Semaphores

CreateSemaphore
ReleaseSemaphore
OpenSemaphore
Example: semaphore.cpp

CreateSemaphore

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.
Windows 95: The lpSecurityDescriptor member of the structure is ignored.

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

Points 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 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.

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 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.

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.

ReleaseSemaphore

The ReleaseSemaphore function increases the count of the specified semaphore object by a specified amount.

BOOL ReleaseSemaphore(
    HANDLE hSemaphore,	// handle of the semaphore object  
    LONG lReleaseCount,	// amount to add to current count  
    LPLONG lpPreviousCount 	// address of previous count 
   );

Parameters

hSemaphore

Identifies the semaphore object. The CreateSemaphore or OpenSemaphore function returns this handle. Windows NT: This handle must have SEMAPHORE_MODIFY_STATE access. For more information, see Interprocess Synchronization Objects.

lReleaseCount

Specifies the amount by which the semaphore object's current count is to be increased. The value must be greater than zero. If the specified amount would cause the semaphore's count to exceed the maximum count that was specified when the semaphore was created, the count is not changed and the function returns FALSE.

lpPreviousCount

Points to a 32-bit variable to receive the previous count for the semaphore. This parameter can be NULL if the previous count is not required.

Return Values

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

Remarks

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 process that calls the CreateSemaphore function specifies the semaphore's 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.
Typically, an application uses a semaphore to limit the number of threads using a resource. Before a thread uses the resource, it specifies the semaphore handle in a call to one of the wait functions. When the wait function returns, it decreases the semaphore's count by one. When the thread has finished using the resource, it calls ReleaseSemaphore to increase the semaphore's count by one.

Another use of ReleaseSemaphore is during an application's initialization. The application can create a semaphore with an initial count of zero. This sets the semaphore's state to nonsignaled and blocks all threads from accessing the protected resource. When the application finishes its initialization, it uses ReleaseSemaphore to increase the count to its maximum value, to permit normal access to the protected resource.

OpenSemaphore

The OpenSemaphore function returns a handle of an existing named semaphore object.

HANDLE OpenSemaphore(
    DWORD dwDesiredAccess,	// access flag 
    BOOL bInheritHandle,	// inherit flag 
    LPCTSTR lpName 	// pointer to semaphore-object name  
   );

Parameters

dwDesiredAccess

Specifies the requested access to the semaphore object. For systems that support object security, the function fails if the security descriptor of the specified object does not permit the requested access for the calling process. This parameter can be any combination of the following values:

Access	Description
`SEMAPHORE_ALL_ACCESS`	Specifies all possible access flags for the semaphore object.
`SEMAPHORE_MODIFY_STATE`	Enables use of the semaphore handle in the `ReleaseSemaphore` function to modify the semaphore's count.
`SYNCHRONIZE`	Windows NT only: Enables use of the semaphore handle in any of the wait functions to wait for the semaphore's state to be signaled.

bInheritHandle

Specifies whether the returned handle is inheritable. If TRUE, a process created by the CreateProcess function can inherit the handle; otherwise, the handle cannot be inherited.

lpName

Points to a null-terminated string that names the semaphore to be opened. Name comparisons are case sensitive.

Return Values

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

Remarks

The OpenSemaphore function enables multiple processes to open handles of the same semaphore object. The function succeeds only if some process has already created the semaphore by using the CreateSemaphore function. The calling process can use the returned handle in any function that requires a handle of a semaphore object, such as the wait functions, subject to the limitations of the access specified in the dwDesiredAccess parameter.

The handle can be duplicated by using the DuplicateHandle 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.