Using Mutex Objects
You can use a mutex object to protect a shared resource from simultaneous
access by multiple threads or processes. Each thread must wait for ownership of the
mutex before it can execute the code that accesses the shared resource. For
example, if several threads share access to a database, the threads can use a
mutex object to permit only one thread at a time to write to the database.
In the following example, a process uses the
CreateMutex function to create a named mutex object or open a handle of an existing mutex
object.
HANDLE hMutex;
// Create a mutex with no initial owner.
hMutex = CreateMutex(
NULL, // no security attributes
FALSE, // initially not owned
"MutexToProtectDatabase"); // name of mutex
if (hMutex == NULL)
{
// Check for error.
}
When a thread of this process writes to the database, as in the next example,
it first requests ownership of the mutex. If it gets ownership, the thread
writes to the database and then releases its ownership.
The example uses the
try-
finally structured exception-handling syntax to ensure that the thread properly
releases the mutex object. The
finally block of code is executed no matter how the
try block terminates (unless the
try block includes a call to the
TerminateThread function). This prevents the mutex object from being abandoned inadvertently.
BOOL FunctionToWriteToDatabase(HANDLE hMutex)
{
DWORD dwWaitResult;
// Request ownership of mutex.
dwWaitResult = WaitForSingleObject(
hMutex, // handle of mutex
5000L); // five-second time-out interval
switch (dwWaitResult)
{
// The thread got mutex ownership.
case WAIT_OBJECT_0:
try {
// Write to the database.
}
finally {
// Release ownership of the mutex object.
if (! ReleaseMutex(hMutex)) {
// Deal with error.
}
break;
}
// Cannot get mutex ownership due to time-out.
case WAIT_TIMEOUT:
return FALSE;
// Got ownership of the abandoned mutex object.
case WAIT_ABANDONED:
return FALSE;
}
return TRUE;
}
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