dbsetuserdata

Saves, in a DBPROCESS structure, a pointer to user-allocated data.

Syntax

void dbsetuserdata ( PDBPROCESS dbproc, LPVOID ptr );

Arguments

dbproc

Is the DBPROCESS structure that is the handle for a particular workstation/Microsoft® SQL Server™ process. It contains all the information that DB-Library uses to manage communications and data between the workstation and SQL Server.

ptr

Is a generic BYTE pointer to the user’s private data space.

Remarks

The dbsetuserdata function lets an application associate user data with a particular DBPROCESS, eliminating the need for global variables. This function is especially useful when an application has more than one DBPROCESS.

Make sure each application allocates the data that ptr points to. DB-Library never manipulates this data; it merely saves the pointer so the application can use it later.

Examples

The following example shows how dbsetuserdata handles deadlock, an occasional occurrence in high-volume applications.This program fragment sends updates to the SQL Server and reruns the transaction when its message handler detects deadlock:

// Deadlock detection:
// In the DBPROCESS structure, save a pointer to a DBBOOL variable.
// The message handler sets the variable when deadlock occurs.
// The result processing logic checks the variable and resends the
// transaction in case of deadlock.

// Allocate the space for the DBBOOL variable and save it in
// the DBPROCESS structure.

dbsetuserdata(dbproc, malloc(sizeof(DBBOOL)));

// Initialize the variable to FALSE.
*((DBBOOL *) dbgetuserdata(dbproc)) = FALSE;

// Run queries and check for deadlock.
deadlock:


// Did the application get here via deadlock?
// If so, the server has already canceled the transaction.
// Start the application again. In a real application,
// the deadlock handling may need to be somewhat more
// sophisticated. For instance, you may want to keep a
// counter and retry the transaction a fixed number
// of times.

if (*((DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
{
    // Reset the variable to FALSE.
    *((DBBOOL *) dbgetuserdata(dbproc)) = FALSE;
}

// Start the transaction.
dbcmd(dbproc, "begin transaction ");

// Run the first UPDATE command.
dbcmd(dbproc, "UPDATE ...");
dbsqlexec(dbproc);
while (dbresults(dbproc) ! = NO_MORE_RESULTS)
{
// application code
}


// Did the application deadlock?
if (*(DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
    goto deadlock;

// Run the second UPDATE command.
dbcmd(dbproc, "UPDATE ...");
dbsqlexec(dbproc);
while (dbresults(dbproc) ! = NO_MORE_RESULTS
{
    // application code
}

// Did the application deadlock?
if (*((DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
    goto deadlock;


// No deadlock -- Commit the transaction.
dbcmd(dbproc, "COMMIT TRANSACTION");
dbsqlexec(dbproc);
dbresults(dbproc);

// SERVERMSGS
// This is the server message handler. Assume that the dbmsghandle
// function installed it earlier in the application.

servermsgs(dbproc, msgno, msgstate, severity, msgtext, srvname, procname, line)
DBPROCESS    *dbproc;
DBINT        msgno;
int            msgstate;
int            severity;
char            *msgtext;
char            srvname;
char            *procname;
DBUSMALLINT    line;
{

// Is this a deadlock message?
if (msgno = 1205)
{
    // Set the deadlock indicator.
    * ((DBBOOL *) dbgetuserdata(dbproc)) = TRUE;
    return (O);
}

// Normal message handling code here.
}

See Also