The CreateSemaphore function creates a named or unnamed semaphore object.

HANDLE CreateSemaphore(
  LPSECURITY_ATTRIBUTES lpSemaphoreAttributes,
                       // pointer to security attributes
  LONG lInitialCount,  // initial count
  LONG lMaximumCount,  // maximum count
  LPCTSTR lpName       // pointer to semaphore-object name
);
 
Parameters

lpSemaphoreAttributes

Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpSemaphoreAttributes is NULL, the handle cannot be inherited.

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.

lInitialCount

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

Specifies the maximum count for the semaphore object. This value must be greater than zero.

lpName

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, this function requests SEMAPHORE_ALL_ACCESS access to the existing object. In this case, the lInitialCount and lMaximumCountparameters are ignored because they have already been set by the creating process. If the lpSemaphoreAttributesparameter 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, waitable timer, job, or file-mapping object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same name space.

Return Values

 

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 function returns a handle to the existing object and GetLastError returns ERROR_ALREADY_EXISTS.

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

Remarks

 

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:

• A child process created by the CreateProcess function can inherit a handle to a semaphore object if the lpSemaphoreAttributes parameter of CreateSemaphore enabled inheritance.
• A process can specify the semaphore-object handle in a call to the DuplicateHandle function to create a duplicate handle that can be used by another process.
• A process can specify the name of a semaphore object in a call to the OpenSemaphore or 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 has been closed.