The CreateSemaphore function creates a named or unnamed semaphore object.
// pointer to security attributes
// initial count
// maximum count
// pointer to semaphore-object name
Windows NT: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new semaphore. If lpSemaphoreAttributes is NULL, the semaphore gets a default security descriptor.
Windows 95: The lpSecurityDescriptor member of the structure is
If lpName matches the name of an existing named semaphore object, this function requests SEMAPHORE_ALL_ACCESS access to the existing object. In this case, the lInitialCount and lMaximumCount parameters are ignored because they have already been set by the creating process. If the lpSemaphoreAttributes parameter is not NULL, it determines whether the handle can be inherited, but its security-descriptor member is ignored.
If lpName is NULL, the semaphore object is created without a name.
If lpName matches the name of an existing event, mutex, or file-mapping object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because event, mutex, semaphore, and file-mapping objects share the same name space.
If the function succeeds, the return value is a handle to the semaphore object. If the named semaphore object existed before the function call, the GetLastError function returns ERROR_ALREADY_EXISTS. Otherwise, GetLastError returns zero.
If the function fails, the return value is NULL. To get extended error information, call GetLastError.
The handle returned by CreateSemaphore has SEMAPHORE_ALL_ACCESS access to the new semaphore object and 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. The following object-sharing mechanisms are available:
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 has been closed.
CloseHandle, CreateProcess, DuplicateHandle, OpenSemaphore, ReleaseSemaphore, SECURITY_ATTRIBUTES
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