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<FONT COLOR="#AOAO99">DAO multi-threading tips</FONT></H3></CENTER>
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<p>This article was contributed by <a href="mailto:bcc27409@vip.cybercity.dk">John Bundgaard</a>.
<P><A HREF="dao_multithreading.zip">Download sample project</A>
</p>
<br>Looks like DAO 3.5 has some support for multi-threading.
<P>As stated by the MFC 4.2 documentation, the Dao database classes are not thread-safe, and that is indeed true.
<P>I recently developed a server application the uses IO completion ports, which by nature, are using a number of threads. Because of that, I had to figure out a way to use DAO in different threads simultaneously.
Here is how I did it:
<PRE><TT><FONT COLOR="#990000">
//
// used a global critical section to prevent multiple threads initializing
// the Dao database simultaneous.
//
CCriticalSection cs;
//
// Some thread function
// (In my server app, 4 thread using this function is started)
//
UINT MyThreadFunc (LPVOID pParam)
{
CSingleLock lock(&cs, TRUE);
//
// Initialize MFC Dao support
//
AfxDaoInit();
CDaoDatabase db;
CSomeDaoRecordset set(&db);
//
// Open the database and recordset.
//
try
{
db.Open(_T("database.mdb"));
set.Open(dbOpenTable);
set.SetCurrentIndex(_T("PrimaryKey"));
}
catch(CException* pe)
{
pe->ReportError();
pe->Delete();
return 0;
}
//
// Allow other threads to initialize Dao
//
lock.Unlock();
//
// OK the database and recordset is now open
//
while( bRunThisThread )
{
//
// NOTE: You do NOT need to lock out other threads vhile calling
// database functions here.
//
WaitForClientRequest();
ParseRequest();
DoDatabaseIO();
}
//
// Now, lock the critical section again
//
lock.Lock();
//
// Close recordset and database.
rs.Close();
db.Close();
// IMPORTANT: do NOT call AfxDaoTerm() !!!!
return 0;
}
</FONT></TT></PRE>
<P>Now, when your application terminates, it will generate a protection error. To prevent that modify your CMyApp::ExitInstance().
<PRE><TT><FONT COLOR="#990000">
int CMyApp::ExitInstance()
{
//
// Insert your own cleanup code here
//
AfxDaoInit();
AfxDaoTerm();
return m_msgCur.wParam;
}
</FONT></TT></PRE>
<P>Notes:
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