Windows apps
Collapse the table of content
Expand the table of content
The topic you requested is included in another documentation set. For convenience, it's displayed below. Choose Switch to see the topic in its original location.

OpenSemaphore function

Opens an existing named semaphore object.


HANDLE WINAPI OpenSemaphore(
  _In_ DWORD   dwDesiredAccess,
  _In_ BOOL    bInheritHandle,
  _In_ LPCTSTR lpName


dwDesiredAccess [in]

The access to the semaphore object. The function fails if the security descriptor of the specified object does not permit the requested access for the calling process. For a list of access rights, see Synchronization Object Security and Access Rights.

bInheritHandle [in]

If this value is TRUE, processes created by this process will inherit the handle. Otherwise, the processes do not inherit this handle.

lpName [in]

The name of the semaphore to be opened. Name comparisons are case sensitive.

This function can open objects in a private namespace. For more information, see Object Namespaces.

Terminal Services:  The name can have a "Global\" or "Local\" prefix to explicitly open an object in the global or session namespace. The remainder of the name can contain any character except the backslash character (\). For more information, see Kernel Object Namespaces.

Note  Fast user switching is implemented using Terminal Services sessions. The first user to log on uses session 0, the next user to log on uses session 1, and so on. Kernel object names must follow the guidelines outlined for Terminal Services so that applications can support multiple users.

Return value

If the function succeeds, the return value is a handle to the semaphore object.

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


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 to 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.


Minimum supported client

Windows XP [desktop apps | UWP apps]

Minimum supported server

Windows Server 2003 [desktop apps | UWP apps]

Minimum supported phone

Windows Phone 8


WinBase.h (include Windows.h)





Unicode and ANSI names

OpenSemaphoreW (Unicode) and OpenSemaphoreA (ANSI)

See also

Object Names
Semaphore Objects
Synchronization Functions



© 2018 Microsoft