Mutexes

Mutexes (mutual-exclusion objects) are synchronization primitives that can be used to protect shared resources from concurrent access. A mutex can be owned by only one thread at a time. When a thread acquires ownership of a mutex, it "owns" the mutex and can protect a critical section of code. If another thread tries to acquire ownership of a mutex that is already owned, the requesting thread will block until the mutex is released. Mutexes are useful for preventing race conditions and ensuring data integrity in multithreaded applications.

Key Concepts

Mutexes provide a simple yet powerful mechanism for synchronizing access to shared resources. Here are some key aspects:

  • Ownership: Only one thread can own a mutex at any given time.
  • Critical Section: The code that accesses a shared resource and is protected by a mutex is called a critical section.
  • Blocking: If a thread attempts to acquire a mutex that is already owned, it will block (wait) until the owner releases it.
  • Recursive Mutexes: Some mutex implementations allow the owning thread to acquire the mutex multiple times without blocking, provided it releases it the same number of times.
  • Named vs. Unnamed Mutexes: Mutexes can be named, allowing them to be shared across processes, or unnamed, for synchronization within a single process.

Proper use of mutexes is crucial for preventing deadlocks and ensuring the stability of your application.

Core Mutex Functions

CreateMutex

HANDLE CreateMutex(LPSECURITY_ATTRIBUTES lpMutexAttributes, BOOL bInitialOwner, LPCTSTR lpName);

Creates or opens a named or unnamed mutex object. If the mutex is created, its initial owned state is determined by the bInitialOwner parameter.

lpMutexAttributes
A pointer to a SECURITY_ATTRIBUTES structure that specifies the security attributes of the mutex. If NULL, the mutex gets default security attributes.
bInitialOwner
If this parameter is TRUE, the calling thread takes ownership of the mutex. Otherwise, the mutex is created in an initially signaled (unowned) state.
lpName
The name of the mutex object. The name is limited to MAX_PATH characters and can contain any character except the backslash character (\).

Return Value

If the function succeeds, the return value is a handle to the newly created or opened mutex object. If the function fails, the return value is NULL. To get extended error information, call GetLastError.

OpenMutex

HANDLE OpenMutex(DWORD dwDesiredAccess, BOOL bInheritHandle, LPCTSTR lpName);

Opens an existing named mutex object. This function allows threads in different processes to access the same mutex object.

dwDesiredAccess
The access to the mutex object. This parameter can be SYNCHRONIZE or MUTEX_ALL_ACCESS.
bInheritHandle
If this parameter is TRUE, then the handle inherited by the new process or thread contains the same value as the hObject member of the STARTUPINFO structure used to create the process. Otherwise, this parameter is FALSE.
lpName
The name of the mutex object to be opened.

Return Value

If the function succeeds, the return value is a handle to the opened mutex object. If the function fails, the return value is NULL. To get extended error information, call GetLastError.

WaitForSingleObject

DWORD WaitForSingleObject(HANDLE hHandle, DWORD dwMilliseconds);

Waits until the specified object is in the signaled state or the time-out interval elapses. A mutex is signaled when it is unowned.

hHandle
A handle to the mutex object.
dwMilliseconds
The time-out interval in milliseconds. Can be INFINITE.

Return Value

If the function succeeds, the return value indicates the event that caused the function to return. Possible return values are WAIT_OBJECT_0 (the object was signaled) and WAIT_TIMEOUT (the time-out interval elapsed).

ReleaseMutex

BOOL ReleaseMutex(HANDLE hMutex);

Releases ownership of a mutex object. This increments the mutex's count. When the count reaches zero, the mutex becomes unowned and is signaled.

hMutex
A handle to the mutex object to be released.

Return Value

If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE. To get extended error information, call GetLastError.

Example Usage

Here's a simplified example demonstrating the use of a mutex to protect access to a shared resource.


#include <windows.h>
#include <iostream>

HANDLE hMutex;
int sharedResource = 0;

DWORD WINAPI WorkerThread(LPVOID lpParam) {
    WaitForSingleObject(hMutex, INFINITE); // Wait for mutex ownership

    // Critical Section: Accessing sharedResource
    sharedResource++;
    std::cout << "Thread " << GetCurrentThreadId() << ": sharedResource is now " << sharedResource << std::endl;
    // End Critical Section

    ReleaseMutex(hMutex); // Release the mutex

    return 0;
}

int main() {
    hMutex = CreateMutex(NULL, FALSE, NULL); // Create an unnamed mutex

    if (hMutex == NULL) {
        std::cerr << "Failed to create mutex. Error: " << GetLastError() << std::endl;
        return 1;
    }

    HANDLE hThread1, hThread2;

    hThread1 = CreateThread(NULL, 0, WorkerThread, NULL, 0, NULL);
    hThread2 = CreateThread(NULL, 0, WorkerThread, NULL, 0, NULL);

    if (hThread1 == NULL || hThread2 == NULL) {
        std::cerr << "Failed to create threads. Error: " << GetLastError() << std::endl;
        CloseHandle(hMutex);
        return 1;
    }

    WaitForSingleObject(hThread1, INFINITE);
    WaitForSingleObject(hThread2, INFINITE);

    CloseHandle(hThread1);
    CloseHandle(hThread2);
    CloseHandle(hMutex);

    std::cout << "Final sharedResource value: " << sharedResource << std::endl;

    return 0;
}

Advanced Concepts

Understanding mutexes involves grasping several related concepts:

  • Recursive Mutexes: Some synchronization primitives, like the Windows CreateMutex function, allow a thread that already owns the mutex to acquire it again. This is useful for functions that might call other functions that also need to acquire the same mutex. However, the thread must release the mutex an equal number of times before it is truly released.
  • Mutexes vs. Semaphores: While both are synchronization primitives, mutexes are typically used for mutual exclusion (only one thread at a time), whereas semaphores can be used to control access to a pool of resources, allowing a specified number of threads to access it concurrently.
  • Deadlocks: A deadlock can occur if two or more threads are blocked indefinitely, each waiting for a resource held by the other. Proper ordering of mutex acquisition is key to avoiding deadlocks.
  • Performance Considerations: Acquiring and releasing mutexes involves kernel transitions and can incur overhead. For high-performance scenarios, consider lighter-weight synchronization mechanisms like atomic operations or spinlocks where appropriate.

See Also