Initializes a critical section object and sets the spin count for the critical section. Spinning means that when a thread tries to acquire a critical section that is locked, the thread enters a loop, checks to see if the lock is released, and if the lock is not released, the thread goes to sleep.

Syntax

BOOL WINAPI InitializeCriticalSectionAndSpinCount(
  __out  LPCRITICAL_SECTION lpCriticalSection,
  __in   DWORD dwSpinCount
);

Parameters

lpCriticalSection [out]

A pointer to the critical section object.

dwSpinCount [in]

The spin count for the critical section object. On single-processor systems, the spin count is ignored and the critical section spin count is set to 0 (zero). On multiprocessor systems, if the critical section is unavailable, the calling thread spin dwSpinCount times before performing a wait operation on a semaphore associated with the critical section. If the critical section becomes free during the spin operation, the calling thread avoids the wait operation.

Windows 2000:  If the high-order bit is set, the function pre-allocates the event used by the EnterCriticalSection function. Pre-allocation guarantees that entering or leaving the critical section will not raise an exception in low memory conditions. Do not set this bit if you are creating a large number of critical section objects, because it consumes a significant amount of nonpaged pool. Note that this event is allocated on demand starting with Windows XP and the high-order bit is ignored.

Return Value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero (0). To get extended error information, call GetLastError.

Remarks

The threads of a single process can use a critical section object for mutual-exclusion synchronization. There is no guarantee about the order that threads obtain ownership of the critical section, however, the system is fair to all threads.

The process is responsible for allocating the memory used by a critical section object, which it can do by declaring a variable of type CRITICAL_SECTION. Before using a critical section, some thread of the process must initialize the object. You can subsequently modify the spin count by calling the SetCriticalSectionSpinCount function.

After a critical section object is initialized, the threads of the process can specify the object in the EnterCriticalSection, TryEnterCriticalSection, or LeaveCriticalSection function to provide mutually exclusive access to a shared resource. For similar synchronization between the threads of different processes, use a mutex object.

A critical section object cannot be moved or copied. The process must also not modify the object, but must treat it as logically opaque. Use only the critical section functions to manage critical section objects. When you have finished using the critical section, call the DeleteCriticalSection function.

A critical section object must be deleted before it can be reinitialized. Initializing a critical section that is already initialized results in undefined behavior.

The spin count is useful for critical sections of short duration that can experience high levels of contention. Consider a worst-case scenario, in which an application on an SMP system has two or three threads constantly allocating and releasing memory from the heap. The application serializes the heap with a critical section. In the worst-case scenario, contention for the critical section is constant, and each thread makes an expensive call to the WaitForSingleObject function. However, if the spin count is set properly, the calling thread does not immediately call WaitForSingleObject when contention occurs. Instead, the calling thread can acquire ownership of the critical section if it is released during the spin operation.

You can improve performance significantly by choosing a small spin count for a critical section of short duration. For example, the heap manager uses a spin count of roughly 4,000 for its per-heap critical sections.

To compile an application that uses this function, define _WIN32_WINNT as 0x0403 or later. For more information, see Using the Windows Headers.

Examples

For an example that uses InitializeCriticalSectionAndSpinCount, see Using Critical Section Objects.

Requirements

See Also

Critical Section Objects
DeleteCriticalSection
InitializeCriticalSection
SetCriticalSectionSpinCount
Synchronization Functions
WaitForSingleObject

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Build date: 11/6/2008