INF: Multithreaded Reentrancy and DB-LIBRARY

• Microsoft SQL Server Programmer's Toolkit, version 4.2

SUMMARY

DB-Library (DB-Lib) functions and routines that access the same DBPROCESS are not reentrant across multiple threads. Therefore, be sure that you serialize all DB-Lib calls that access the same DBPROCESS in multithreaded applications you develop.

In applications where each thread uses a separate DBPROCESS, it is not necessary to serialize the DB-Lib calls. However, there is one important exception: You must serialize any calls that involve global variables. You can do this by using a synchronization method such as a flag variable, semaphore, or event.

MORE INFORMATION

The following guidelines will help you determine where to protect global variables:

• Protect error and message handling functions installed by dberrhandle and dbmsghandle, because these are global to the entire DB-LIBRARY application. These should be protected with a separate synchronization object to avoid colliding with the protection around certain DB-LIBRARY calls below.
• Protect DB-Lib functions that do not take a DBPROCESS pointer or a DBCURSOR handle as the first parameter. For example, dberrhandle and dbsettime.
• Protect DB-Lib functions wherever you pass a null DBPROCESS pointer. For example, dbsetopt.
• Protect the dbopen function because it returns a DBPROCESS pointer. DBPROCESS pointers can potentially be null and can therefore use DB-LIBRARY global variables.

For OS/2

• // Make this variable global to the entire application HSEM semDblib;
• // This code would be in a thread that uses DB-LIBRARY DosSemRequest(&semDblib, SEM_INDEFINITE_WAIT); pDbproc = dbopen(pLoginRec, "myserver"); DosSemClear(&semDblib);

• // make this variable global to the entire application HANDLE hOpenEvent;
• // create the event handle at application startup

     // have it set on creation, with auto-reset
     hOpenEvent = CreateEvent(NULL,FALSE,TRUE,NULL);
  
  

• // this code would be in a thread that uses DB-LIBRARY

     // it waits for other threads to complete, opens a
     // connection, then sets the event so other threads
     // can continue
     WaitForSingleObject(hOpenEvent,INFINITE);
     pDbproc = dbopen(pLoginRec,"myserver");
     SetEvent(hOpenEvent);
  
  

• // close the event handle at application exit CloseHandle(hOpenEvent);