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.
See Also