Database programming with multithreads


I developed an database application using VC++ and Micorsoft Access driver as the ODBC source. There are 2 threads in the application both accessing the same table. The first thread writes data in the table and the second thread reads and deletes data from the table. Here is a snapshot of the code of the second thread:

while (!attackRecSet->IsEOF())
{
      .
      .
      .
      .
      .
      .
      try
      {
            attackRecSet->Delete();
            attackRecSet->MoveNext();
      }                                          //try
      catch(CDBException *e)
      {
            MessageBox(NULL, e->m_strError, "Error", MB_OK);
            e->Delete();
      }                                          //catch            
}                                                //while      

An error message occurs because of the delete function and the thread stops working. the error message is:

" Could not update; currently locked by user 'admin' on machine ISLAM_COMPUTER'. "

and when I press OK the following message occurs for ever (it reoccurs if I press the OK button):

" Invalid cursor position; no keyset defined. "

If I commented the delete function the thread will work perfectly. I searched the MSDN for an explanation but with no use. Could anyone tell where could I find an explanation for this error.

Islam Hegazy
islhegAsked:
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nchristy0Connect With a Mentor Commented:
you need to make sure that one process waits while the other one has access to the database, this is a typical concept in operating systems...

you need to use a semaphore

try this

(this is pseudocode)

prior to accessing the database call P(S), after leaving call V(S)
class Semaphore() {
private value = 0;
private queue of waitting processes;
public function P()
public function V()
}
consutructor() {
value = 0;
}

P() {
 value--;
 if(value < 0) {
          // add process to queue of waitting processes
         // block the process
  }
}

V() {
  value++;
  if(value <= 0) {
     // get 1 process from queue of waitting processes and unblock the process
  }
}

this is the most well known solution to your problem

hope it helps
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