CreateSemaphore

A version of this page is also available for

Windows Embedded CE 6.0 R3

4/8/2010

This function creates a named or unnamed semaphore object.

Syntax

HANDLE CreateSemaphore(
  LPSECURITY_ATTRIBUTES lpSemaphoreAttributes,
  LONG lInitialCount,
  LONG lMaximumCount,
  LPCTSTR lpName
);

Parameters

• lpSemaphoreAttributes
  [in] Set to NULL.

• lInitialCount
  [in] Specifies an initial count for the semaphore object. This value must be greater than or equal to zero and less than or equal to lMaximumCount. The state of a semaphore is signaled when its count is greater than zero and nonsignaled when it is zero. The count is decreased by one whenever a wait function releases a thread that was waiting for the semaphore. The count is increased by a specified amount by calling the ReleaseSemaphore function.

• lMaximumCount
  [in] Specifies the maximum count for the semaphore object. This value must be greater than zero.

• lpName
  [in] Long pointer to a null-terminated string specifying the name of the semaphore object. The name is limited to MAX_PATH characters and can contain any character except the backslash path-separator character (\). Name comparison is case sensitive.

  If lpName matches the name of an existing named semaphore object, the lInitialCount and lMaximumCount parameters are ignored because they have already been set during the creation process.

  Each object type, such as memory maps, semaphores, events, message queues, mutexes, and watchdog timers, has its own separate namespace. Empty strings ("") are handled as named objects. On Windows desktop-based platforms, synchronization objects all share the same namespace.

Return Value

A handle to the semaphore object indicates success. If the named semaphore object existed before the function call, the GetLastError function returns ERROR_ALREADY_EXISTS. Otherwise, GetLastError returns zero.

NULL indicates failure. To get extended error information, call GetLastError.

Remarks

The handle returned by CreateSemaphore can be used in any function that requires a handle to a semaphore object.

Any thread of the calling process can specify the semaphore-object handle in a call to one of the wait functions. The single-object wait functions return when the state of the specified object is signaled. The multiple-object wait functions can be instructed to return either when any one or when all of the specified objects are signaled. When a wait function returns, the waiting thread is released to continue its execution.

The state of a semaphore object is signaled when its count is greater than zero and nonsignaled when its count is equal to zero. The lInitialCount parameter specifies the initial count. Each time a waiting thread is released because of the semaphore's signaled state, the count of the semaphore is decreased by one. Use the ReleaseSemaphore function to increment a semaphore's count by a specified amount. The count can never be less than zero or greater than the value specified in the lMaximumCount parameter.

Multiple processes can have handles of the same semaphore object, enabling use of the object for interprocess synchronization. To share an object, a process can specify the name of a semaphore object in a call to the CreateSemaphore function.

Use the CloseHandle function to close the handle. The system closes the handle automatically when the process terminates. The semaphore object is destroyed when its last handle is closed.

Windows Embedded CE does not implement OpenSemaphore. Use the CreateSemaphore function to open a semaphore.

Example

*pHandle = CreateSemaphore(pSemaphoreAttributes, InitialCount, MaximumCount, pName);

Requirements

See Also

Reference

Synchronization Functions
ReleaseSemaphore

Other Resources

CloseHandle