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| 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.
The following example uses the CreateMutex function to create a mutex object and the CreateThread function to create worker threads.
When a thread of this process writes to the database, it first requests ownership of the mutex using the WaitForSingleObject function. If the thread obtains ownership of the mutex, it writes to the database and then releases its ownership of the mutex using the ReleaseMutex function.
This example uses structured exception handling 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.
#include <windows.h>
#include <stdio.h>
#define THREADCOUNT 2
HANDLE ghMutex;
DWORD WINAPI WriteToDatabase( LPVOID );
void main()
{
HANDLE aThread[THREADCOUNT];
DWORD ThreadID;
int i;
// Create a mutex with no initial owner
ghMutex = CreateMutex(
NULL, // default security attributes
FALSE, // initially not owned
NULL); // unnamed mutex
if (ghMutex == NULL)
{
printf("CreateMutex error: %d\n", GetLastError());
return;
}
// Create worker threads
for( i=0; i < THREADCOUNT; i++ )
{
aThread[i] = CreateThread(
NULL, // default security attributes
0, // default stack size
(LPTHREAD_START_ROUTINE) WriteToDatabase,
NULL, // no thread function arguments
0, // default creation flags
&ThreadID); // receive thread identifier
if( aThread[i] == NULL )
{
printf("CreateThread error: %d\n", GetLastError());
return;
}
}
// Wait for all threads to terminate
WaitForMultipleObjects(THREADCOUNT, aThread, TRUE, INFINITE);
// Close thread and mutex handles
for( i=0; i < THREADCOUNT; i++ )
CloseHandle(aThread[i]);
CloseHandle(ghMutex);
}
DWORD WINAPI WriteToDatabase( LPVOID lpParam )
{
DWORD dwCount=0, dwWaitResult;
// Request ownership of mutex.
while( dwCount < 20 )
{
dwWaitResult = WaitForSingleObject(
ghMutex, // handle to mutex
INFINITE); // no time-out interval
switch (dwWaitResult)
{
// The thread got ownership of the mutex
case WAIT_OBJECT_0:
__try {
// TODO: Write to the database
printf("Thread %d writing to database...\n",
GetCurrentThreadId());
dwCount++;
}
__finally {
// Release ownership of the mutex object
if (! ReleaseMutex(ghMutex))
{
// Deal with error.
}
}
break;
// The thread got ownership of an abandoned mutex
case WAIT_ABANDONED:
return FALSE;
}
}
return TRUE;
} |
questions sur les mutex, les singletons, boost etc.
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