Using Semaphore Objects
In the following example, a process uses a semaphore object to limit the
number of windows it creates. First, it uses the
CreateSemaphore function to create the semaphore and to specify initial and maximum counts.
HANDLE hSemaphore;
LONG cMax = 10;
LONG cPreviousCount;
// Create a semaphore with initial and max. counts of 10.
hSemaphore = CreateSemaphore(
NULL, // no security attributes
cMax, // initial count
cMax, // maximum count
NULL); // unnamed semaphore
if (hSemaphore == NULL)
{
// Check for error.
}
Before any thread of the process creates a new window, it uses the
WaitForSingleObject function to determine whether the semaphore's current count permits the
creation of additional windows. The wait function's time-out parameter is set to
zero, so the function returns immediately if the semaphore is nonsignaled.
DWORD dwWaitResult;
// Try to enter the semaphore gate.
dwWaitResult = WaitForSingleObject(
hSemaphore, // handle of semaphore
0L); // zero-second time-out interval
switch (dwWaitResult) {
// The semaphore object was signaled.
case WAIT_OBJECT_0:
// OK to open another window.
break;
// Semaphore was nonsignaled, so a time-out occurred.
case WAIT_TIMEOUT:
// Cannot open another window.
break;
}
When a thread closes a window, it uses the
ReleaseSemaphore function to increment the semaphore's count.
// Increment the count of the semaphore.
if (!ReleaseSemaphore(
hSemaphore, // handle of semaphore
1, // increase count by one
NULL) ) // not interested in previous count
{
// Deal with the error.
}
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