CreateSemaphoreA function (winbase.h)

Creates or opens a named or unnamed semaphore object.

To specify an access mask for the object, use the CreateSemaphoreEx function.


HANDLE CreateSemaphoreA(
  [in, optional] LPSECURITY_ATTRIBUTES lpSemaphoreAttributes,
  [in]           LONG                  lInitialCount,
  [in]           LONG                  lMaximumCount,
  [in, optional] LPCSTR                lpName


[in, optional] lpSemaphoreAttributes

A pointer to a SECURITY_ATTRIBUTES structure. If this parameter is NULL, the handle cannot be inherited by child processes.

The lpSecurityDescriptor member of the structure specifies a security descriptor for the new semaphore. If this parameter is NULL, the semaphore gets a default security descriptor. The ACLs in the default security descriptor for a semaphore come from the primary or impersonation token of the creator.

[in] lInitialCount

The 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.

[in] lMaximumCount

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

[in, optional] lpName

The name of the semaphore object. The name is limited to MAX_PATH characters. Name comparison is case sensitive.

If lpName matches the name of an existing named semaphore object, this function requests the SEMAPHORE_ALL_ACCESS access right. 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, 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 namespace.

The name can have a "Global" or "Local" prefix to explicitly create the 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. Fast user switching is implemented using Terminal Services sessions. Kernel object names must follow the guidelines outlined for Terminal Services so that applications can support multiple users.

The object can be created in a private namespace. For more information, see Object Namespaces.

Return value

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.


The handle returned by CreateSemaphore has the SEMAPHORE_ALL_ACCESS access right; it can be used in any function that requires a handle to a semaphore object, provided that the caller has been granted access. If a semaphore is created from a service or a thread that is impersonating a different user, you can either apply a security descriptor to the semaphore when you create it, or change the default security descriptor for the creating process by changing its default DACL. For more information, see Synchronization Object Security and Access Rights.

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. The count can never be less than zero or greater than the value specified in the lMaximumCount parameter.

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. Each time a thread completes a wait for a semaphore object, the count of the semaphore object is decremented by one. When the thread has finished, it calls the ReleaseSemaphore function, which increments the count of the semaphore object.

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](/windows/win32/api/synchapi/nf-synchapi-opensemaphorew) 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.


For an example that uses CreateSemaphore, see Using Semaphore Objects.


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header winbase.h (include Windows.h)
Library Kernel32.lib
DLL Kernel32.dll

See also





Object Names




Semaphore Objects

Synchronization Functions