are actually working on their own copies of the variable, which leads to inconsistent results.

Identifying the Issues

Incorrect Usage of Semaphores:

The original code incorrectly initialized the semaphore multiple times with sem_init(), which is only intended for anonymous semaphores. This led to undefined behavior.

Non-Shared Counter Variable:

The iCounter variable is not shared between the two forked processes. Each process maintains its own separate copy of the variable, which means increments from one process do not reflect in the other.

The Solution: Using POSIX Shared Memory and Semaphores Correctly

To resolve these issues, we can utilize POSIX shared memory (shm_open) to create a shared counter that can be accessed by both processes. Additionally, we need to ensure that our semaphore is correctly initialized and used to manage access to this counter.

Step-by-Step Implementation

Here’s a complete reference to how we can make these adjustments in the code:

Include Required Headers:
Ensure that you include the headers for IPC, semaphores, and shared memory.

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Setup Shared Memory and Semaphore:
Initialize the shared memory for the counter and the semaphore for synchronization.

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Create and Map Shared Memory:
Implement shared memory allocation with proper error handling.

Fork Processes:
Create two processes that will increment the shared counter.

Semaphore Operations:
Each process must wait on the semaphore before incrementing the counter and signal when done.

Updated Code Example

Here’s the complete reworked code following the discussed principles:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In this guide, we explored the critical aspects of using global semaphores for synchronization in concurrent programming using C. By solving the issues of semaphore misuse and process-level scope, we were able to successfully create a synchronized system that accurately reflects the expected outcome. Always remember to keep inter-process communication channels such as shared memory in mind to ensure your data is consistent across multiple processes.

Try compiling and running the amended code. You should see an output indicating that the Value of iCounter = 2000. This is a clear demonstration of how effective synchronization c